Few counties in England present so great a diversity of scenery
as Somerset, and none possesses a greater variety of geological
formations. Although the land nowhere attains the dignity of
a mountain, yet the older rocks which form the more solid
framework of the county stand out boldly on Exmoor and in the
Quantock Hills of west Somerset, in Crook Peak and other portions
of the Mendip Hills, and rise to a lesser elevation on Broadfield Down
in north Somerset. Along the borders of the county to the east and
south-east there are ranges of secondary rocks, connected north of Bath
with the Cotteswold Hills, and extending southwards in a series of
escarpments to the neighbourhood of Wincanton and Yeovil, and thence
along the Blackdown Hills to near Wellington. They form a kind of
crescent, from the centre of which the older rocks of Mendip proceed
westwards. Between these hilly grounds fertile vales and rich moorlands
stretch irregularly inland from the shores of the Bristol Channel.
The stratified and igneous rocks met with are as follows:—
|Period||Formation||Character of the Strata||Approximate thickness in feet|
|Recent and Pleistocene||Alluvium||Silt, clay, and peat||10 to 60|
|Blown Sand||Fine sand||20|
|Marine Sand (Burtle Beds)||Shelly sand and shingle with bands of sandy limestone||12|
|Cave Deposits||Red earth and stalagmite with bones||10|
|Valley Gravel||Gravel, sand, and loam||10 to 30|
|Plateau Drift||Angular and pebbly gravel||2|
|Cretaceous||Middle Chalk||Soft limestone with few flints||150|
|Lower Chalk||Soft limestone and marl; phosphatic nodules at base||200|
|Upper Greensand||Calcareous grit, chert, sandstone, and sands||100 to 180|
|Gault||Bluish-grey clay||70 to 90|
|Oolitic||Corallian||Oolitic limestone, calcareous sandstone and sand||120|
|Oxford Clay||Stiff grey clay and shale, with septaria||400|
|Forest Marble and Bradford Clay||Flaggy oolitic limestone, sand and sandstone, and clay||100|
|Great Oolite||Oolitic and shelly limestone||80 to 110|
|Fullonian or Fullers' Earth||Grey marly clay, with fullers' earth and argillaceous limestone||150|
|Inferior Oolite||Oolitic and shelly limestone||40 to 80|
|Jurassic||Midford Sands (Passage Beds)||Buff sands with bands and nodules of calcareous sandstone||20 to 150|
|Liassic||Upper Lias||Dark clay and pale argillaceous limestone||10 to 40|
|Middle Lias||Hard brown ironshot limestone, sands and sandy shale||100|
|Lower Lias||Dark blue clay and limestone||up to 350|
|New Red Sandstone Series or Permo-Triassic||Rhætic Beds||White limestone, black shale, and grey marl||50|
|Upper (Keuper) Marls passing locally into Dolomitic Conglomerate||Red and variegated marl and conglomerate||up to 750|
|Upper Sandstone||Red sandstone||200|
|Conglomerate||Conglomerate with limestone and other fragments||75|
|Lower Marls||Marl with sandstone||120|
|Lower Sandstone and Breccia||Red sandstone and breccia||500|
|Carboniferous Coal Measures||Upper Coal Measures||Shales and sandstones with seams of coal||2,200|
|Pennant Grit||Chiefly red and grey sandstones||2,500 to 3,000|
|Lower Coal Measures||Shales and sandstones with seams of coal||2,800|
|Millstone Grit||Hard compact sandstone, with shales||500 to 900|
|Carboniferous Limestone Series||Upper Limestone Shales||Shales with bands of limestone and grit||500 to 600|
|Carboniferous Limestone||Hard blue and grey limestone with nodules of chert||1,500 to 3,000|
|Lower Limestone Shales||Soft greenish-grey shales with bands of limestone||320 to 500|
|Old Red Sandstone and Devonian||Upper Old Red Sandstone||Hard red sandstone and quartzose conglomerate||over 2,000|
|Upper, Middle, and Lower Devonian||Hard sandstones and grits, slates and occasional limestones||thickness unknown|
|Igneous Rocks (of various ages)||Volcanic Ashes and Lava|
With such a list of formations the great diversity of soil and feature
in Somerset will readily be understood. The soils indeed are but little
modified by material drifted from a distance; they are due so directly
to the outcropping formations that geological boundary lines may be
traced across ploughed fields from the evidence of weathered fragments
of the underlying rocks. On the elevated moorlands of Exmoor and the
Quantocks the heather, dwarf oak and whortleberry abound, and the red
deer in places runs wild. On the tablelands of Mendip and Broadfield
Down, amidst wild and cultivated tracts, we find excellent pasturage for
sheep. The Oolitic and Liassic uplands are partly under plough cultivation, while the slopes of these stonebrash hills are studded with orchards.
The clay vales of Lias and New Red Marl form rich pastures, and the
levels in particular are noted grazing and dairy lands, one of the products
being the famous Cheddar cheese. More than half the land is under
permanent pasture, and the county is essentially an agricultural one; but
it is by no means wholly so—there are mines of coal and iron, famous
building stones, fullers' earth, peat moors or turbary lands, lime and
cement works, and formerly there were important mines of lead and
zinc of which the refuse heaps are now worked.
As might be expected Somerset has had a considerable share in
the development of geological knowledge. As early as 1719 the relations of its Coal Measures to the overlying 'Red earth' and 'Lyas' were
pointed out by John Strachey, while towards the close of the eighteenth
century William Smith, 'the father of English Geology,' was actively
engaged in setting out and superintending the works for the Somerset
coal canal, thereby gathering much of that knowledge of the succession of the strata and of their characteristic fossils which formed so
important a part of the foundation of geological science. (fn. 1) Subsequently
Buckland, W. D. Conybeare, Thomas Weaver and William Lonsdale
determined the geological structure of various portions of the county in
the masterly way characteristic of those old workers. The Geological
Survey under the direction of De la Beche depicted on maps the superficial areas over which the many different strata are exposed, a task aided
by William Sanders of Bristol, who afterwards personally surveyed a large
part of Somerset between Bristol and the Mendip Hills, and published in
1862 a map on the scale of 4 inches to a mile.
The researches of numerous other observers during the latter half
of the nineteenth century largely increased our knowledge, and among
these we are indebted especially to Charles Moore of Bath. Since his
death local interest in geology has ever been fostered by the Rev. H. H.
Winwood, also of Bath. (fn. 2)
Geologists are far from unanimous with regard to the age and
correlation of some of the grits and slates met with in that large and
picturesque moorland region of west Somerset which extends from the
Quantock Hills on the east to Exmoor on the west. (fn. 3)
The rocks classed as Devonian are there so folded and faulted that
no reliable estimates can be made of their thickness, but the main lithological groups can be traced across country through Exmoor with generally an east and west strike to the Quantocks, where the general trend
is north-west and south-east; and these divisions, first marked out by
Etheridge, have been surveyed in more detail by Mr. W. A. E. Ussher.
Later researches have however thrown considerable doubt on the regularity of the succession which they depicted. (fn. 4)
On parts of Exmoor and the Brendon Hills (1,344 feet high) there
is a group of much folded and highly cleaved slates, known as the
Morte Slates, which for many years were regarded as an unfossiliferous set of beds overlying the Ilfracombe group in the Middle
In 1890, however, the late Dr. Henry Hicks discovered fossils in
these Morte Slates at Woolacombe in north Devon, and elsewhere in
the neighbourhood. Although the specimens were much distorted and
in a bad state of preservation, he was able to recognize Lingula, Orthis,
Stricklandinia, Cardiola, and some others of Silurian aspect, and he maintained that the rocks were the oldest in north Devon. (fn. 5) Continuing his
researches into west Somerset, Dr. Hicks was successful in finding fossils
in the Morte Slates of the Oakhampton slate quarry near Wiveliscombe,
and in calcareous bands in the slates at Treborough on the Brendon
Hills. Here he obtained the trilobites Homalonotus and Dalmanites, also
Strophomena, Chonetes, and other forms suggestive of the Lower Devonian
period. (fn. 6) These observations led Dr. Hicks to conclude that in the
Morte Slates we have life-zones belonging to both Upper Silurian and
Lower Devonian, and although his views have not as yet been fully
accepted, his discovery of fossils in beds previously regarded as barren
is of the utmost importance, while his contention that the beds in west
Somerset are Lower, rather than Middle, Devonian has received the
support of the Rev. G. F. Whidborne and Mr. J. E. Marr.
The famous iron ores of the Exmoor and Brendon Hills occur in
these slaty rocks, and for the most part above certain calcareous bands
which are impersistently seen in the slates. The late Sir Warington
Smyth observed that these 'ores of iron occur both in regular strata and
in veins; and it is evident that they were known and worked by our
forefathers, although the common tradition of their having been opened
by the Romans, and the term "Roman Vein" attached to one locality
would seem, I think, of less weight than the probability of their having
been wrought about the time when Queen Elizabeth invited German
miners to England, supported as it is by the name Eyesen Hill by which
one of the ancient workings is still known.' (fn. 7) Nodules of iron ore have
been met with in the shales in the Exe valley north-east of Simonsbath,
but the ore has been obtained mainly from lodes which have been extensively worked at Raleigh's Cross in the Brendon Hills, where one lode
is from 2 to 20 feet wide. The lodes yield brown iron ore, with also
goethite and much manganese ore, occasionally copper pyrites, and some
hæmatite (red iron ore) at the surface, whilst at a depth the brown ore
passes into sparry ore (carbonate of iron). According to Smyth a series
of parallel fissures must originally have been opened in the slaty rocks,
nearly concordant with the general planes of stratification. These, after
being filled with carbonate of iron, some quartz and fragments of the
containing rock, were dislocated, while much of the ore was changed
from carbonate to peroxide of iron. The occurrence of pebbles of
hæmatite in the New Red strata of west Somerset seems to indicate
that the infilling of the fissures was of earlier date than those red rocks.
Elsewhere among the older Devonian Rocks the Foreland Grits,
comprising the fine grained red, grey and purplish grits, extending from
the Foreland in Devonshire to Porlock Hill, rise up in North Hill and
Grabbist Hill to the west and south of Minehead, and again appear in
Dunster Park. On the coast they form bold precipitous fronts to the
stormy seas of the Bristol Channel. Resting apparently upon these grits,
though in faulted relationship with them in west Somerset are the
slaty grits and schists known as the Lynton Beds, which yield Spirifer
hystericus, and appear at Oare and the north end of Luccot Hill. The
connection between these rocks and the Morte Slates has yet to be
The Lynton Beds are surmounted by a great series known as the
Hangman Grits, which include fine and coarse grained grits, speckled
with red, and contain some slaty bands and occasional fossils such as
Natica and Myalina. These rocks, which may possibly be equivalent to
the Foreland Sandstones, form fine moorland ranges, including the prominent Dunkery Beacon (1,707 feet), Croydon Hill (1,251 feet), and
the higher portions of the Quantocks with Wills Neck (1,261 feet).
Above the Hangman Grits is a series known as the Ilfracombe
Slates, comprising slates with occasional bands of limestone, which are
sometimes conspicuous, but elsewhere occur in strings, and these in many
places have been entirely dissolved away along the outcrop.
These rocks are described by Mr. Ussher as forming high longbacked ranges, with frequent conical hills of lesser elevation than the
grits below and above them. They extend from Exmoor towards the
Brendon Hills, and occur on the eastern borders of the Quantocks,
where at Great Holwell, Asholt and Dodington the limestones are best
developed. Here the Rev. H. H. Winwood found a number of fossils,
including crinoids, brachiopods, and the coral Favosites cervicornis. Gritty
beds locally occur in this division as noted by Mr. Ussher, and these
appear in inliers among the New Red rocks east of the Quantocks.
Still higher in succession is a third series of grits, the Pickwell
Down Sandstones, variously coloured green, grey and purple grits with
slates, which form barren moorlands intersected by the river Barle
north-west of Dulverton, and extending eastwards by Winsford and
Exton Hills and over Haddon Down.
To the south of Dulverton there is a tract of the higher Devonian
slaty and gritty rocks grouped as Baggy and Pilton Beds, which pass
upwards into the Lower Culm Measures near Brushford. These Lower
Carboniferous rocks, so well known to the south at Bampton in Devonshire, do not enter conspicuously into the geology of west Somerset,
although strata of equivalent age occur elsewhere in the county as noted
In the great mass of Devonian rocks, which represent the hardened
sands and muds of ancient sea bottoms, there are evidences of local volcanic activity in certain ashy beds which occur at the base of the Ilfracombe Series between Cockercombe and Adscombe on the Quantocks.
In the Morte Series there is a rock referred to by Mr. Ussher as
the Bittadon felsite, which outcrops east of Exton Hill at Armoor and
Withiel Florey 'apparently at or near the same horizon' (fn. 8) ; while a
syenitic rock was observed many years ago by Leonard Horner in the
Ilfracombe Series at Hestercombe, north of Taunton, where the adjacent
slates have been altered into a kind of hone stone.
The economic products of the Devonian rocks include the iron ores
before mentioned, as well as slates and other materials employed for local
building purposes and road metal.
If the Lower Devonian rocks merge into the Upper Silurian, so do
the Upper Devonian into the Carboniferous, and there is every probability that the usually accepted Devonian group of Devonshire is in
reality a more comprehensive system than the Old Red Sandstone which
it has been taken to represent. Mr. Ussher indeed has remarked 'that
the Upper Devonian Beds may be in part at least equivalent to what is
called the Lower Carboniferous Slate Series of the south of Ireland; and
if so, such correlations as the Pickwell Down Beds with the Upper Old
Red Sandstone are natural.' The strata may be grouped as follows :— (fn. 9)
|North Devon and West Somerset||East Somerset|
|Upper Culm Measures||Coal Measures Millstone Grit||Carboniferous|
|Lower Culm Measures||Upper Limestone Shales Carboniferous Limestone|
|Upper Devonian||Pilton and Baggy Beds||Lower Limestone Shales|
|Pickwell Down Sandstone||Upper Old Red Sandstone|
|Middle Devonian||Ilfracombe Beds|
|Lower Devonian and Silurian||Hangman Grits Lynton and Foreland Beds and Morte Slates|
UPPER OLD RED SANDSTONE
The Old Red Sandstone which forms a nucleus to the Mendip
Hills, and rises at intervals from near Frome to Black Down north of
Cheddar, consists for the most part of red and variegated sandstone, with
sandy marls and towards the upper part beds of quartzose conglomerate.
The conglomerates are conspicuous between Downhead and Beacon Hill
to the north-west of Shepton Mallet, where in places they have been
disintegrated into a loose gravel.
The Old Red Sandstone also occurs at Portishead and along the
shores of the Avon near Abbots Leigh. Remains of the fish Holoptychius
have been recorded from Portishead, otherwise but few traces of fossils
have been found in this formation in Somerset.
On the Mendips the rock forms tracts of moorland together with
pasture and arable ground, but the soil, a reddish stony loam, is not
rich. Springs issue at the junction with the Lower Limestone Shales,
which flank the Old Red Sandstone and hold up water in the
joints and weathered portions of that formation. Economically the
rock is not much used, except for building walls and for local road
On the summit of the Mendips, extending westwards from Downhead to Beacon Hill, there are masses of igneous rock (andesite) sufficiently hard to be quarried for road metal at Moon's Hill south of Stoke
Lane. Attention was first called to these exposures by Charles Moore,
and they have since been frequently examined. According to Sir A.
Geikie and Mr. A. Strahan they appear to rise intrusively through the
Old Red Sandstone, and may possibly belong to the Carboniferous volcanic series referred to later on.
CARBONIFEROUS LIMESTONE SERIES
The lowest Carboniferous strata in east Somerset are the Lower
Limestone Shales which occur between the Old Red Sandstone and
the Carboniferous Limestone. They form passage-beds and are well
shown above Burrington Combe on the Mendips and in the Avon
gorge. In the ascending series we find that the higher beds of Old
Red Sandstone alternate with shales and thence merge into the main
mass of greenish grey and bluish-grey shales, above which the shales
alternate with bands of limestone, which pass gradually into the Carboniferous Limestone.
Fossils are not uncommon in these beds; they include the trilobite,
Phillipsia, also many fish-remains, crinoids, bryozoa, and brachiopods such
as Spirifera, Chonetes and Strophomena.
The shales which occur between the harder masses of Old Red
Sandstone and Carboniferous Limestone have suffered erosion so that
they occupy belts of lower ground often wet and boggy from the direct
rainfall or from springs which issue from the bordering rocks. In bygone days these shales, which are sometimes black, have led to fruitless
trials for coal.
The Carboniferous or Mountain Limestone forms some of the
more picturesque parts of Somerset, and it well deserves the name of
'Scar Limestone,' which it received in the north of England on account
of the numerous scarps of rock which characterize the land. In Somerset
it forms the mass of the Mendip Hills, with its many charming rocky
gorges, combes and caverns, as at Cheddar, Ebbor, Burrington and
Frome on the Mendips, at Cleeve and Brockley on Broadfield Down.
While however the rock frequently appears at the surface, there is elsewhere a rich red soil, a residue from the decomposition of the rock, and
this supports an excellent herbage for sheep.
A fine section of the rocks is to be seen in the gorge of the Avon,
where it cuts Leigh Down in Somerset, and Clifton and Durdham
Downs in Gloucestershire. The upward succession from the Old Red
Sandstone and Lower Limestone Shales may be traced from Cook's Folly
towards Clifton, where the Upper Limestone Shales and Millstone Grit
appear. The Carboniferous Limestone is there to a considerable extent
repeated by the Great Clifton fault, which has thrown down the beds on
the north to an extent estimated by Prof. C. Lloyd Morgan at 1,100
feet. (fn. 10) This great dislocation has produced much disturbance in the
strata. The lower portion of the Carboniferous Limestone, seen in the
Black Rock quarry, comprises dark bituminous limestone crowded with
crinoid remains, Spirifera, etc.; higher up there occur bands with many
fish remains, then a series of oolitic limestones, and some shaly beds;
while in the upper beds we find layers with Terebratula hastata and
Productus, others with the trilobite Phillipsia, also Orthoceras, Bellerophon,
Murchisonia and Euomphalus, and corals such as Lithostrotion and Cyathophyllum. The upper beds are worked in the Great quarry of Durdham
Down, where fluor-spar occurs in crevices of the rock.
Near the Suspension Bridge at St. Vincent's Rocks the famous
Hotwell Spring of Clifton issues from the higher beds of the Limestone
'at the rate of 60 gallons per minute, at a tolerably uniform temperature
of 70° Fahr.' Its principal constituent is carbonate of lime; it contains
also salts of soda and magnesia, but only to the extent of 44 grains to
the gallon. (fn. 11)
The thermal waters of Bath which issue through the Keuper and
Rhætic Beds probably arise also from the Carboniferous Limestone. (fn. 12)
Judging by their temperature, 104 to 120° Fahr., Prestwich suggested
that the depth from which they come is about 3,500 feet. Hence they
may be regarded as natural artesian wells, fed perhaps to some extent
from the sea, which in the Severn estuary is bordered in places by
Carboniferous Limestone, and this rock also rises up in the Holmes
islets. The chief ingredient in the waters is sulphate of lime, with also
sulphate and chloride of sodium, and chloride of magnesium, etc.
Altogether they contain from 144 to 168 grains per gallon, and the
discharge is about 385,000 gallons daily. Traces of copper and other
metals have been found in the Bath waters, and in connection with them
it is interesting to bear in mind that almost every element is contained
in sea water, while in the Carboniferous Limestone of Walton near
Clevedon traces of gold as well as silver have been found. It should also
be mentioned that at Bathampton Spa tepid saline waters were encountered in a boring, and saline waters were tapped in a coal-shaft at
The Bath waters often bring up sand as well as nuts, insects and
even fossils from the alluvial deposits and other strata through which
they emerge; moreover they leave a deep red ochreous deposit, and
tinge with a red colour the bordering rocks.
The surface of the Carboniferous Limestone is so fissured and
jointed that water readily finds its way downwards, and impurities also
are readily washed in; hence while considerable reservoirs of water
occur in fissures and caverns below ground, there is no certainty of
finding a copious and wholesome supply at any particular spot.
Since British and Roman times ores of lead and zinc (galena and
calamine), as well as ochres, have been obtained from the Carboniferous
Limestone and Dolomitic Conglomerate. In the time of Edward IV.
there were established certain 'Laws and Orders of the Mendip Miners,'
which contain interesting clauses connected with the working of the
ores. (fn. 13) The extent of the old workings is shown by the 'hills and
hollows' that occur here and there on the Mendip plateau, from Green
Ore, Egar (or Eaker) Hill and Priddy, northwestwards to Charterhouse
Warren, Rowberrow and Shipham.
In old times shafts were sunk to depths of 6 or 12 fathoms, and
rarely to 20 or 30 fathoms. The ores were sometimes found in regular
courses or grooves, whence the old miners were known as 'groovers,'
and the mineral veins were found especially in the Carboniferous
Limestone, and occasionally in the Dolomitic Conglomerate. At other
times the ores occurred in large masses or hulks lying horizontally;
in these cases in the Dolomitic Conglomerate. Particular accounts
of the methods of mining have been published by Giles Pooley and
others. (fn. 14)
For many years no fresh material has been obtained from the rocks,
lead being procured in sufficient quantities from the slags and slimes left
by the old workers. Large accumulations of this kind have been found
here and there at Stoke Hill, near Priddy, Charterhouse, East Harptree,
and in the Tar valley near Chewton Mendip. In all cases the deposits
are found on or near areas of Lower Limestone Shales, as in such situations water was held up and could be utilized for washing the material
that had been brought from the mines. The fine silty material or slime,
when calcined, yields about 5 per cent of lead, and the slags yield about
20 per cent, the average being about 12½ per cent. The slags left by
the Romans have proved to be the richest.
In 1890 manganese-ore and iron-ore (hæmatite) were worked at
Dursdon at the head of Ebbor Gorge. Trenches excavated in the
Dolomitic Conglomerate showed nests of manganese-ore, associated with
a little copper-ore, cerussite (carbonate of lead) and mendipite (oxychloride of lead). The occurrence of mendipite was interesting, as it
had hitherto only been recorded from Churchill. Quite recently Mr.
L. J. Spencer has detected crystals of leadhillite (sulphate and carbonate
of lead) in the ancient slags.
A small mass of Carboniferous Limestone, at one time thought to
be Devonian, appears at Cannington Park, north-west of Bridgewater.
It has yielded Lithostrotion and other Carboniferous fossils. (fn. 15) Its relations
to the Devonian rocks near by are not clear owing to coverings of Trias,
and although there is no evidence of unconformity Mr. Ussher is
inclined to think that a pre-Triassic fault of great magnitude may occur
there and stretch eastwards between Glastonbury and Shepton Mallet.
This might affect the underground extent of Coal Measures, and there
fore he would commend trial-borings for coal at Highbridge, Burnham,
Wedmore, or Mark, rather than in the area to the south of the supposed
fault. (fn. 16)
There is evidence of volcanic activity in Carboniferous times in
Somerset as in Derbyshire, and further particulars have recently been
gathered by Sir A. Geikie and Mr. A. Strahan, who observe that successive outflows of vesicular lava and showers of volcanic detritus were here
and there spread over the floor of the sea in which the Carboniferous
Limestone was deposited. (fn. 17) No evidence of any central vent was observed,
but it is probable there were orifices near Weston-super-mare, Uphill
and Wrington Warren. Near Weston the volcanic rocks appear at
Spring Cove and at Middle Hope.
The Upper Limestone Shales, although not represented on the
Geological Survey map, were long ago recognized by Buckland and
Conybeare; they comprise shales with bands of limestone and grit, and
are exposed in the Avon gorge, where their thickness is estimated at
600 feet by Prof. Lloyd Morgan.
Representatives of these beds should be looked for between Mells
Green and Ashwick, as well as near Emborrow and to the south-west of
Ebbor Rocks, where doubt has been thrown on the occurrence of Lower
Limestone Shales locally marked on the Geological Survey map. It
would be interesting to find evidence of beds corresponding in character
with the chert beds and culm limestones with Posidonomya becheri, which
occur near Bampton and Burlescombe in Devon.
These Lower Carboniferous strata were all deposited in marine
areas, with in places bands and reefs of coral.
This formation comprises beds of hard close-grained grit or quartzite with ferruginous specks, and with subordinate beds of shale and
occasional seams of coal. It has received the name of 'Farewell Rock,'
from the fact that when reached in mining there is no longer any chance
of getting profitable coal.
Fossils mostly in the form of casts are met with, and they have
been described in an essay by Fort-Major Thomas Austin. (fn. 18) Shallowwater conditions prevailed at this period, the formation being probably
of estuarine origin.
The stone has in places been quarried for building material and
road stone, and here and there it has yielded a good deal of hæmatite. It
appears in the Avon gorge by the Hotwells, on the western side of
Ashton Vale, at Leigh Down near Winford, and between Binegar and
Mells on the northern side of the Mendip Hills. Traces occur near
Ebbor and Dinder on the southern side of the range. The absence of
any outcrop of Millstone Grit near Clevedon, Clapton-in-Gordano and
Tickenham may be due to faulting.
The Carboniferous Limestone and other strata, including the Coal
Measures, have been bent into folds, in the hollows of which we find
those portions of the conformable Coal Measures that have escaped
Three of these Coal basins are exposed or partially exposed at the
surface: (1) That of Radstock, which is connected with the Bristol
Coalfield; (2) that of Nailsea; and (3) that of Clapton-in-Gordano.
To the south of the Mendips there is doubtless another tract of Coal
Measures beneath the vale which extends from Brent Knoll to Glastonbury and Evercreech.
The limits of the basins to the north of the Mendips can be fairly
well defined, but those of the probable basin on the south are wholly
unknown (see p. 10). Nevertheless there is no unexplored tract in
England from which coal might so confidently be expected as in this
southern area beneath the Lias and New Red rocks, at a depth approximately of about 1,000 feet; and it will be observed that to the north of
the Mendips the Coal Measures occupy the lower grounds and are there
largely concealed by Lias and New Red rocks which were deposited to
some extent in a broad channel hollowed out of the softer Carboniferous
The scenery of the coal districts in Somerset is far more agreeable
than in some other coalfields in this country, because so much of the
area is covered with the fertile Red Marls and other secondary strata.
These indeed are locally so reduced in thickness that coal is worked near
Radstock beneath the Inferior Oolite.
Some picturesque parts of the Coal tract are formed by the Pennant
Grit which stands out boldly in wooded heights near Temple Cloud,
Pensford, and along the Avon between Keynsham and Bristol. These
sandstone rocks hold much water, but it is not as a rule serviceable for
domestic supplies. The soil, a sandy loam, is not very fertile, nor is
that on the shaly portions of the Coal Measures which give rise to a
yellow clay. In places where there is much iron ore in the strata the
soil is a bright red, often brighter in colour than that yielded by the
New Red rocks.
For our knowledge of the Coal Measures we are indebted largely
to the observations of the late G. C. Greenwell, J. Anstie, and J. Prestwich, but chiefly to Mr. J. McMurtrie of Radstock, who is still at
work in the area.
The strata are generally divided as follows:—
|Sandstones and shales with eight seams of coal Red shales|
Sandstones and shales with six seams of coal usually worked, and others
|about 1,000 feet about 150 feet|
about 1,000 feet
|Radstock, High Littleton, Pensford|
Farrington Gurney, Clutton
|Pennant Grit||Chiefly grey, green and red sandstones, with occasional seams of coal||about 2,500 feet||North of Brislington, Pensford, Temple Cloud, Babington and Holcombe|
|Lower Division||New Rock and Vobster Series||Sandstones and shales with numerous seams of coal and some ironstones||about 2,800 feet||Long Ashton and Bedminster; Newton St. Loe and Twerton; Nailsea, Vobster|
The district about Radstock is noted for the variety and excellent
preservation of the fossil plants in the Coal Measures.
In the Upper Series the best seam of coal does not exceed 2 feet
4 inches in thickness; in the Lower Series there is a seam 8 feet thick.
As remarked by Mr. McMurtrie it is mainly owing to its geographical
position, being (at present) the most southerly coalfield of England,
that the thin seams met with, especially in the Upper Series, have been
worked with profit. He adds that 'one favourable feature connected
with the Somerset and Bristol coalfield deserves to be mentioned,
namely, the absence of fire-damp, which in practice has not been met
with at all in the Upper Division, and in the Lower Division is chiefly
confined to the deeper seams of the Vobster and the Nettlebridge valley,
Ashton Vale and Bedminster, so that the district has to a large extent
been spared from those disastrous explosions which unhappily occur in
other parts of Great Britain.' (fn. 19) The deepest mine at Farrington is
1,794 feet, where some of the lower seams in the Farrington series are
now being worked.
Many remarkable dislocations and disturbances have affected the
Coal Measures, especially in the vicinity of the Mendip Hills, where
the beds are so crushed and folded that the same seam has been passed
through two or three times in a shaft. The softer Coal Measures have
been much more contorted than the bordering hard rocks. The Radstock slide-fault or overlap-fault is a dislocation or overthrust whereby
the strata are locally duplicated over a breadth of 350 yards in a fairly
horizontal manner, the vertical displacement amounting to about 60
Connected with these disturbances and with the folds which occur
in the Mendip Hills are the isolated masses of Carboniferous Limestone
which occur at Luckington and Vobster. These masses have been
attributed to overfolding from the Mendip Hills, to an independent
inverted and faulted anticline, and to a thrust-plane. (fn. 20)
Although the general structure of the Mendip Hills is usually
expressed in the term 'Mendip anticlinal,' the tableland is in reality
formed of a series of denuded anticlines which trend in an easterly and
westerly direction, and thus do not coincide with the north-westerly and
south-easterly direction of the range. There is evidence of at least five
folds, the summits of which in four instances, Beacon Hill, Pen Hill,
North Hill and Black Down, have been laid bare sufficiently to expose
the Old Red Sandstone. As in the case of other noteworthy folds in
the secondary rocks of the south and south-east of England, the strata
on the northern sides of these anticlines usually plunge downwards more
steeply than on the southern side, and there are vertical strata and
symptoms of overfolding at Churchill Batch, near East End, Leigh-uponMendip, and in the Steep Holme which is a continuation of the Black
In the Clapton-in-Gordano district there are also tracts of Carboniferous Limestone overlying Coal Measures, and the structure has in
Professor Lloyd Morgan's opinion been produced by a double system of
faulting. (fn. 21)
The disturbances in the Somerset coalfield are of a character similar
to those well known to have affected the coalfields of Belgium and the
north of France, where faults, contortions and overthrusts are the rule
rather than the exception.
The highest strata in the coalfield, as remarked by Mr. McMurtrie,
occur probably near Pensford, where there is a considerable thickness of
unproductive strata above the Radstock series. (fn. 22)
The formation was laid down over an extensive estuarine and
fluviatile area, with marshy tracts where luxuriant growths of giant clubmosses, horsetails (Equisetaceæ) and ferns furnished material for the
seams of coal.
There is abundant evidence of great unconformity between the
Carboniferous and other older rocks and the secondary strata. Excellent
examples are to be seen in the vales near Frome and Wells, and in the
Avon gorge near Bristol; moreover remarkably even eroded surfaces of
highly inclined strata are sometimes preserved beneath a mass of Dolomitic Conglomerate, Lias, or Inferior Oolite. These old platforms occur
at various levels beneath the main plain of denudation of the Mendips,
and for the most part represent different stages of erosion, although
locally they may be displaced by faults.
Geological Map of West Somerset
The break between the Coal Measures and the New Red rocks was
marked by considerable disturbance of the older rocks, the Mendip
anticlines and coal basins were produced with their attendant dislocations,
and the whole area suffered vast denudation. Ramsay calculated that
the loss from Mendip amounted in places to 6,000 feet. (fn. 23) In any case
the present tableland is a plain of erosion, dating originally from PermoTriassic times and being perhaps again and again buried up and revealed
and further wasted away during later periods.
The secondary strata commencing with the New Red rocks were
for the most part deposited in a subsiding area so that newer layers
stretched over a wider tract and overlapped those previously laid down.
NEW RED SANDSTONE SERIES OR PERMO-TRIASSIC
Resting for the most part irregularly on the worn surfaces and
upturned edges of the older rocks is the great series of New Red rocks,
which in west Somerset probably include strata of Permian and Triassic
ages. These were for the most part accumulated in a large inland sea
area, an arid region with sand drifts and saline waters.
A traverse of the country from Wiveliscombe to Taunton shows
the following succession:—
|Keuper||Upper red and variegated Marls||Taunton|
|Upper Sandstone||Wellington, Milverton, Fitz-head|
|Bunter||Conglomerate||Thorn St. Margaret|
|Permian||Lower Marls with occasional Sandstone||Wiveliscombe|
|Lower Sandstone and Breccia|
These divisions are again met with in the vale which extends to
Stogumber, but in Mr. Ussher's opinion the higher strata overlap and
entirely conceal the lower as we proceed from Williton westwards to
Porlock. (fn. 24) Throughout this district the rocks are a good deal faulted, and
although the main sequence is well established the subdivisions are themselves subject to so much local change that it is difficult to interpret some
The Breccias are made up mainly of fragments of Devonian slate
and grit, and are well seen near Stogumber. They are irregularly interstratified with sandstone. The Lower Marls are used for brick making
The Conglomerates are of a more mixed character than the Breccias
and contain, in addition to fragments of grit and slate, large pebbles of
Devonian and Carboniferous Limestone which are picked out in the
quarries for lime burning; while the rock itself is worked for building
purposes, as at Tipnoller near Wiveliscombe and Vellow near Stogumber.
In aspect it resembles the Dolomitic Conglomerate of the Mendips, but
is in the main of older date.
The Upper Sandstone forms a pleasant country with deep sandy lanes
near Wellington, Milverton, Halse, Williton and Minehead, and also in
places along the southern and eastern sides of the Quantocks. It may
be seen again near Yatton, Chew Magna, Brislington and St. Anne's by
Bristol. Both sandstones and conglomerate are usually good waterbearing strata.
The Upper (Keuper) Marls which extend over the vale of Taunton
border the Devonian rocks along the western side of the Quantocks with
a steep easterly dip. Here however we find no evidence of an ancient cliffmargin, and probably the older rocks of Quantock were not above water
during the Triassic and Liassic periods. The Marls extend to Carhampton, Dunster, Minehead and Porlock, but they are best exhibited in the
cliffs on either side of Watchet and also on the foreshore, where they
occur in faulted relationship with the Rhætic Beds and Lower Lias.
Here much gypsum or alabaster occurs in the cliffs: it appears in
nodules, bands and also in veins which traverse the beds in all directions.
It is found also near Somerton.
Over this intervening area the Marls fringe the marsh lands and
form islands in it. At Compton Dundon a boring was carried to a
depth of 519 feet in search of Coal Measures, but although the locality
is a promising one the boring was not carried deep enough to reach the
base of the Upper or Keuper Marls.
Approaching the Mendip Hills we find evidence of marginal
accumulations in the Dolomitic Conglomerate—beds of breccia and conglomerate formed mainly of angular and rounded fragments of Carboniferous Limestone, cemented sometimes by a dolomitic (or magnesian
limestone) matrix, more often by ferruginous matter and carbonate of
lime. Much of the limestone material is evidently due to subaerial waste
of the land, being talus or angular detritus cemented at or near the spots
where it accumulated, while other portions have been more or less rolled.
Remnants of this Dolomitic Conglomerate occur along the higher
grounds of Mendip as well as along the borders of the range, where the
marls and conglomerate dovetail one into the other ; while in the
Radstock coal field to the north of the Mendips the conglomerate is
generally found at the base of the marls.
These facts indicate that while the conglomerate originated at the
early stages of the Keuper period, its formation continued along the
margin of the area of deposition until the close of the period.
At Draycot near Cheddar the stone has been quarried for building
purposes, while near West Harptree some hard siliceous beds occur in
association with the marls and conglomerates.
Fine sections of Conglomerate have been opened in the Avon
gorge, where Dinosaurian remains were found. Here too, as near
Wells and elsewhere, celestite (sulphate of strontian) and baryto-celestite
have been found, as well as 'potato-stones,' geodes lined with calcite
and quartz crystals (known locally as 'Bristol diamonds'). The Marls
extend over the Vale of Wrington and Ashton Vale, and everywhere they
yield a fertile soil, supporting good meadow and pasture land as well as
At Winford there are 'reddle pits,' or ochre and umber works, in
ferruginous deposits which occur at the base of the New Red rocks and
in joints and hollows of the subjacent Carboniferous rocks.
The Rhætic Beds, the latest stage of the Trias, form a connecting
link between that system and the Lias. In general they comprise the
|White Lias with hard compact limestone known as the Sun Bed at top, and Cotham or Landscape Marble at the base||10 to 25|
|Black shales with Avicula contorta, occasional bands of limestone, sandy layers, and one or more bone-beds||12 to 30|
|Green and grey marls which shade downwards into the red and variegated marls of the Keuper||10 to 30|
These beds are well exhibited on the coast at Blue Anchor,
Watchet, St. Audries and in many an inland section, notably at Queen
Camel, Puriton, Shepton Mallet, Uphill, Saltford and Pylle Hill south
of Bristol. They occur in the escarpment of the Polden Hills, where as
elsewhere the junction of the green and grey marls with the underlying
red marls is often conspicuously shown in the ploughed fields.
The green and grey marls are linked by some authorities with the
Keuper marls into which they graduate, but they indicate the beginning
of the change which ushered in the Rhætic fauna, they have yielded
similar fish remains, while the black marly bands and the pale limestone
bands seen in the green and grey marls at St. Audries foreshadow the
black shales and White Lias.
The fossils indicate conditions of an inland salt lake subject to
irruptions of the sea, a phase which occurred between the inland sea
or lacustrine conditions of the Keuper and the open sea conditions of
the Lias. Among the characteristic fossils are Avicula contorta, Pecten
valoniensis, Cardium rhæticum, Pleurophorus and Gervillia, as well as fish
remains and plant remains (Naiadita). (fn. 25)
In fissures of the Carboniferous Limestone near Frome many
Rhætic fossils were obtained including the oldest known mammal
Microlestes, and teeth of the fishes Acrodus, Hybodus, Gyrolepis and
One of the most remarkable bands in the Rhætic series is the Landscape Marble so well known to visitors at Clifton near Bristol. (fn. 26) The
stone, which is from an inch or two to about 8 inches in thickness,
occurs at the base of the White Lias and is sometimes represented by a
persistent layer of banded limestone of compact texture. Where this
limestone occurs in isolated and more or less nodular masses with a
crinkly surface, there the aborescent markings which pervade the stone
are met with. The darker patches which represent the landscape are
tinted by carbonaceous matter, and the features have been attributed to
the escape of bubbles of gas; while the crinkly surfaces and some
modifications of the landscape, which varies considerably, appear to be
due to the changes produced amid the variously tinted calcareous mud
during its solidification.
Sandy beds with Pullastra arenicola occur on Harptree Hill and near
Wedmore, a locality where there is a hard shelly limestone used for
A sulphur spring at Queen Camel rises probably at the base of
the White Lias from the decomposition of pyrites in the Rhætic shales.
Limited supplies of good water are sometimes obtained from the White
The Lias formation occupies a considerable portion of Somerset
and is one which from its fossiliferous character offers many attractions
to the collector. The beds were formed in marine areas and under somewhat varying conditions as regards depth, the sandy beds in the Middle
Lias being shallow-water deposits. The argillaceous limestones were to
some extent due to the mechanical waste of old cliffs formed of Carboniferous Limestone.
The Lower Lias almost everywhere rests conformably on the White
Lias of the Rhætic Beds, but near Bristol and northwards it rests directly
on the Cotham or Landscape Marble, and there probably the higher Rhætic
Beds have been subject to contemporaneous erosion. The Lower Lias
comprises two main divisions—the lower of limestones with alternating
bands of clay or shale presenting a 'riband-like appearance'; the upper of
clay and shale with occasional bands or nodules of limestone.
The fossils, which are many and varied, include large saurians, fishes,
many ammonites, belemnites and other mollusca, corals, crinoids, etc.
So marked in vertical distribution are many of the ammonites that
they have been taken to indicate life-zones, and these, although not
rigidly separated one from another, are yet fairly constant, and the
order of sequence is never inverted. (fn. 27) The following are the main
zonal divisions :—
|Ammonites capricornus||Mainly clays or shales with occasional bands and nodules of limestone and much pyrites.|
|" semicostatus||Alternating limestones and clays or shales, the limestones sometimes preponderating.|
In the southern part of the county the Lower Lias is exposed along
a scarp overlooking the vale of Taunton where the lower division is
quarried here and there as at Thurlbeer and Curry Rivel.
The higher clayey beds extend over the area below Castle Neroche,
at Ashill and Isle Abbots, to the west of Ilminster and near Chard. At
the last-named locality the presence of dark bituminous shales led many
years ago to a fruitless trial for coal. Similar trials were also made at
Badgworth and near Glastonbury. Further on at Langport and Somerton the stone beds are exposed in many places, and at Keinton Mandeville they are largely quarried for paving and building stone.
At Queen Camel the limestone beds are well developed, being over
100 feet thick, but their thickness is less along the Polden Hills where
the stone is extensively quarried, especially at Street. This locality is
famous for its saurian remains, Ichthyosaurus and Plesiosaurus, some of
them described in a rather fanciful work, The Great Sea Dragons, by
Thomas Hawkins, who at one time resided at Sharpham Park, and
collected many specimens which are now in the Natural History
Museum at South Kensington.
At Dunball near Puriton Blue Lias lime and cement are manufactured. Here some of the lower beds are pale and resemble White
Lias, but the mass of the limestones is little more than 20 feet thick, the
higher beds with Ammonites bucklandi, elsewhere mainly limestone, being
locally represented by clays and shales.
The clayey division is present over a large area from Ilchester to
Castle Cary and Evercreech, and at Marston Magna the famous
Ammonite Marble, consisting of nodular masses of limestone packed
with small examples of Ammonites obtusus and A. planicosta, was at one
At Pylle by Pennard Hill the limestones are seen at the lime and
cement works, and the country northwards by Pilton to Shepton Mallet
shows many a quarry in a region much broken by faults.
Approaching the Mendip Hills near Shepton Mallet we come upon
curious pale granular and shelly limestones precisely like the Sutton
Stone of Glamorganshire, yielding Ammonites johnstoni, Pecten pollux,
Lima, Ostrea multicostata, etc. Like the beds in South Wales the Lias
assumes different characters in proximity to the Carboniferous Limestone,
which locally formed shore-lines. Here and there also we find conglomeratic beds with pebbles of the older rock.
On the summit of the Mendips south of East Harptree there are
cherty beds yielding an assemblage of Lower Lias fossils similar to that
above mentioned, with also Ammonites planorbis. Ochre has been worked
from the ferruginous clay partings associated with the chert.
In west Somerset, along the coast from near Stolford by Kilve to
Watchet and Blue Anchor, and inland near Selworthy west of Minehead,
we find interesting exposures of the Lower Lias limestones. Again on
the western extremity of the Mendips near Uphill we find a tract of
Lower Lias, Rhætic Beds, and Red Marls faulted against the Carboniferous Limestone.
North of the Mendips near Radstock and Paulton the Lower Lias
is much attenuated as pointed out by Moore, who considered that the
hill range formed a barrier which prevented the accumulation of much
sediment. The zones are crowded within small limits, there is evidence
of reconstruction of layers and of pauses in deposition accompanied by
phosphatic nodules and phosphatized fossils. In some cases, as E. B.
Tawney observed, 'there really was not sufficient sediment to be the
burial ground of distinct zones of life.' In this region Spirifera walcotti
is a common fossil. There are also bands of ironshot limestone, the
grains in which were found by Dr. G. J. Hinde to be mostly fragments
of echinoderms. Further north there are modifications of the Lower
Lias limestone at Downside on Broadfield Down, where there is a massive
sparry rock with Cardinia, Modiola and Ostrea, approaching in character
some of the beds seen near Shepton Mallet.
At Twerton and Weston near Bath, at Saltford and Keynsham,
there are fine quarries in the Lower Lias limestone and many ammonites
have been obtained. Hence arose the tradition that St. Keyna of Keynsham resided in a wood full of serpents, and her prayers converted them
The limestones as a rule form a brashy clayey soil whereon turnips
are grown, while the clays form land suitable for pasture, wheat and
The formation is not calculated to yield a large amount of water
owing to the intercalation of shales, but wells at Street, Somerton and
other places have afforded limited supplies. Saline waters have been
met with at Horton near Ilminster and at Alford Well, Castle Cary;
chalybeate water at Capland Spa near Ashill ; and sulphuretted water at
Shapwick and Burnham.
The Middle Lias occupies a lesser area than the Lower Lias,
being usually exposed on the slopes of escarpments. It comprises the
Hard iron-shot limestone known as the Rock-bed or Marlstone.
Micaceous sands with indurated masses of calcareous sandstone.
Laminated sands and clays passing down into blue micaceous clays.
The lower beds are characterized by small examples of Ammonites
margaritatus, but the more sandy or loamy beds yield as a rule few fossils,
and when present they are often tender and difficult to preserve.
The Marlstone has yielded a large series of fossils for our knowledge of which we are especially indebted to C. Moore. The commoner fossils are Ammonites spinatus, Pecten æquivalvis, Gryphæ cymbium,
Rhynchonella tetrahedra and R. acuta.
The beds are exposed in many a quarry opened for road metal near
Ilminster, South Petherton, Yeovil, Pennard Hill and Glastonbury, and
they occur also on Brent Knoll.
North of the Mendips in Somerset the Marlstone rock-bed is not
often to be seen, and the beds in general appear to be mainly represented
by clays in the neighbourhood of Bath ; but it is possible that here as
near Yeovil the thin rock-bed has been dissolved away along the outcrop.
At Ilminster the Marlstone was found by Moore to yield locally 15 or
16 per cent of metallic iron. Elsewhere in the midland counties the
calcareous beds have been more wholly replaced by iron ore, and have
been extensively worked.
Springs are thrown out from the sands, which usually yield a fair
amount of water. The Holy Well and Chalice Well at Glastonbury are
thus supplied, the water being chalybeate, while at Dillington near
Ilminster there is a saline well.
The Upper Lias comprises at its base bands of pale earthy and
nodular limestone and clay, and higher up clays and sandy shales which
pass up into the Midford Sands. The formation has attained much fame
as a fossiliferous division from the labours of Moore, who collected
extensively from the basement beds. In the neighbourhood of Ilminster
and onwards to that of Yeovil he discovered many saurians and remarkable fishes, as well as cephalopods, insects and crustaceans; and among
them were specimens of fossil cuttle-fishes with the ink-bag containing
fossil sepia. These fossils he obtained from yellow limestone nodules,
the shape of which roughly conformed to the enclosed organism, so that
he could often predict before breaking the block the nature and even the
genus of the fossil that was preserved. Many of the nodules however
yield no fossils.
Ammonites annulatus, A. communis, A. bifrons and A. serpentinus
(falcifer) are usually to be found in the basement beds of the Upper Lias.
The limestone was formerly used for building purposes at Yeovil. The
beds occur here and there on Pennard Hill, at Glastonbury Tor and
Brent Knoll, where the stone and its fossils have been used for road
North of the Mendips there are few clear sections of the Upper
The Midford Sands, so named from the hamlet of Midford about
3 miles south of Bath, form a connecting link between the Lias and the
Oolites. Where the sequence is complete there is a gradual passage
from the Upper Lias clays through sandy clays into yellow micaceous
sands. These sands contain bands and nodular masses of sandy and shelly
limestone, which mark the incoming of those conditions which characterized the Inferior Oolite. The hard bands are conspicuous in the railway cuttings near Yeovil, while the more shelly bands occur at North
Perrot, and coalesce at Ham Hill to form a mass of famous freestone, a
brown stone of pleasing appearance, worked since Roman times, and seen
to advantage in the old mansions of the district. Deep sandy lanes or
hollow ways characterize the country near Yeovil and Crewkerne, and
these trackways, which are often bounded by thick hedgerows, are deeply
excavated beneath the bordering ground, the erosion being due mainly
to the wear and tear of rain and streams along old bridle-paths.
The lower beds of the Midford Sands yield Ammonites jurensis, A.
striatulus, and A. variabilis; the higher beds are usually much more
fossiliferous and yield A. opalinus and more abundantly Rhynchonella
cynocephala, Terebratula infra-oolitica and other brachiopods. In many
localities we may obtain no fossils. The sands occur along the borders
of the oolitic hills from Crewkerne to near Doulting ; they cap the Tor
at Glastonbury and Brent Knoll, and appear less persistently to the north
of the Mendips. Springs are thrown out at the junction with the Lias
The Inferior Oolite, which consists of a variable series of limestones,
shelly and oolitic, forms a conspicuous belt in the county.
The lower portion is characterized by Ammonites murchisonæ (named
after Lady Murchison), and the higher portion by A. humphriesianus and
As we proceed from the neighbourhood of Yeovil and Bruton
northwards the lower beds disappear, and near the Mendips and further
north by Radstock and onwards to Bath only the higher portion with
A. parkinsoni is represented, except in the outlier of Dundry.
In the southern part of the county near Yeovil the series is but
poorly developed as regards thickness, but it is remarkably fossiliferous,
as is the Lias under similar conditions.
The beds are quarried near Crewkerne, Misterton and Haselbury in
south Somerset, while along the eastern borders the higher beds are
quarried in many places from Milborne Port to Castle Cary and onwards,
and they yield in abundance Rhynchonella spinosa.
Near the Mendips the Doulting stone is extensively worked; it
consists of oolitic and sparry limestone—much of the latter being detrital
and formed of fragments of crinoids cemented by calcite. Near Frome
the Inferior Oolite becomes conglomeratic where it rests on the older
rocks of Mendip. Northward it occurs through the Radstock district to
Bath, but it presents no features of special interest. In some of the
quarries near Bath the stone exhibits cavities due to the dissolution of
At Dundry Hill (765 feet) we find a conspicuous outlier of Inferior
Oolite where the main subdivisions are well represented and have yielded
a rich harvest of fossils. The principal freestone, a hard pale brown and
imperfectly oolitic stone, is in the highest strata (zone of Ammonites
parkinsoni) with coral beds on top; lower down there are richly
fossiliferous iron-shot oolites with A. sauzei, A. brocchii and many other
forms of mollusca; lower still are beds with A. concavus, A. murchisonæ,
etc., and representatives of the Midford Sands with A. opalinus, A.
aalensis, A. striatulus, etc. (fn. 28)
Mr. W. H. Hudleston has pointed out the close connection
between the Dundry Inferior Oolite and that of the neighbourhood
of Sherborne in Dorset, and this clearly indicates an open sea communication during the period around what is now the western end of the
Mendip Hills. (fn. 29)
Springs are abundant at the base of the Inferior Oolite and Midford
Sands. Chalybeate springs occur at Lyncombe Spa, Bath, and at Goathill, south of Milborne Port; while at East Chinnock between Yeovil
and Crewkerne saline water has been met with.
GREAT OOLITE SERIES
Above the Inferior Oolite there is a great clay formation known as
the Fullonian or Fuller's Earth because at Midford and Wellow there
occur in it beds of economic fuller's earth 4 to 7 feet thick. In mass the
formation is 150 feet thick and more, and it includes bands of soft earthy
limestone some 20 or 30 feet thick in places, known as the Fuller's Earth
Rock. These bands of rock as well as the clays yield numerous fossils,
amongst which are Ammonites subcontractus, many lamellibranchs such as
Ostrea acuminata, Isocardia and Pholadomya, and brachiopods such as
Rhynchonella varians and Waldheimia ornithocephala.
The formation is well developed near Crewkerne, where it is
worked for brick-making, and it extends from near Milborne Port to
near Bruton on the slopes beneath the fine escarpment formed of the
Forest Marble, and thence onwards by Frome to the neighbourhood of
Bath. The Fuller's Earth Rock is seen in railway cuttings near Milborne Port and south-east of Shepton Montague, also at Egford Bridges
near Frome and Lansdown, Bath. The soil as a rule is cold and wet,
but it yields good meadow land and well timbered hedgerows.
The peculiar detergent properties of the fuller's earth are due rather
to the physical character than to the chemical composition of the earth;
it is not plastic like ordinary clay, but falls to a powder under water.
The Great or Bath Oolite forms an important and conspicuous
part of north-east Somerset, in the high grounds of Lansdown, Little
Salisbury Hill, Bonner Down, Bathampton Down (672 feet) and Odd
Down. It consists of a mass of limestones, with some marly and clayey
partings, and the stone beds are oolitic and shelly.
Broadly speaking there are two main divisions, the upper comprising
false-bedded shelly oolites and marls, some freestone and very hard rag
beds; and the lower comprising fine oolitic beds comparatively free from
shells and furnishing most of the best freestone. The full thickness to
the east of Bath is from 100 to 110 feet.
The mass of the freestones which have been worked are included in
a thickness of about 50 feet, but the main freestone or ground bed is
from 12 to 14 feet thick and is the lowest bed worked. The beds
diminish in thickness towards Bradford-on-Avon and finally disappear to
the south-west of that town, near Farleigh Hungerford and Norton St.
Philip. The evidence leads to the conclusion that the Great Oolite has
suffered erosion locally and to a certain extent contemporaneously during
the succeeding period of the Bradford Clay and Forest Marble. (fn. 30)
Although the fauna and flora of the Great Oolite period are rich
and varied when we take into account the Stonesfield Slate, yet that
portion of it though feebly represented by certain beds on Lansdown is
not locally very fossiliferous.
The shelly beds and rags are largely made up of comminuted shells,
fragments of echini and crinoids, and there are many bryozoa and minute
gasteropods in some layers. The principal fossils are Ostrea sowerbyi,
Pecten vagans, Lima cardiiformis and Terebratula maxillata, while here and
there, as on Farley Down above Bathford, and Combe Down, a coral bed
yielding many species has been described by Mr. R. F. Tomes. (fn. 31)
Although Bath stone is quarried on Combe Down, Odd Down and
other places, by far the larger quantity of stone is obtained at a depth by
shafts and tunnels at Box, Corsham and Bradford-on-Avon in Wiltshire. (fn. 32)
Old terraces of cultivation known as lynchets occur on some of the
oolitic scarps in the Avon valley, as well as on the Liassic slopes of
Brent Knoll; and Canon H. N. Ellacombe has drawn attention to the
former presence of vineyards on the slopes of Claverton Down and other
places near Bath. (fn. 33)
The Forest Marble is remarkably false bedded and current bedded.
No more variable formation is to be met with. In its changeful series
we find clays and shales, sands and sandstones, shelly and oolitic limestones and much lignite. The curious track-marks and ripple-marks on
the surfaces of many slabs show that it was deposited under shallow-water
conditions. Ochreous clay-galls probably originated from rolled masses
of clay, while the structure of the oolitic beds, the grains being irregularly mingled with comminuted shells and lignite, or scattered in a sandy
as well as in a calcareous matrix, suggest the notion that they may have
been derived from the local erosion of the Great Oolite.
At the base of the Forest Marble there is usually found in the south-west of England a bed known as the Bradford Clay about 10 to 12
feet thick, with at its base a fossiliferous band yielding the pear encrinite
(Apiocrinus parkinsoni), and many brachiopods such as Waldheimia digona,
Terebratula coarctata, etc. (fn. 34) Although not well exposed in Somerset it
has been observed at Farleigh Hungerford, Charterhouse Hinton, and in
a boring for coal at Buckland Dinham.
The limestones which as a rule form a central mass in the Forest
Marble, underlaid and overlaid by clays and shales with bands of limestone, stand out in a bold escarpment at Windmill and Charleton Hills
near Charleton Horethorne, at Bratton and Scale Hill near Bruton, near
Wanstrow, and thence by Cloford to Frome. Many a quarry may be
observed and the stone is extensively used at Frome. In this neighbourhood the occurrence of sandy beds is noticeable, and they are to be seen
at Buckland Dinham, and especially at Charterhouse Hinton, where
large concretionary masses or 'doggers' of sandstone occur. The beds
were noticed by William Smith under the name 'Hinton Sand and
Sandstone.' On the Wiltshire borders some of the thin bands of limestone have been employed as stone tiles for roofing.
This formation was so named because it yields a brashy soil suitable
for the growth of corn, and where exposed in quarries it is a rubbly
yellow limestone which in most localities yields numerous fossils.
Ammonites macrocephalus is characteristic, but more abundant though
seldom well preserved are many lamellibranchs such as Avicula echinata,
Myacites, Gresslya, Pecten vagans and Pholadomya; brachiopods, especially
Terebratula intermedia and Waldheimia obovata; and the echinoderm
Echinobrissus clunicularis. These may often be picked up on the ploughed
fields. The formation was probably slowly deposited in tranquil waters
and at a greater depth than other members of the Great Oolite series.
The Cornbrash is exposed at Closworth, Sutton Bingham, East
Coker and again at Henstridge, Templecombe, Wincanton and Upton
Noble, where hard limestone suitable for building purposes is obtained.
Further on it is seen at Marston Bigot near Frome, at Road and Tellisford.
The Cornbrash yields water in shallow wells above the clays of
the Forest Marble. It will be observed that a succession of villages on
this account arose on the formation, but owing to unsatisfactory sanitary
arrangements these supplies have nowadays often to be condemned. At
Templecombe a boring carried to a depth of 172 feet encountered saline
This great clay formation, so well developed near Oxford, forms the
extensive vale of pasture and meadow land below Penselwood, and thence
by Witham Park to Berkley near Frome.
It is a marine formation divided for convenience into zones
characterized by species of ammonites, which in upward succession
are Ammonites calloviensis, A. ornatus and A. cordatus. Large examples
of Gryphæa dilatata occur in the upper beds. The lowest zone includes
locally some bands of calcareous sandstone known as the Kellaways Rock,
from Kellaways in Wiltshire, where its numerous fossils early attracted
attention. The rock bands are nowhere conspicuous in Somerset, but
their occurrence at South Brewham has been recorded.
Septaria are found in the formation, and the dark shales, which are
sometimes bituminous and contain a good deal of lignite, led to a fruitless
trial for coal at Brewham.
These rocks, which are well exposed in north Dorset, extend into
Somerset at Cucklington and Stoke Trister to the east, and in an outlier
at Higher Hatherley, to the south-west of Wincanton. They are the
highest Jurassic strata exposed in Somerset.
There is a gradual passage upwards from the Oxford Clay, and large
oysters Gryphæa dilatata are found in the sandy clays which mark the
incoming of the Corallian. These clays give place to sands with concretionary masses of calcareous sandstone, which yield Ammonites perarmatus, A. cordatus, etc.
The upper Corallian Beds include oolitic freestones, sandy and shelly
limestones, clays and sometimes ferruginous beds, as near Sturminster
Newton in Dorset and Westbury in Wiltshire. In these upper beds
Ammonites plicatilis, Chemnitzia heddingtonensis, Cidaris florigemma and
Echinobrissus scutatus are usually to be found.
The strata are water-bearing, but at Horwood Wells, south-east of
Wincanton there are saline waters which probably issue from lower strata.
Prior to the overspread of the Upper Cretaceous rocks the Jurassic
strata were tilted, faulted and folded, and to a great extent eroded. These
phenomena took place after the deposition of the Wealden strata and
during or prior to the accumulation of the Lower Greensand, while
afterwards during the Upper Cretaceous period a more extensive plain of
denudation was formed when Gault, Upper Greensand and Chalk were
spread successively across the worn outcrops of the Jurassic and Triassic
Of all the present escarpments none of course were formed until
after the Cretaceous strata were so far removed as to expose the Jurassic
The Gault and Upper Greensand, which have been grouped
together under the name Selbornian by Mr. Jukes-Browne, (fn. 35) appear in
Somerset along the northern borders of the Blackdown Hills south of
Wellington; also near Chard and Crewkerne; again at Penselwood and
Kilmington north-east of Wincanton, and thence they extend to near
The Gault itself, grey sandy micaceous clay with occasional phosphatic nodules, is the clayey lower portion of this group. It is from
70 to 90 feet in thickness in east Somerset near Penselwood, but it is
hardly perceptible in the Blackdown Hills, where the sandy conditions
of the Upper Greensand prevail and replace the clayey beds. These
beds in east Somerset represent only the Lower Gault zones of Ammonites interruptus and A. lautus. They have also yielded A. splendens, A.
beudanti and A. denarius, but although not well exposed they have been
recognized along the borders of the Greensand hills from near Penselwood to the east of Frome, in a band which at one time was regarded
as Kimeridge Clay. A brickyard at Flintford near Rodden has been
opened in the Gault.
The Upper Greensand is more important, and forms a bold scarp in
the Blackdown Hills overlooking the vale of Taunton, and extending
thence by Buckland St. Mary to Chard and Crewkerne.
As observed by Mr. Jukes-Browne the thickness increases from
about 100 feet near Crewkerne to 180 feet near Chard, and it is about
150 feet in east Somerset. It comprises in descending order calcareous
sandstone, chert beds, and green, grey and buff sands; and among other
fossils it has yielded Ammonites rostratus, Arca, Pleuromya, Exogyra conica,
Pecten (Neithea) quadricostata, P. quinquecostata and Cardiaster fossarius.
The Upper Greensand is an important water-bearing formation,
springs being thrown out at its base where it rests on the Lias near
Chard or on the Gault north of Wincanton. Bruton is supplied from
springs near Kilmington. The region is naturally a healthy one, though
in some places pollution of water arises from defective sanitary arrangements.
Penselwood has long been famous for the old Pen pits whence
nodules of hard cherty sandstone were formerly dug for the manufacture of whetstones. The pits from which they were dug have been
regarded as early British pit-dwellings, and much has been written on
this subject. (fn. 36)
The soil on the Greensand is variable, that near Kilmington being
more fertile than it is near Crewkerne and Chard.
The Chalk occupies small areas in Somerset, including the western
extremity of Salisbury Plain and tracts between Chard and Crewkerne.
The Chloritic Marl, a fossiliferous glauconitic marl from 1 to 2 feet
thick, occurs at the base of the Chalk at Snowdown by Chard, (fn. 37) and
Kilmington. It yields Ammonites rotomagensis, A. varians, Holaster subglobosus, var. altus, also the sponge Stauronema carteri, and it contains phosphatic nodules.
The Lower Chalk, including the Chalk Marl, is about 200 feet
thick, and occurs near Chard, Crewkerne, and at Long Knoll, while
above it on White Sheet Down there are beds of Middle Chalk with
the Melbourne Rock at the base. (fn. 38)
PLEISTOCENE AND RECENT
A great break occurs between the Chalk and the next records
which have been preserved in Somerset. The Chalk as a deep sea
deposit no doubt overspread the entire county. Whether any Eocene
strata extended over the area it is impossible to say; the nearest repre
sentatives occur within twenty miles of the eastern borders, while
patches of flint shingle, probably of Eocene derivation, lie on the Chalk
at Chitterne St. Mary near Heytesbury. (fn. 39) Eocene pebble beds indicate
that locally the Chalk was upraised to form land areas, and this being
the case agents of erosion partly marine and estuarine, largely subaerial,
must have commenced the work of destruction which ever since, amid
changing conditions and with greater or less potency, has been going on.
The Miocene period, characterized by great earth movements which upraised Chalk and Eocene strata, was one during which subaerial erosion
occurred on a large scale. The Severn drainage may have been defined;
and there can be little doubt that the mass of the Chalk and other
Cretaceous strata were removed from the greater part of Somerset before
The removal of these formations unbared a plain of Jurassic and
Triassic strata through which perhaps only in west Somerset the older
rocks protruded, the inequalities elsewhere being filled up. Along this
plain, and fed by springs from the Chalk and Greensand and ultimately
from other sources, the Avon and its tributaries in the north, the Brue
and Parret in the south, have carved out their courses. The Avon cut
through hard and soft rocks, deeply trenching its course along what is
now the Avon gorge before the bordering lowlands of Lias and New
Red Rocks had been excavated (fn. 40) ; and this lower part of its course was
naturally deepened before the river had completed its picturesque channel
through the Oolites above Bath. The long continued erosion has enabled
the rivers to reach what is termed the base level of erosion over the
greater length of their courses, and their channels are subject now to
widening rather than to deepening processes.
Waste along the present hills and scarps is seen in the landslips
which now and again occur along the oolitic hills near Bath. Thus at
Hedgemead below Lansdown, and again at Beechen Cliff on the south
side of Bath, the Inferior Oolite has slipped over the Lias clays; while
at Bathampton and Claverton there have been slips of Great Oolite over
the unstable foundation of Fuller's Earth Clay.
Knolls like Glastonbury Tor and Brent Knoll are striking monuments of denudation. Others near Ilminster, Montacute, South Cadbury and Castle Cary, not far removed from the main escarpment,
indicate how the severance of such outliers may take place. Subterranean drainage and erosion in the first instance lead to the formation
of underground channels in the impervious Lias clays beneath the porous
Midford Sands and the limestones of the Inferior Oolite. The limestones
themselves may be in part wasted by chemical dissolution, and if channels are formed in subjacent strata slight subsidences must take place
here and there, and pave the way for the disconnection of portions of
the main limestones to form outliers. The subsequent more complete
isolation of the severed masses is due to the superficial and subterranean
erosion by rain and streams. In the case of Brent Knoll and Glastonbury Tor the influence of estuarine waters that once spread over the
Somerset levels must have helped to complete the denudation. The
slightly basin-shaped arrangement of the strata in both these outliers has
helped in their preservation. The effects of subterranean erosion are
also seen where valley gravel rests on furrowed surfaces of clays such as
those of the Lias near Bath.
Elsewhere along the Mendip plateau and that of Broadfield Down
the effects of chemical as well as mechanical erosion are evident in the
deep ravines and caverns. A few of the ravines may have been initiated
in the Triassic period, and have been re-excavated and deepened in later
times, but Cheddar, Ebbor, Burrington, Brockley and Cleeve and most
others probably belong in the main to the post-Pliocene period. Many
of the great fissures on Mendip originating along joint planes or lines of
fracture have been so enlarged as to form a series of caverns, and in
some cases where erosion has proceeded further a deep ravine has
resulted. The solvent action of carbonated water, aided by frost and
streams, mainly contributed to produce the effect.
In the case of Cheddar the dip has exercised much influence on the
shape of the cliffs, the one side with the inward dip standing up boldly
and precipitously, while the other, the dip slope, affording abundant
evidence of disintegration, showers of detritus now and then falling on
to the road and in old times into a deep natural gully. The cliffs are
perhaps the finest of their kind in England, and they rise at one point
to a height of 420 feet above the level of the road.
Several of the streams which issue at the foot of the Mendip Hills
take their rise from swallet holes in the Carboniferous Limestone of the
plateau. Among these are St. Andrew's and other springs at Wells, the
Axe (derived from Priddy) which issues at Wookey Hole, and the
These waters flowing through limestone areas carry away much
carbonate of lime in solution, but under certain conditions they redeposit
it on the roofs and floors of caverns in the form of stalactites and stalagmites, or elsewhere in pipes; and in some instances, as near Coleford,
Darshill near Shepton Mallet, and Chilton-upon-Polden, there are deposits of tufa left by petrifying springs.
While the Avon was cutting its rocky course the area of the Bristol
Channel was evidently a great vale of Lias which spread from Watchet
to the Glamorganshire coast, while the Inferior Oolite of Dundry and
and the Midford Sands of Brent Knoll were connected with the main
escarpments. This broad vale was trenched by the Severn drainage and
afterwards widened by estuarine and marine action. The plain of marine
erosion is now well seen in the wide rocky platform which is exposed at
low tide at Watchet, but the sea must in Pleistocene times have cut its
way up to some of the Limestone cliffs near Weston-super-Mare, as there
we find traces of raised beaches.
In this brief sketch no reference has been made to the Glacial
period, prior to which the main features throughout England were
produced. Evidences of the Great Ice Age in Somerset are few and
local. Unlike many other areas it was not buried beneath a mantle of
ice, nor overspread with deposits of boulder clay or drift gravel. For
this reason the soils of Somerset are for the most part local, bearing a
direct relation to the sub-strata. No doubt the area has been affected
by Pleistocene climatic changes and fluctuations in level, but from later
Tertiary until comparatively recent times it has been subject chiefly to
waste by rain, rivers and sea rather than to deposit.
The striated surfaces of Devonian sandstone observed near Porlock
by W. C. Lucy, and the 'terminal curvature' of the slaty rocks near
Wiveliscombe noted by D. Mackintosh, (fn. 41) are features which may have
been produced by local agents.
Relics of early stages in denudation are met with in the shape of
chalk flints, and pebbles of quartz and quartzite, which have been found
here and there on the surface and in chinks of the Oolites on Farley,
Bathampton and Claverton Downs. Small patches of gravel have also
been observed on Clevedon Down.
Valley gravels here and there fringe the rivers and occur in scattered patches in the vales, but the only extensive superficial deposits are
those which occupy the alluvial moorlands.
Along the Avon valley there are here and there small areas of river
gravel, and these have yielded in places remains of Pleistocene mammalia such as we find in the older deposits of the Thames valley. These
include mammoth or Elephas primigenius, also E. antiquus, rhinoceros, musk
ox and reindeer. (fn. 42) Thus at Freshford, Bathampton, Bath and Keynsham
there are deposits of gravel made up of local materials, the limestones of
the Lias and Oolites, together with harder materials of the Carboniferous
group and Old Red Sandstone derived from tributary streams which rise
in the Mendips. At Larkhall, Bath, as much as 30 feet of gravel and
loamy beds have been observed.
Valley gravel occurs at Wells and Wookey, along the Brue valley
near Bruton and Castle Cary, and over the Lias vale bordering the rivers
Yeo and Cary near Ilchester and Langport, at Babcary, Milton, Puddimore and other places. Patches of clayey gravel largely composed
of chert detritus from the Greensand hills occur from near Chard to
Broadway, at Ashill and other places.
At Taunton the valley gravel has yielded rhinoceros and Irish elk,
as well as remains of alder, ash and oak timber; and in this neighbourhood one Palæolithic implement was obtained by W. Bidgood. In the
vale extending to Williton there are patches of gravel largely derived
from the New Red conglomerates.
Possibly of the same age as some of the valley gravels into which
they may have merged, are the remnants of raised beaches made up
mainly of Carboniferous Limestone pebbles with occasional flints, and
preserved on platforms of the limestone at Anchor Head and Woodspring Hill north of Weston-super-Mare. (fn. 43) They lie from 20 to 30 feet
above high-water mark, and are covered by blown sand, by rubble or
talus of ancient date, and by recent talus. The beaches yield Mytilus
edulis, Cardium edule, Tellina balthica, Ostrea edulis, Buccinum undatum and
Littorina littorea. All these species and others such as Scrobicularia are
found in the so-called Burtle Beds. These comprise sands with occasional pebbles, sometimes cemented into hard bands, which occur in
patches on the Lias or Red Marl in the moorlands at Burtle between
Glastonbury and Highbridge, and at Middlezoy and Chedzoy near
Bridgewater. Whether these Burtle Beds were contemporaneous with
the raised beaches or were due to later incursions of the sea over the
low lands is not certain.
The older rubble drift above the raised beaches has yielded remains
of hyæna, bear, mammoth and rhinoceros. Hence these deposits may
be compared with some of the valley gravels, with the older cave deposits, and with bone fissures such as occur on the Oolitic hills near Bath,
where remains of rhinoceros, bison and lemming have been obtained. It
must be remembered however that the Pleistocene period was a long one
and attended by considerable physical changes, and that in South Wales
Mr. R. H. Tiddeman has pointed out that subsequent to the raised
beaches and the infilling of some of the caverns in Gower the local
boulder drift was accumulated. (fn. 44)
Many caverns occur in Somerset, the more noteworthy being those
of Cheddar, which contain fine stalactites, Wookey Hole, Burrington,
Banwell and Uphill. (fn. 45) The Lamb Cavern near East Harptree is an
extensive one, in which formerly there were lead workings.
The hyæna den of Wookey Hole, worked out by Professor W.
Boyd Dawkins, is a fissure in the Dolomitic Conglomerate distinct from
the large cavern in the Carboniferous Limestone. It has yielded remains
of hyæna, cave lion, bear, badger, mammoth, rhinoceros, Irish elk, etc.,
together with implements of flint and chert of somewhat rude workmanship. Many of the bones bear evidence of having been gnawed by
hyænas. Professor Dawkins expressed the opinion that at the time of
the occupation of these caves a great plain extended over much of what
is now the Bristol Channel. The land must indeed have stood higher
previous to the accumulation of the more recent alluvial deposits,
probably to the extent of 50 or 60 feet. Proof of this is to be gathered
from the depth of these more recent deposits, and from the evidences of
old forest growth at their base at Porlock (fn. 46) and elsewhere. The marshlands as near Bridgewater are made up of a succession of fluviatile and
estuarine mud deposits, gravel and sand, and peat, with evidence of plant
growth at more than one horizon. Remains of an old forest at Stolford
were long ago recognized. Occasionally the peat is found in a compressed and indurated condition known locally as pill coal.
Peat is extensively dug to a depth of 14 or 15 feet on the moors
between Glastonbury and Highbridge, especially near Ashcot, Shapwick and Edington. It is largely composed of the common sedge,
bog moss, reeds and rushes.
The alluvial deposits have yielded remains of existing land and
freshwater mollusca, and of such animals as Bos taurus var. longifrons,
roe deer, goat, sheep, horse and dog.
The estuary of the Severn is noted for the extensive mud flats
bordering it at low tide, and which are liable, as near the mouth of the
Parret, to constant changes. The turmoil of the tidal waters agitates
this silty material, and Weston-super-Mare has gained the name of
'Weston-super-Mud' from the fact that at low tide there is an extensive
area of sandy silt and the waters are clouded with the suspended matter.
This is borne down to a large extent by the rivers which drain into the
Severn Sea, but is partly derived from the waste of the cliffs. (fn. 47)
At Bridgewater the material is utilized in the formation of the
celebrated scouring bricks, named Bath bricks from the original manuturer, Mr. Bath. Tanks or slime-batches have been constructed on the
borders of the Parret, into which the tidal waters are admitted and the
slime is allowed to subside. This slime contains about 70 per cent of
fine siliceous matter.
The marshlands over considerable areas lie beneath high water
level, in some places as much as 8 or 10 feet, but they have since
Roman times been more or less protected by artificial embankments,
while the dunes of blown sand between Burnham and Woodspring,
north of Weston-super-Mare, have likewise checked the inflow of the
sea. The rivers too have been embanked and their courses have locally
been modified, but in times of long continued heavy rain the dykes and
sluggish rivers are unable to carry off the land drainage which is held up
by the tidal waters; and from these causes and the occasional bursting of
a bank or overflow of a stream the marshes are sometimes inundated for
many square miles. (fn. 48) The fine silty material then deposited helps to
fertilize these large areas of dairy and grazing ground. In times of
flood Athelney, a low hill which rises from the alluvial moors between
Langport and Bridgewater, is liable to become an island; but Glastonbury Tor, the Isle of Avalon, has probably never been surrounded by
water in the historic period, though situated in the midst of marshes
In ancient times the uplands were extensively wooded, and remains
of the Mendip forest above Wells and Cheddar, of Selwood forest which
stretched from Frome to Wincanton, and of the forests of North Petherton, Neroche and Exmoor which occupied much of southern and western
Somerset were long unenclosed, some portions until the middle of the
eighteenth century. The most prominent heights were early fortified,
and the history of these earthworks and of the still older lake dwellings
near Glastonbury link geology with human history. Indeed since Roman
times some geological changes have taken place, and alluvium and peaty
beds have been accumulated to a depth of 12 feet at Bath and on the
levels of Burnham and Huntspill.