OEOLOGY OF THE AVON BASIN. 



9 



marls lying on ilie tipturncd edgos of tlio lower beds of ilio 

 Old Red Sandstone. 



The junction of the two series is well seen at the point 

 marked / on the map, whore the l<]aston JJrook lias cut 

 down through the ],)olomitic Conglomerate into the Old 

 lied Sandstone. The Dolomitic Conglomerate is hero of the 

 characteristic largely-fragmental texture. The junction 

 may also Ix; seen under different conditions at the point 

 marked g, where the new road just constructed on the 

 Charlton estate joins the older and steeper road. Here the 

 somewhat decomposed Dolomitic Conglomerate reposes on 

 the upturned edges of the ancient gravel beds of the Old 

 lied Sandstone. These ancient gravel beds are well seen in 

 a quarry about 100 yards farther along the road towai-ds 

 Charlton House. They resemble those exposed on the sliore 

 line of the Portbury-Clevedon ridge, W. of Woodhill ]3ay. 

 A better example of a consolidated gravel of ancient date 

 could sca,rcoly be f(3un.d. 



The reason for the coalescence of the Portbury and 

 Failands ridges is now evident. The Dolonritic Conglomer- 

 ate which forms the former hero lies on the harder beds of 

 the Old Red Sandstone which give rise to the latter. The 

 softer beds have not been exposed to the modern processes 

 of denudation. It was, indeed, eaten into by pre-Mesozoic 

 denudation; but over the eroded edges the Dolomitic Con- 

 glomerate was deposited. The Dolomitic Conglomerate ridge 

 marks the old shore line of the Triassic sea or lake. At 

 Coiiyo-,u- Hill the softer Old Red beds had probably not 

 boon so far denuded ; and over them no protecting layer of 

 Dolomitic Conglomerate was laid down. But farther W. 

 the pre-Moso/.oic waves had planed down the lower and 

 softer beds, and played upon the harder ridge, against 

 which the Dolomitic Conglomerate was thus deposited. 



