Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/126

88 but over the country traversed by this valley the Upper Glacial is usually underlain by the Middle Glacial, which is often in great thickness, some of the wells, as we learnt, going through upwards of 70 feet of it. As elsewhere in East Anglia, however, the Contorted Drift occasionally protrudes through the Middle Glacial and shows itself on the highest ground overlain by the Upper Glacial. At Guist also is a fine section of the Contorted Drift over the pebbly sands ; but mostly in this part of Norfolk the pebbly sands have thinned out and the Contorted Drift rests on the Chalk direct.

That portion of the Wensum valley which extends on either side of the line of section VII., affords several instances of what we regard as the same bed as that marked a in sections V. and VI. The character of the bed in this district differs somewhat from that which it presents nearer Norwich, where traversed by the lines of the last-mentioned sections ; for instead of being, as there, a tough clay full of chalk debris, it here generally consists of a greenish gritty deposit more sand than clay ; but in one exposure this is overlain by clay exactly like that at Cringleford, Trowse, and Thorpe already described, with a sheet of glaciated chalk (torn from the glaciated chalk of the valley-floor) interposed between the two. The bed in every exposure that we have met with rests on Chalk in this glaciated condition ; and, indeed, the Chalk in all the Norfolk valleys, where not protected by the Lower Glacial sands, or the Crag, presents this feature, indicative, as it seems to us, of the action of the interglacial ice upon it. So completely is the condition of the Chalk changed by this action in some places that, instead of affording a porous surface of light land, it has become retentive and water-sodden, of which striking examples may be found in the bottoms on Bridgham, Eoudham, and Croxton Heaths between East Harling and Thetford, where, the Contorted Drift having been interglacially denuded, the Middle Glacial sand has been deposited on this glaciated chalk, which has been again laid bare by Postglacial denudation. Where this has taken place the chalk surface holds water as well as the most tenacious clay. The explanation of the different character presented by this bed a about Lyng and Elsing (that is to say, in the portion of the Wensum valley illustrated by section VII.) seems to us to be found in the different condition of the beds out of which the valley is here excavated having furnished a different pabulum for the valley ice to degrade. Thus nearer Norwich the Chalk rises high up the valley-sides (see section V.), the Contorted Drift there has a good deal of clay in it (forming of itself a true brick-earth), and patches of the Chillesford Clay (though these