486 S. V. WOOD, JTJN., ON THE TTEWER 



Outside the broken line the formation is usually underlain by 

 the sand and gravel c in those parts of its area which are shown 

 as sea in Map 2 ; except that towards the close of the formation, 

 in consequence of a change in the mode in which the ice acted, 

 which I shrJl describe in the sequel, the ice has, in the same way in 

 which it has done in the valley-bottoms through which at the close 

 of the formation it issued to the sea, destroyed the gravel and such 

 beds of Stage II. as there maj^ have been there, and left its moraine 

 resting directly upon the older Tertiary and Mesozoic strata. The 

 way in which the clay sweeps down over the edges of successive 

 Pliocene beds through which these valleys have been cut, and on to 

 the older formatiou which constitutes the valley-bottom, I shall 

 more fully describe in the sequel. 



The clay is thickest where not underlain by any Pliocene forma- 

 tion, that is to say where it has been heaped up on or against 

 some of those islaiids shown in Map 2, which were overwhelmed 

 or enveloped by the ice, as, for example, near the centre of the 

 division between Sheets 51 and 47, where the railway excavations 

 and well at Horseheath showed it to be 120 feet thick ; in the 

 south-east of 52, where it has been banked up against the j^eo- 

 comian sand-hill of Potton, and where the well at Longstow 

 railway-station traversed 160 feet of it. In the centre of the 

 same sheet also, over the islands lying between the Ouse and 

 the Xen, I heard of wells having been sunk through an equally deep 

 mass of it ; and it lies deep against the Wold in Sheet 83. Its 

 thickness where it overlies Pliocene formations, on the contrary, 

 seldom reaches 50 feet, and is more frequently under 20. 



The contact or junction of the formation thus described with the 

 gravel c beneath it varies in character, and I have selected the cases 

 represented in figures X. to XIY. for its illustration. 



In fig. X. the sand and gravel is not quite abruptly succeeded by 

 the clay, but passes into it by a few inches of material which is an 

 intermixture of both. In fig. XL, after the deposition of a small thick- 

 ness of the clay, the formation of the sand and gravel has been re- 

 sumed, and the clay thus become interstratified in the sand and 

 gravel. The process appears to have been tranquil and uninter- 

 rupted beyond the brief resumption of gravel deposit. Fig. XII. 

 shows similar features in these respects, but varpng in the inter- 

 stratified band of the clay being thicker and in its slightly bending 

 down into the sand and gravel. The lower bed of the clay in this 

 section was much more chalky than the upper, which would show 

 that in the interval between the deposition of the first portion and the 

 next, and during which the intermediate band of c was accu- 

 mulated, the character of the moraine reaching the spot had some- 

 what changed so far as regarded the proportion of chalk in it. The 

 band of c thus intervening here was loamy*. 



There can, I think, be no question that these instances show that 

 by some means the moraine of which the clay is composed was in- 



* I am not sure that the clay of this figure may not be the Till, and the sand 

 beueath b 1 instead of c, but I think not. 



