xviii SUPPLEMENT TO THE CRAG MOLLUSCA. 



masses accumulated ; and that the Fauna of this Till, a glacier-fed deposit nearly 

 destitute of Molluscan remains, is represented by the shells occurring in the sands of 

 Crostwick, Rackheath, Belaugh, &c. If we follow the coast section (W) we see that this 

 stratified formation, the Till, is lenticularly developed ; that is, it attenuates greatly 

 towards either extremity of the Cliff section, being in its greatest thickness about 

 Trimingham and Sidestrand. Not only so, but we see throughout the Coast section, that 

 the pebbly sands are interbedded with the Till, and that where this is thickest these pebbly 

 sands begin to change their character, becoming very silty and inter stratified with bands of 

 ash-coloured Till, full of chalk debris, while the pebble seams are thin and intermittent. 1 

 The Till is found in well sinkings for a few miles inland of the North Norfolk coast, but it 

 does not come to the surface, and it is not present where the pebbly sands come out along 

 the valleys of the Bure and of its tributary rivulets (see Section N). 



This clay, 6a, often strongly stratified, which was once supposed to be identical with 

 the great Boulder clay (No. 9), thus turns out to be a mere estuary deposit, rapidly 

 attenuating in every direction from its centre. Part of the shores of this estuary we can 

 define with approach to accuracy by the shingle masses and pebble beaches which lined 

 them, while the abundance of rolled chalk lumps, and especially the great sheets of chalk 

 several feet thick (VI of Section W) that are interstratified in the Till, prove that one 

 side of this estuary must have been fed by a glacier, some part of which, at least, was in 

 contact with the chalk district. 



At the Hasboro' or Eastern extremity of the Coast section, there exists an uncon- 

 formity between the Till and the overlying contorted drift, which is here uncontorted and 

 finely stratified ; but, so far as the churning up of the two formations at the other, or 

 Western extremity of the Coast section allows of observation, the Till there seems to 

 pass into the contorted drift without interruption. Probably the unconformity of the 

 Till at the Hasboro' extremity was due merely to some current, as we find the base of 

 the contorted drift itself filling up, in the form of sand, hollows in the Till, and this sand 

 unconformably overlain by the main stratified mass of that drift, as shown in Sections I 

 and II. 



The Contorted Drift. — The most extensive deposit of the Lower Glacial series is the 

 contorted drift of Sir Charles Lyell (No. 7). This bed along the Norfolk coast changes 

 its aspect materially. In the Western part of the section it becomes more chalky, by 

 an intermixture of fine chalk debris and chalky silt with the red mud of which the 

 Eastern half is mainly composed ; and in this Western portion, masses, sometimes of 

 enormous size, of marl, or reconstructed chalk (VII of Section W), are enveloped in it. 

 A study of these masses shows clearly that the agency by which they were introduced 

 was that to which the contortions were due, since masses of this material are usually 



1 They are here also much charged with lignite and peat debris, as is the Till itself in some places. This 

 lignite and peat were douhtless swept off the old forest-covered surface by the land ice, then beginning to 

 gather, and carried into this estuary. 



