488 ' S. V. WOOD, JTJN., ON THE NEWER 



bottoms, and is fringed at its outer edge with spaces in which 

 the gravel has been more extensively cut out — such, for instance, 

 as part of the valleys covered by the clay in fig. VI., which at 

 the northern extremity of the line of this figure have the gravel 

 under the clay, but which, as the ice advanced over island No. 3, 

 have had the gravel entirely ploughed out of them, so that d rests 

 directly upon the Eocene. Exactly the same thing is shown by the 

 channel represented by the valley of the Roding which separates 

 island No. 3 from the next island westward ; for at the northern end 

 of this channel, close to the dividing line between Sheets 1 and 47, 

 the gravel in full thickness (as in fig. YI.) underlies the clay, but 

 thence to the southernmost edge of the Chalky Clay the whole 

 gravel (except patches of crushed pebbles at the Theydons, at Staple- 

 ford Tawney, and at Willingdale) has been wholly ploughed out. 

 In the channel between this island again and the next west of it, 

 which was constituted by the valley of the Lower Lea, the same 

 cutting-out has occurred — the gravel underlying the clay in the 

 north of this channel (where it enters Sheets 47 and 48), but being 

 absent at the southern end, except high up against the edge of the 

 island in the east of Sheet 7, where (at Finchley) it had been covered 

 by moraine before the ice shrank in this cutting-out way into the 

 Lea valley, as I shall presently show it did in the case of all the 

 East-Anglian valleys. 



"Where the ice overwhelmed the islands the moraine was sometimes 

 forced into their surface, where this consisted of soft material. 

 Fig. XV., taken from an islet of emerged hS, shown in Map 2 (on 

 the south of the Waveney, and in the north centre of Sheet 50), is 

 an instance of this. 



In fig. X. A I have, on a larger scale, represented the transition 

 from the gravel to the clay where, from being at the present time 

 exposed by slips in the railway-cutting on the east side of Wester- 

 field station, it can be examined minutely. 



In this case the upper portion of the sand and gravel c, perfectly 

 horizontal and undisturbed, and containing lumps of rolled chalk 

 derived from the moraine, is overspread by a horizontal band of 

 yellowish-white marl (d') from 1 to 1| inch thick. Upon this band lies 

 one of sand similar to the sand of the sand and gravel c below, and 

 of about the same thickness as the marl band ; and upon this again 

 comes another band of marl c?' identical in character and thickness 

 with the first ; and on that the amorphous morainic clay d rests. 

 This clay here, as it frequently does elsewhere, contains small pockets 

 of gravel, c\ 



These features appear to me to indicate the following process, 

 viz. : — First, and while the moraine bank was at some distance from 

 the spot, rolled chalk-lumps were carried from it and imbedded in 

 the sand and gravel as this was accumulating. Then, as the moraine 

 approached very near, chalky silt, produced from the attrition of 

 the chalk which makes up so large a proportion of the morainic 

 clay, was carried into the sea by the stream of fresh water which 

 arises from the thawing of the surface of the land-ice and of the 



