﻿542 S. V. Wood, fun. — American and British Surface- Geology. 



l)etweeii 1100 and 1200 feet/ it is clear thalt either at this time 

 or subsequently the submergence must have much increased north- 

 wards and westwards. As, however, the mollusca of the Middle 

 Glacial sands presents so decidedly an older and more Crag-like 

 facies, and even the Upper Glacial at Dimlington and Bridlington 

 contains the Crag forms Nucula Cobboldice and Tellina ohliqua, 

 while the Moel Tryfaen and Lancashire beds contain none but forms 

 still living in British seas, and one or two more that live in the 

 Arctic Sea immediately north of the Shetlands, we can hardly 

 escape the inference that either a submergence of the Pennine 

 region and North Wales took place after the accumulation of the 

 earliest portion of the Upper Glacial (the chalky clay), or else that 

 during this accumulation those mountain regions were enveloped in 

 ice which, until the gradual passing away of the glacial conditions 

 removed it (and therefore after the chalky and purple clays had 

 accumulated), prevented the access of the sea and the formation of 

 marine beds. 



The chalky Upper Glacial reaches in Lincolnshire (Ponton) to 

 elevations somewhat exceeding 400 feet, and in Herts of 550 feet, 

 and the glacier to which it was due, and whose submarine recession 

 most of its accumulation in my view accompanied, descended over 

 Lincolnshire, but could hardly have extended much to the west of the 

 meridian which touches the westernmost boundary of that county ; 

 because nearly all the material which makes up this clay is derived 

 from the beds of that county, from the Trias to the Chalk inclusive.^ 

 At its greatest extension it existed in the form of inland-ice partly 

 below the sea-level, jiTst as now occurs in the case of some of the 

 inland-ice of Greenland and of parts of Spitzbergen, its eastern edge 

 resting upon the high ground forming the westernmost parts of 

 Norfolk, Suffolk, and Essex, its southern on the north border of 

 Herts, and its western, as already defined. Within these limits, 

 although here and there a few sporadic occurrences of gravel under 

 the chalky clay may be detected, and which I refer to some very 

 localized current action issuing from the ice in its retreat, the 

 chalky clay rests directly on the older rocks ; while on every side of it, 

 except the north, where the glacier connected with the place of its 

 genesis, the sands and gravels which I term Middle Glacial occur 

 extensively, though not uniformly. On its west side this glacier 



1 Darbyshii-e, Geol. Mag. I860, Vol. II. p. 293. 



2 The Upper Glacial of Middlesex and Essex contains little other chalk than the 

 hard form of that material characteristic of Yorkshire and Lincolnshire ; but in 

 Norfolk a good deal of soft chalk is intermingled, and in the case of the morainic 

 clay in the valleys of that county produced, as mentioned further on, by glaciers pro- 

 truding tongue-like from the great mass of inland-ice, the chalk debris seems mostly 

 of the soft character of the valley floor on which it rests. The fact that the chalk 

 so abundant in the Glacial clay of Middlesex and Essex is all of the hard kind (at 

 least if any of the soft is present it is so occasional that it has escaped my notice), 

 seems to me to go a long way in proof of this clay not being what Mr. Geikie makes 

 it out to be, — the submoraine in situ of a glacier which reached to the Thames 

 Valley ; for if so the chalk debris would be that derived from the counties of Essex, 

 Middlesex, Herts, Cambridge, Sufiolk, and Norfolk exclusively, instead of, as it 

 appears to be, chalk derived in chief part, if not exclusively, from Lincolnshire. 



