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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 19, 



don Clay capped by tlie Lower Bagshot sand and Bagshot pebble- 

 beds, and when, in tbe indentations and inlets so eroded, a thick- 

 ness considerably less than the mass of this formation further to the 

 north, in Norfolk, Suffolk, and north Essex, had been accumulated. 



Pigs. 1-3. — Diagram Sections showing the relations of the Middle 

 and Upjper Glacial deposits to the Valley of the Thames. 



Valley of the 

 Chelmer, 



Nortli side of the Valley of the 



Thames valley. Chelmer. 



5. Middle Glacial beds. 



6. Upper Glacial clay, occasional patches 

 of shingly gravel. The horizontal 

 lines indicate the sea-level of each 

 successive stage. 



North side of the 

 Thames yalley. 



1. Chalk. 



2. Lower London Tertiaries, 



3. London clay. 



4. Lower Bagshot sand. 

 4'. Bagshot pebbles. 



Fig. 2 shows the commencement of the submergence which, with 

 a total change in the sediment, introduced the Upper Glacial forma- 

 tion (or Boulder-clay). During this incipient submergence, and 

 after the change of sediment had occurred, the coast-erosion car- 

 ried the shore back from the points marked a, at which it stood 

 at the close of the Middle Glacial formation, to the point marked 

 h ; the new deposit of Boulder- clay was spread over the interval, 

 being underlain here and there only by a feeble bed of shingly 

 gravel (6'), very distinguishable from the thick sands and gravels 

 of the Middle Glacial period, and marked in figs. 2 and 3. Fig. 3 

 shows the stage when complete submergence had been etiected, and 

 a thickness of Upper Glacial clay spread over the entire area. As 

 this deposit, where it has most escaped denudation, still retains a 

 thickness of 160 feet, as in Cambridgeshire and Huntingdonshire, 

 and presents so uniform a character of slow and steady accumulation 

 by the outspread of water-borne clay- sediment, accompanied by the 

 dropping of chalk-debris from ice, there is but little room to doubt 

 its having spread over the counties of Surrey and Kent, as weU as 

 Essex, in great thickness, although, from its attenuation by denu- 



