Essay on Geology. 93 



which the Avon flows, between Keynsham and Pile Marsh, traverses 

 this sandstone and presents very good sections of it. On the north of 

 the Avon, the coal strata are generally exposed on the surface of the 

 gronnd, throughout our coal basin, which in that direction extends as far 

 as Cromehall ; here the margin of carboniferous lime bends round it, then 

 turning southwards by VVickwar and Chipping Sodbury. But near tiie 

 latter town, the superior deposits of red marl and lias overflow, as it were, 

 into the coal basin,* and cover the strata composing it. These newer 

 deposits extend as they advance to the south-west, so as to overlay and 

 conceal almost the whole of the basin between the Avon and the Mendips. 

 This occasions the pits in this district to be often sunk to a great depth, 

 before the true coal measures are touched ; at Clandown, for instance, a 

 shaft was sunk through the whole thickness of the lias and red marl, more 

 than one hundred fathoms 3 the total depth at which it is now worked 

 .being near two hundred. Such a statement as this would in most other 

 coa! districts be rejected as incredible. The coal measures are, however, 

 stript of this covering, or, to use the technical word, denuded, in the vale 

 of Pensford, lying at our feet on the south-east, as we stand on Dundry, 

 and again at Nettlebridge, close to the north-eastern extremity of the 

 Mendips. We have here the best opportunities of studjing the phgenomena 

 of vallies of excavation, as explained in our former outline, p. 15. The 

 collieries about Nettlebridge also strikingly exhibit the disturbances which 

 have been produced by the elevation of the Menrlip Hills ; the strata are 

 often quite vertical, sometimes thrown quite over through an angle exceed- 

 ing 90°, and commonly very greatly contorted, being twisted in the shape 

 of a Z, &c. 



We should next direct the attention of our observer to the phsenomena 

 of the more recent overlying rocks before mentioned, as exhibited in the 

 district now beneath our eyes ; and the very first we have to examine will 

 forcibly illustrate some of the most important points in our general outline. 

 The violent convulsions which elevated tlie Mendips, and shattered and 

 contorted, in the manner we have just noticed, all the adjacent strata, must 

 necessarily have occasioned vast masses of torn and fractured rocks ; and 

 as the ocean appears, from tlie nature of the subsequent and overspreading 

 deposits, to have still covered all this district at the period in question, 

 these submarine explosions and movements must have produced the most 

 violent waves and currents. By these means, the piles of wrecks to which 

 we have alluded, would have been drifted asunder, and distributed in the 

 form of a broad sheet of shingle, over the general surface ; this mass bcin"- 

 of course thickest towards the base of the hills by whose elevation it was 

 occasioned, and from the ruins of whose strata it was derived. Now we 



• In the romantic defile of Wick rocks, however, the carboniferous liracstoiif 

 again, for a short interval, emerges from hencatli this tleposit. 



