PLIOCENE PERIOD EST ENGLANB. 487 



troduced tranquilly over a sea-bottom in which, sand and gravel had 

 up to this time been accumulating ; and the question therefore arises 

 in what way this took place. In the Geol. Mag. for August 1872, 

 p. 364, Prof. Nordenskiold gives illustrations of the various modes 

 in which the Greenland and Spitzbergen ice passes to the sea. In 

 fig. 6 of his illustrations he shows it in no great thickness, pre- 

 senting a low and shelving face to the sea, and preceded by a mud-bank 

 sloping also to the sea and identical in shape with the mass marked 

 d in the figure (XYII.), which I have copied from Messrs. Crosskey 

 and Woodward's notice of sections in the neighbourhood of Bir- 

 mingham, presently described. This mode N'ordenskiold describes 

 as having given place in one instance, within the space of two 

 years, to the cutting-out mode of entry, which he shows by his 

 figure 5. 



This advancing mud-bank appears to me to have been the method 

 by which the morainic material constituting the Chalky Clay was 

 laid so tranquilly on the gravel of the bottom of the sea over which 

 the ice was creeping, while it also overwhelmed the islands with 

 which that sea was studded ; and that in this way a cushion was 

 formed for the ice, which protected the gravel from destruction by it ; 

 until by the great thickening of the ice nearer its source (^. e. within 

 the broken line of Map no. 1), it crushed out both the protecting 

 cushion and the gravel also, forcing it all onward as new moraine, 

 which it in some parts piled up against and upon the islands which 

 it had overwhelmed, and in others left in situ ; or until, from the 

 mode in which it issued to sea having changed to the cutting-out 

 one shown by JSTordenskiold's figure 5, the gravel and the moraine 

 at first laid over it were near those issues cut out by the ice to be 

 replaced by moraine accumulating in places beneath it. Where the 

 broken line crosses the north-west corner of Sheet 50 this process 

 is distinctly apparent ; for here, at Knattishall, d resting on c pre- 

 sents the same condition of stiff heavy clay which is universal 

 outside this line ; but immediately west of Knattishall it changes 

 abruptly, and resting on the chalk without any of c under it, d pre- 

 sents, over the part where it extends for several miles to the west of 

 this village, the condition of a light- coloured gritty moraine, which 

 some of the pits show to include streaks and masses of this stiff" 

 heavy dark- coloured usual form of the clay twisted up in it. It is evi- 

 dent that all this moraine has been formed from the destruction of c 

 and of d in the original form in which it was deposited over c, as well 

 as probably from the destruction of h 3, with the addition of chalk 

 from the floor on which these rested. This general crushing-out 

 has given rise to the features within the broken line of Map 1, and 

 the cutting-out I shall describe as prevailing over the parts where 

 the ice at the close of the formation issued to the sea ; for the change 

 from the mud-bank mode of entry into the sea to this cutting-out 

 mode seems to have occurred at one particular time over the eastern 

 area of the formation ; so that the clay with gravel under it, which 

 occupies most of that part of the area outside the broken line which 

 appears as sea in Map 2, has the gravel generally cut out of the valley- 



