PLIOCElfE PEEIOD IN ENGLAND. 505 



generally in the valleys, and that therefore the gravel occupying 

 that valley pretty uniformly up to a level of about 100 feet above 

 Ordnance datum is not the gravel c cropping out from beneath the 

 Chalky Clay, but gravel posterior to this clay. 



The transition shown by this Appleford-bridge section from the 

 Chalky Clay to the gravel which rests upon it in the valley-bottom 

 seems, from the description of Mr. J. M. "Wilson*, to be entirely 

 analogous to that which occurs in the railway- cuttings near Rugby, 

 in the valley of the Avon, where that gentleman describes this clay 

 (which there makes a similar plunge into the valley of the Avon, 

 cutting- out the gravel c) as changing imperceptibly into the quartzite 

 gravel, which I am about to describe in Stage Y. as extending over 

 the western edge of the formation. 



In the excavation of the trough, which is occupied partly by the 

 upper portion of the Bain river, and partly by the Steeping river 

 in Lincolnshire, however, we get what seems to me to be one of the 

 most conspicuous results of the ice melting. This deep and steep - 

 sided trough is formed on its north-east sides by the escarpment of 

 the Chalk wold, shown by the dotted line in Sheets 83 and 84, and 

 on the west by that massive deposit of the Chalky Clay rising to a 

 nearly equal height with the Wold, of which I have spoken in 

 connexion with the marl masses in Stage II. The petty streams 

 of the Bain and Steeping which occupy it could in no conceivable 

 conditions of climate have possessed the volume necessary to have 

 wrought its excavation, even if the general facts pointed in that di- 

 rection ; but we have seen, in the case of the East- Anglian valleys, 

 that rivers, so far from having had any thing to do with their exca- 

 vation, have only filled them up to their present alluvium level. The 

 excavation of this picturesque trough of the Bain and Steeping seems 

 to me to have resulted thus. 



The sections referred to in the footnote f will show that the head 

 of this trough commences where the Chalky Clay is bedded up to and 

 buries the Wold escarpment. This and the fact that the Wold forms 

 one side and the Chalky Clay the other side of the trough, indicate 

 that when the Chalky-Clay ice began to dissolve the clay lay 

 continuously up to the escarpment along the line of the trough. The 

 Wold itself is bare of the clay, the whole of the vast mass of the 

 chalk degraded from it by the ice having been swept over its 

 escarpment into the depressed area of Central Lincolnshire ; so that 

 what is nevertheless a part of the Chalky-Clay moraine consists of 

 reconstructed chalk, pure enough to be burnt for lime. So long as 

 the ice continued to move it kept up this supply of moraine 

 from the Wold, and no fissure was allowed to form between the 

 W old escarpment face and the moraine thus swept over it ; but 

 when, from the diminution or cessation of the movement of the ice, 



* Report of the Eugby School Nat. Hist. Soc. for 1873, p. 10. 



t For the structure of these valleys, see sections of paper of Mr. Eome and 

 myself in Quart. Journ. Geol. Soc. vol. xxiv. p. 161 ; also plate of sections accom- 

 panying paper by myself in Greol. Mag. for January 1878. 



Q.J.G.S. No. 144. 2 m 



