238 Messrs. Buckland's and Conybeare's Observations on the 



the grit dips to the south-east^ conformably to the limestone, at an angle 

 of 45°. 



Along the northern escarpment of Leigh Down we might expect to find the 

 old red sandstone emerging from beneath the limestone ; and such is the case 

 throughout the eastern half of the chain from Portbury, where its section 

 exhibits a perfect regularity of structure [see Plate XXXII, fig. 3.J to the 

 Avon. Along this interval the old red sandstone is generally separated from 

 the limestone by a slight valley, marking the course of the interposed shale. 

 Near Charlton House, not far from Portbury, several abrupt, finely-wooded 

 and picturesque valleys descend from the sandstone towards the north. The 

 old red sandstone is here more than a mile in breadth ; but near Abbot's 

 Leigh, as you approach the Avon^ the newer red sandstone and the dolomitic 

 conglomerate come so near to the limestone, as to have only a narrow stripe of 

 old red sandstone exposed. 



The western half of the chain, from Clevedon to near Fortbury, presents 

 remarkable anomalies along its northern escarpment. The local phenomena 

 are such, as clearly indicate that a great fault ranges along the edge of this 

 part of the escarpment, affecting a very considerable subsidence of the strata ; 

 so that the coal-measures, depressed to the level of the old red sandstone, ap- 

 pear to occupy its place, and seem to dip beneath mountain limestone, on 

 which in fact they repose. The limestone beneath the coal-measures having 

 subsided as well as the coal-measures themselves, on re-emerging forms a se- 

 cond calcareous ridge, called Walton Down, to the west of Leigh Down, 

 though not parallel to it. This is the ridge already mentioned, as diverging 

 from Leigh Down near to Clevedon. The line of fault forms the third side 

 of a triangular area, of which the two others are Leigh Down and Walton 

 Down. It will be seen, on con- 

 sulting the annexed sketch of the 

 horizontal plan of the district, that 

 the subsided mass having wheeled 

 round through an arc of nearly 30°, 

 has its eastern extremity thrown 



forward nearly 3 miles towards 8.^^'""' '^~ ~^~^~W^ ^-^-^ ^--i::^'* 



the north. ^^^^E!gf- 



The anomalous appearances attending this fault are well exhibited in the 



northern part of the ravine traversing Leigh Down at Sir A. Elton's park. 



After passing the limestone, which bounds the ravine towards the south, the 



sides abruptly change to the Pennant coal -grit*, full of coaly matter, and 



* A description of this coal-grit will be found in the sequel of this chapter. 



