PLIOCENE PEEIOD IN EN^GLAND. 491 



against the upper edge of the bank of d, where this bank, after 

 the ice had disappeared, was left as the foreshore. 



According to Messrs. Crosskey and Woodward, the level of the 

 canal here, where the junction of c with d occurs, is 460 feet 

 above Ordnance datum. This was the sea-bottom ; but the level 

 of the sea-top is indicated by the gravel e, and this was as much 

 higher than the bottom as that gravel resting on d is above the place 

 where it rests on c beyond the tail of the bank, which is about 

 10 fathoms. 



It is evident that where, or when, moraine is beneath the glacier, 

 the ice cannot degrade. If degradation then takes place it must be 

 effected by the motion, not of the ice, but of the moraine itself; 

 and we see an instance of this in fig. XX., where, during the 

 shrinkage of the ice into the Gipping valley, as shown in fig. I a, 

 this moraine has ploughed out the sand ; but as we find large spaces 

 within the central limits of the Chalky-Clay formation uncovered, or 

 nearly so, by the moraine — such as those in Sheets 69, 64, 65, and 

 51 — it is evident that it has been stripped from this part as the ice 

 advanced, to contribute to the material of the moraine which has 

 been accumulated nearer to the outer edge of the ice. 



It is only on the assumption that the moraine, after its first de- 

 position, formed a protecting cushion to the subjacent rock, that I can 

 find an explanation of the character of the clay in Sheet 1 ; for the 

 clay here contains only the hard chalk of Yorkshire and Lincolnshire, 

 or of those layers near the base of that formation which crop out 

 in the west of Sheets 46 and 51. ISTow, unless the soft chalk of 

 Sheet 47 had been first covered by a cushion of moraine, and thus 

 protected from degradation, could the material from it have escaped 

 forming a part, and that, indeed, much the greater part, of the 

 chalk contained in the clay in Sheet 1 ? The case is similar with 

 Sheets 47, 48, and 50. 



It is in this way, then, only that the ice could, it seems to me, 

 have passed over those parts where newer PHocene beds, antecedent 

 to the Chalky Clay, remain undestroyed ; for even in the valleys there 

 this destruction has occurred from the thickening of the ice, as I shall 

 show presently. Much of the ice, therefore, outside the broken line 

 seems to have had no great thickness. 



Where this antecedent cushion of moraine has been cut through 

 by the ice, the floor of older formations is disturbed ; and where this 

 is hard it is sometimes grooved, and over this disturbed floor later 

 moraine has been brought. This feature is very conspicuous in the 

 bottoms of the Norfolk valleys, where they are not concealed by 

 gravel and alluvium ; but these more recent deposits have fiUed up 

 those valleys to so considerable an extent since the ice passed down 

 them as to conceal to a great degree the phenomena produced by the 

 Chalky-Clay ice at the period of its greatest extension. 



In Map 2 (PL XXI.) those parts which rise well above the level of 

 the junction of the clay with the sand and gravel c beneath it, in their 

 immediate neighbourhood^ are in shade, and so appear as islands 



21.2 



