GLACIAL BEDS. 507 



The surface of the Boulder Clay is frequently ' piped ' in the same way as the 

 Chalk, a feature evidently due to a similar cause, namely, the dissolution of the 

 chalky matter by carbonated water. 



Tlie Chalky Boulder Clay is well developed over the greater part of South 

 Norfolk, and over much of Suffolk and Essex, in the country around Tivetshall, 

 Eye, Framlingliam, Halstead, Thaxted, Dunmow, Braintree, the Easters, and the 

 Rodings, but it has not been met with south of the Thames, nor in the Thames 

 Valley, although it extends to its northern margin near Brentwood and Epping. 

 It occurs at Finchley, at Bricket Wood near Watford, and other places north of 

 London, and around Buntingford, Biggleswade, Huntingdon, Horncastle, etc. 

 (See p. 500.) 



The character of this deposit is fairly uniform over a large area, but while spread 

 out in extensive sheets over the higher grounds of South Norfolk, Suffolk, and 

 Essex, it rests on different beds, and often descends to the bottom of the valleys 

 on a level with the Alluvium. In Norfolk it is remarkable that over the greater 

 part of the area where the Chalky Clay is developed, there is very little of the 

 Lower Boulder Clay beneath it. The sequence of Upper, Middle, and Lower 

 Glacial is the exception and not the rule. 



A fine section of the Boulder Clay resting on Middle Glacial Sands is seen in 

 the cliff between Kessingland and Pakefield ; and we have at Corton ' the evidence 

 of Upper and Lower Boulder Clays. At Corton the latter bed is a very feeble and 

 irregular accumulation compared with the Lower Glacial Clays at Happisburgh 

 and Cromer, while in that area no Chalky Boulder Clay appears in the cliff-sections. 



Inland in Norfolk there are a few pit-sections which show the sequence of 

 Chalky Boulder Clay, Middle Glacial sand, and stony loam or Lower Boulder 

 Clay, as near Strumpshaw Hill, Moulton, Upton, and South Walsham ; but most 

 of these sections sliow in the same pit the sand tapering away, and tht-n the two 

 Boulder Clays come together, and their separation is not a happy task. Fortu- 

 nately in East Norfolk the Lower Clay maintains its character of a brown stony 

 loam, so that whenever a section is exposed, it may usually be distinguished from 

 the more chalky Upper Boulder Clay in that area ; but westwards the Lower 

 Glacial Clays become so like the Chalky Boulder Clay, that from the evidence 

 of pit-sections, they cannot be separated one from the other. - 



Looking at the beds, however, in a general way, we discern that two great 

 divisions may be recognized — the Cromer Till and Contorted Drift, grouped 

 together as Lower Boulder Clay, followed by sands and gravels ; and the Chalky 

 or Upper Boulder Clay, with associated sands and gravels, which will be described 

 further on. The Lower Boulder Clay, as Mr. John Gunn has remarked) is 

 characterized by boulders of an igneous type, and it often contains Pleistocene 

 shell-fragments. It may have been derived from the north and north-east. The 

 Upper Boulder Clay is characterized by Jurassic detritus, and was probably derived 

 from the north-west. But the most important distinction, perhaps, is in the 

 disturbances to which the Lower Glacial Beds have been subjected by the agent 

 which formed the Chalky Boulder Clay. 



Sometimes the Chalky Boulder Clay rests on a substratum of sand that shows 

 little or no disturbance ; and, as a rule, with such junctions, the lower part of the 

 Boulder Clay is sandy, and the line of division, though sometimes undulating, is 

 distinct. Where the Boulder Clay rests on clay, the same distinct line may also 

 be observed; but almost invariably where the underlying clay contains streaks or 

 nests of sand, these are very much contorted. The fact is that the clay without 

 these nests of sand would not well exhibit the contortions ; while where sand alone 

 is seen under the Boulder Clay, contortions may be absent because the deposit was 

 of too yielding a nature to be acted upon. In one or two localities where marked 

 lines of bedding occur in the sand, there are evidences of disturbance. On the 

 whole, the beds beneath the Chalky Boulder Clay of Norfolk are more frequently 

 contorted than not, and hence it may be concluded that the agent which formed 

 this Clay was instrumental in disturbing the strata over which it was accumulated. 



^ See J. H. Blake, Horizontal Sections, Sheet 128 (Geol. Surv.). 

 ^ P. Geol. Assoc, ix. Ill ; see also F. J. Bennett, Proc. Norwich Geol. Soc. 

 i. 252. 



