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Notes on the Structure of Mendip – Part 2

by Keith Murray

Blackdown

Welch has aptly described this structure as resembling a molar tooth on its side, the root pointing to the west.  Over folding on the northern limb has produce vertical and even inverted strata, the latter accounting for the limestone outlier at Churchill.  The western end of the core is overlain by post-Carboniferous rocks in orange-red (f6) when the area was presumably a bay bounded by the limestone outcrop.  Where the orange-red is blue-dotted it indicates the beach deposit conglomerate which often contains such large boulders of Carboniferous Limestone that it can be mistaken for that rock in small exposures.  Where the Shale/Black Rock Limestone boundary is exposed narrowly on the steeply dipping northern limb, it crosses the Twin Streams above Burrington. The shales swing round the nose of the feature to Charterhouse where the outcrop broadens owing to the more gradual dip of the strata and forms the low marshy ground just north of the Lower Farm.

Its junction with Limestones gives rise to the well-know swallets of the area.  Further west, a swallet is mentioned in the literature as being within the limestones of the Shales succession to the north of Tynnings Farm.  The Shales form the lowest part of the Hale Coombe and the boundary is mapped as far west at Crook Peak.

Off the nose of the pericline, and in limestones much higher in the succession, the Lamb Leer Fault shows a displacement of strata of 200ft. in the cavern.

North Hill

Overfolding here has produced the Harptree outlier in the same manner as the Churchill outlier off Blackdown.  In addition, it seems as though the western end of this pericline was held up by the nose of Blackdown while continued pressure from the south-west split North Hill along the Stock Hill and Biddle faults, the rocks between the two having been displaced upwards.  East of the road from Mineries Pool to the Miners Arms deposits of conglomerate overlie the Carboniferous and older rocks.  However, the Shale/Limestone boundary swings round from the Stock Hill Fault just west of the Miners Arms in a rough semi-circle interrupted by the Priddy Fault(s) to rejoin the Stock hill to the east of the Mineries Pool.  The faulted junction to the north-east of Priddy takes in Swildons and Nine Barrows cave systems, while both Eastwater and St. Cuthbert’s enter at the unfaulted junction, each under a low cliff of Black Rock Limestone.  Between Stock Hill and Biddle Faults the junction runs close to Cuckoo Cleeves and ends on Tower Hill in which area it is reported to be marked by a line of swallets.

Waldegrave Swallet, in the beach conglomerate, is cunningly contrived to deceive as the section of bedded limestone at its head is a displaced limestone boulder(s).  Just east of the Stock Hill Fault, it is almost in the centre of the uplifted sandstone core encroached upon by Conglomerate and a good half mile from the nearest limestone outcrop.  All the same in 1950, a depth of 28ft. of water worn limestone was reported although no water was seen in the swallet at the time.

Pen Hill

This structure was likewise split – by both Biddle and Slab House Faults – and at least the portion to the east of the Biddle Fault forced so hard against the eastern flanks of North Hill pericline that limstones were fractured and driven over each other along the Emborough Thrust and the Lower Limestone Shales appear to have been squeezed out altogether east of Hillgrove.  Only a patch of the Shales survive to the south-west of Whitnell Farm. West of Hill Grove a very narrow band of steeply dipping if not inverted Shales extends to Rookham.  The south-western edge of Pen Hill pericline has more recent rocks banked up against it, the Shales only appear to the West of Horrington.  Across the Biddle Fault they occupy a wide band of marshy ground to Nab House, dipping under Black Rock Limestone all the way.

Beacon Hill

Only the western end of this pericline appears on sheet 280, and plenty of displacements are shown. Further east, in the Heale area, the fold is split along north-south lines, the eastern section of the southern limb being thrust forward and overfolded on top of the northern limb to form the famous inverted Coal Measures klippe at Vobster.

Of Lower Limestone Shales, the railway cutting north of Maesbury Station contains the classic section described by Sibly (1906, p338) comprising the top two-thirds of the series.. (It is interesting to note that he comments ‘Maesbury, mis-spelt Masbury by the Railway Authorities’).  Some 600 yards west of the station the Shales/Limestone boundary is terminated by a fault.  To the east, the boundary, sometimes faulted, sometimes obscured by superficial deposits, runs through Stoke Lane to end just west of Whatley.  (Details of these eastern outcrops are described from a map published by Welsh in his paper of 1933).  The southern limb has a few stream exposures of the Shales in the Thrupe area and the Shale/Limestone boundary has been mapped north of Waterlip and south of Heale.  Along the Heale fault, to the west of the Heale-Downhead road, small exposures of the junction have been mapped and the swallets have been indicated (Welsh, 1933, p22).  East from Downhead the junction is displaced to the south in Asham Wood and its finally obscured some 400yds. S.E. of Dead Woman’s Bottom.  A minute area showing the junction has been mapped in Whatley Coombe just easy of Coombe Farm.

A mention of the spectacular Mendip gorges is appropriate – were they formed by collapse of underground caverns or by normal stream downcutting? Or by both?  An ingenious theory put forward by Reynolds (1927, p3) on account of late Ice Age deposits found within caves in the lower parts of the gorges and streaming from the mouths of these gorges is that melt waters cut the ravine before the ground water was sufficiently thawed to permit underground drainage.

It will be noticed that no mention of Vaughan’s zonal classification used as the basis of Welch’s mapping appear in contemporary descriptions or on maps, although it was used in the Regional Guide (Kellaway and Welch) of 1948.  In Kellaway and Welch (1955) a comparison of the Lower Carboniferous rocks of Mendip, Bristol, Chepstow and the Forest of Dean was made.  Unfortunately, it was found that the rocks of the Bristol area could not be described satisfactorily in terms of the zonal classification, moreover, the required fossil assemblages were not everywhere present, hence it was necessary to re-classify on a different basis.  This was done largely on the physical nature of the rocks themselves, and as the result was in good agreement with the zones where they could be identified, the alternative and more universal classification was adopted.

A similar, but farther reaching, problem arose with regard to the Lower Limestone Shales.  In the Bristol area there are passage beds between the Old Red Sandstone and the Lower Limestone Shales which do not occur on Mendip or in the Forest of Dean. In the Avon Gorge section these passage beds were included within the Lower Limestone Shales in the Carboniferous System, the base of which had originally been ascribed by Buckland and Conybeare (1824) to the base of the Shales.  Hence the passage beds are Carboniferous.  Where they appear in the Portishead area, however, they are inseparable from the Old Red Sandstone by the nature of their deposition with the O.R.S. beds.  Moreover both the passage beds and the Shales where they occur in North Devon and Belgium show affinities with the Old Red Sandstone.  Quite apart from this, the top of the Old Red Sandstone in the Forest of Dean, where there are no passage beds, contains fossils which are found in the Lower Carboniferous rocks to the south.

There is, therefore, a considerable argument for the transfer of the local Old Red Sandstone – Carboniferous boundary to the base of the Black Rock Limestone.

As there is already evidence (Green and Welch, 1965, p.171) that the Old Red sandstone and Lower Limestone Shales may be in hydraulic continuity in two localities on Mendip, it is perhaps fitting to conclude by lending speleological support to the argument for altering the established but debated boundary between the Devonian and the Carboniferous Systems in the area.

 

BLACKDOWN – N. Limb outcrop narrower because of steeper dip

NORTH HILL – N. Limb out outcrop inverted.

PEN HILL – N. Limb without shales in region south of thrust

Sketch Sections To Account For Variations In Northern Limbs Of Periclines

Bibliography

Buckland, W. and Conybeare, W.D. 1824.  Observations on the south-western coal district of England. Trans. Geol. Soc. (2) 1, 210-316.

De La Beche, H.T. 1846.  On the Formation of the Rocks of South Wales and South-western England. Mem. Geol. Surv.1

Green, G.W. and Welch, F.B.A. 1965.  Geology of the Country around Wells and Cheddar. Mem. Geol. Surv. 1 – inch sheet 280.  +=

Kellaway, G.A. and Welch, F.B.A. 1948. Bristol and Gloucester District, 2nd edition, British Regional Geology, Geol. Surv.  +=

Kellaway, G.A. and Welch, F.B.A. 1955.  The Upper Old Red Sandstone and Lower Carboniferous Rocks of Bristol and the Mendips compared with those of Chepstow and the Forest of Dean.  Bull. Geol. Surv. Gt. Britain., No.9.  1- 21.

Kellaway, G.A. and Welch, F.B.A. (Reynolds, S.H.) 1927. The Mendips. Geography, 13, 169-76.

Reynolds, S.H. and Vaughan, A. 1911.  Faunal and Lithological Sequence in the Carboniferous Limestone Series (Avonian) of Burrington Coombe, ( Somerset).  Quart. J. Geol. Soc. 67, 342-92.

Sibly, T.F.  1906.  On the Carboniferous Limestone (Avonian) of the Mendip area ( Somerset) with especial reference to the Palaeontological Sequence.  Quart. J. Geol. Soc., 62, 324-80.

Vaughan, A. 1905.  The Palaeontological Sequence in the Carboniferous Limestone of the Bristol Area.  Quart.  J. Geol. Soc. 61, 181-305.

Welch, F.B.A.  1929. The Geological Structure of the Central Mendips.  Quart. J. Geol. Soc., 85, 45-76.

Welch, F.B.A. 1932. The Geological Structure of the Blackdown Pericline.  Proc. Bristol Nat. Soc. (4), 7, 388-96.

Welch, F.B.A. 1933. The Geological Structure of the Eastern Mendips.  Quart. J. Geol. Soc., 89, 14-52.

Geological Map – I inch Sheet 280 (Wells), 1963.  +=

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Ed. Note: - The publications marked thus + are in the BEC Library.

Those marked = are currently available.