352 



NA TURE 



\Aug. 26, 1875 



An able ]-iaper on this subject, with Map and Sections, by my 

 friend Mr. Etheridge,' F.R.S., will be found in the papers of 

 the Cotteswold Club. 



Dolomilu Conglomerate. — Weaver described this formation as 

 composed pnncipally of "rounded and angular fragments of 

 limestone, exceeding the size of the head, with fragments also 

 of quartz and hornstone. These are all cemented together by a 

 calcareous paste, which is frequently of a marly nature — or of 

 a carbonate uf lime either of an earthy or compact structure ; " 

 the cement is generally magnesian, and in this there are many 

 cavities frequently lined with crystals of calcareous spar and 

 quartz, and also with the sulphate of strontian. 



This remarkable formation forms a kind of irregular broken 

 fringe, hanging on the flanks of the older rocks, and resting 

 unconformably upon them. We shall meet with this con- 

 glomerate again in connection with the beds in the Mendip Hills, 

 and in the Clifton section. 



Ne'v Red Sandstone. — The upper and central members of the 

 New Red Sandstone are found near Tortworth ; they consist 

 chiefly of red clay and marl. 



Avicula contorta beds have been found by the Earl of Ducie 

 in the form of the Bone Bed, the series resting on the inclined 

 edges of the Carboniferous Limestone. 



2. Mendip Hills. 



The Mendip Hills proper extend from Bleadon Hill near 

 Hutton on the west, to Elm and Whatley on the east ; and they 

 strike nearly due west and east, and are about thirty miles in 

 length, with an average breadth of five to six miles. They con- 

 stitute the southern base of the Bristol Coal Field, or the base 

 of an almost equilateral triangle, formed by the Palaeozoic rocks, 

 comprising the area from Purton Passage and Tortworth to the 

 south slopes of the Mendips ; this includes the outlier Bream 

 Down, which is only a westerly prolongation in the Severn, 

 separated from the main range of the Mendips by the alluvial 

 flat of the estuary of the Axe. 



The Lithology of the Mendips consists of Old Red Sandstone, 

 Carboniferous Limestone, and Trias, the latter represented 

 chiefly by the Dolomitic Conglomerate, which lies unconform- 

 ably on the Old Red and Carboniferous, flanking nearly the 

 entire range of hills, and in places capping their summits. 



Numerous islands of Carboniferous Limestone surrounded by 

 Triassic rocks occur east of Wells and south of Croscombe, also 

 encircled by fringes of Dolomitic Conglomerate, of which Church 

 Hill, Worminsler, and Knowl-foot Hill are examples ; these 

 outliers testify to the southern extension of the Carboniferous 

 Limestone beneath the New Red Sandstone and Lias south of 

 the Mendips, and lend us aid in determining the probable posi- 

 tion of deep-seated Coal Measures similar to those at Vobster, 

 Collord, Ed ford, Holcombe, &c., north of the Mendip range. 



The lower flanks of the northern portion of the range are 

 covered by the New Red Sandstone, that of the south being a 

 mere strip traversed by the Wells and Axbridge Railway, the 

 peat plains and bogs of Sedgmoor covering them up to a certain 

 level to the east of the meridian of Glastonbury. The Lias 

 occupies an extensive plain, masking likewise the older rocks 

 beneath. 



Old Red Sandstone forms the oldest stratified rock, and is, 

 strictly speaking, the axis of the Mendip Hills. It is exposed 

 in four well-marked areas along the highest ridge: — (l) Black- 

 down ; (2) North Hill and Pen Hill ; (3) Beacon Hill ; and (4) 

 Downhead Common, which is the largest exposed tract. The 

 intervening areas are occupied by a mantle of Carboniferous 

 Limestone, which arches over and covers the underlying Old 

 Red, denudation having yet spared the limestone. 



The Old Red is exposed along two anticlinal axes, these being, 

 indeed, the chief cause of its exposure ; the axes being post- 

 Carboniferous and pre-Triassic, are not traceable beneath or 

 where the patches of Dolomitic Conglomerate and cherty Lias 

 cover up the Old Red Sandstone and Carboniferous Limestone, 

 as at Harptree Hill, Rowham, Shipham, &c. 



The most northerly anticlinal brings up the fine range of 

 Blackdown, on the north, soutli, and east of which occur the 

 Lower Limestone Shales resting on Old Red. 



The northern dip of the anticlinal is higher than the southern, 

 being in places as high as 54° in the north, whilst in the south it 

 does not exceed 20". This anticlinal is traceable from near the 

 exposure of the igneous rock at Uphill, along Bleadon Hill, 

 thence under the New Red Sandstone to Padingham, and Dolo- 



• Procssdings of the Cotteswold Naturalists' Field Club, p. 28, 1865. ■ 



mitic Conglomerate and Calamine beds of Shipham, through the 

 Old Red Sandstone of Blackdown, and on through the Carboni- 

 ferous Limestone of Lamb-bottom, where it is lost under the 

 cherty Rhaetic beds of Harptree Hill. P>om Little Elm on the 

 extreme east, to Masbury Castle nearly due west of the range, 

 the Old Red is again exposed for three miles, which is likewise 

 due to the anticlinal axis. 



At Masbury Castle we lose trace of this S.E. anticlinal, and a 

 second and parallel one to that of Blackdown occurs, ranging 

 from the Old Red of North Hill through the Carboniferous 

 Limestone of Stoke Warren, and last under the Dolomitic 

 Conglomerate of North Draycott. This may join the great 

 anticlinal near Egar Hill. We thus see that the strike of the 

 Mendips was induced by a force which has brought out its oldest 

 rock to the surface, and thereby produced the present physio- 

 graphy of the bold range of hills we are now considering. 



Carboniferous Limestone surrounds the exposed and concealed 

 nucleus of Old Red, and is conformable therewith both in dip 

 and strike. The Carboniferous Limestone has grand development 

 in the Mendips, and constitutes the great mass of the chain, 

 having a continuous spread of five miles between Westbury 

 Beacon and Abbey, also between Croscombe and Emberrow. 

 The Lower Limestone Shales are nowhere more finely exposed 

 than around and resting on the upper members of the Old Red 

 Sandstone, and are highly fossiliferous throughout, the beds 

 being crowded with Strophomena, Chonetes, Spirifera, Polyzoa, 

 the ossicula of Crinoids, and many Trilobites, presenting a 

 strong contrast to the barren beds of the Old Red on which they 

 conformably rest. The Shales are well developed around Black- 

 down, especially to the east of Charterhouse, at Rowbarrow and 

 Piiddy, west of North Hill, and Nine Barrows; and east of 

 Egar Hill they attain a thickness of 500 feet, and are extremely 

 rich in organic remains. They present an extended outcrop from 

 Masbury to Stoke Lane, and Leigh upon Mendip, and in the 

 Downhead beds near Asham Woods. The local development 

 of these argillaceous beds of the lowest division of the Carboni- 

 ferous Limestone first gave origin to the name Lower Limestone 

 Shales. They are almost special to the west of England, and 

 are exposed on both flanks of the Mendip range. On them 

 rest the thick-bedded strata of the Carboniferous Limestone, 

 which is everywhere traceable for thirty miles from Oldford, the 

 gorge of the Vallis to Elm on the east, to the distant headland 

 of Bleadon in the west, and everywhere abounding more or less 

 with organisms which form the leading fossils in its beds. 



Coal Measures. — On the northern flank of the Mendips, 

 between Binegar and Wells, and resting on the Millstone Grit, 

 highly faulted and contorted, are the well-known Coals of Vob- 

 ster, Holcombe, Pitcot, &c., that portion on the west at Stratton 

 on the Fosse, Downside, &c. being covered by Dolomitic Con- 

 glomerate, the eastern side at Newbuiy and Vobster being over- 

 lain by the same rock and the Inferior Oolite. There is no 

 reason why we should not conclude that the Coals of the northern 

 side once extended across the Mendips and now lie deeply buried 

 along the south parts of the range. At Ebber rocks, west of 

 Wells, we have evidence of the Millstone Grit resting on the 

 Carboniferous Limestone ; and the elevation of the Mendips 

 being post-carboniferous, lends an additional reason for the 

 occurrence of the Coals of the northern area to the south of the 

 Mendips, and beneath the Lias and Peat plain of Glastonbury, 

 Castle Carey, the Pennards, and the Poldon Hills. No Coal 

 area in the United Kingdom is so disturbed and folded both 

 along its strike and on the dip of the Coals as those of North 

 Mendips; and like the Coals of the " Mons Coal-field" in 

 Belgium, which exists under similar conditions, the seams are 

 vertical and thrown over, so that the same seams are passed 

 through by shafts two or three times. The Vobster and Hol- 

 combe Coal-seams are the same as those at Ashton and Kings- 

 wood near Bristol, Twerton near Bath, and probably the same 

 as those at Yate. They underlie the whole area between the 

 Mendips and Bristol, and are probably the same that occur at 

 Kingswood and underlie the Pennant at Coal-pit Heath. 



'J he Tiias. — Two divisions of this group are greatly developed 

 around and upon the Mendips, especially the inferior or Dolo- 

 mitic Conglomerate, a peculiar and local condition of the base 

 of the Keuper Sandstone of the Bristol and South Wales Coal- 

 fields, chiefly that portion of the latter which extends from 

 Cardiff to Bridgend. The entire range of the Mendips is sur- 

 rounded by Dolomitic Conglomerate ; and ten or twelve patches 

 still remain as unconformable undenuded masses of that formation 

 resting upon the older rocks forming the massive range of the 

 Mendips. This remarkable deposit completely covered the 



