Glaciation of Black Combe District. 



899 



Evidence is given to show that the Clay-with-Flints and Gravelly 

 Drift were formed by an ice-sheet which came from the north or 

 north-west over the Chiltern Hills. Only the clean upper layers of 

 ice surmounted the escarpment, and this produced the Clay-with- 

 Flints and Gravelly .Drift from the 'head' (Chalk and Eocene 

 waste), which had been accumulating for ages. 



At that time the Thames flowed, at the foot of the Eocene 

 escarpment, from Bourne End through Beaconsfield and Bickmans- 

 worth to Watford. The ice-sheet blocked the river-channel between 

 Bourne End and Bickmansworth about the time of maximum cold 

 and diverted the Thames southwards at Bourne End. The river 

 beyond Watford was further blocked by the Eastern Drift, which 

 has left Boulder Clay near Hatfield and at Einchley. The section 

 of the river between Bickmansworth and the Eastern Drift had its 

 direction of drainage reversed, and the water escaped at Bick- 

 mansworth by a new channel which became the Colne. 



On the melting of the ice, Fluvioglacial Gravels (Plateau Gravels 

 of some writers) were left over a great area. These gravels are 

 composed chiefly of Eocene and Cretaceous materials derived from 

 the Gravelly Drift, but also contain Bunter pebbles which have been 

 brought down the Thames. The Bunter pebbles are particularly 

 abundant in a band between Bourne End and Watford; to the 

 south only a few, presumably washed out of the band, are found. 

 They are believed to show that the Thames in times of flood returned 

 to its former course. 



The floods from the melting ice, added to the waters of the 

 Thames and Colne, produced, by denudation of the Eocene clays, the 

 great flat through which the Thames now flows, east of Maidenhead, 

 and which, opposite Iver, is 8 miles wide. 



After the retreat of the ice, the Wye and Misbourne extended 

 their channels over the Fluvioglacial Gravel flat, and some other 

 small streams were formed. 



2. ' Some New Lower Carboniferous Gasteropoda.' By Mrs. Jane 

 Longstaff (nee Donald), F.L.S. 



March 27th, 1912.- 



-Dr. Aubrey Strahan, F.B.S., President, 

 in the Chair. 



The following communications were read : — 



1. ' The Glaciation of the Black Combe District (Cumberland).' 

 By Bernard Smith, M.A., F.G.S. 



After a brief discussion of previous work and literature, a short 

 sketch is given of the geological structure of the district. 



With the exception of the western coastal plain the main topo- 

 graphical features are pre-Glacial, but they have been either 

 subdued or accentuated by glaciation. The chief pre-Glacial 

 drainage-lines determined those of the present day. 



Evidence is given to show that, during the flood-tide of glacial 

 tion, the whole district was swamped beneath an ice-sheet formed 

 by the confluence of Lake District and Irish Sea ice, the summit 



