Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/122

84 84 S. V. WOOD, JTJN., AND F. W. IIAEMEE ON THE is merely part of the interglacially denuded valley, which was deeper than the present, and extends down the river beneath the marshes, and the Postglacial gravel which underlies them ; for Mr. Samuel Woodward, in his ' Geology of Norfolk ' (1833), mentions that what he calls a subsided mass of the same blue clay as that of Strumpshaw Hill (the ordinary Upper Glacial clay of the district) was found beneath the marshes of the Yare in digging the canal which joins the Yare and Waveney. The Yare below Norwich is formed by the confluence of the Wensum (the principal stream) with it ; and in describing the geological features of the two valleys we take a line of section (V.) across them near to the confluence of their streams. The removal of the Lower Glacial sands and Contorted Drift along the slope of the interglacial valley portrayed in the cut has taken place very extensively in places along both these valleys — a circum- stance which, until we came to perceive the fact of this interglacial denudation, was a source of infinite perplexity to us in mapping the beds and working out their succession in this part of Norfolk, as was also the occurrence of sections of clay with chalk debris in the bottoms of these valleys and resting on the Chalk, which resembled in their physical composition the wide-spread Upper Glacial ; and it was not until one of us had the opportunity of making excavations which proved that this valley-clay in one instance at least (Cringle- ford, in section Y.) passed under the Middle Glacial, that we could get any satisfactory evidence as to its geological position. Thus in a paper published in the Journal of the Society for 1867 (vol. xxiii. p. 88), one of us described two exposures of a bed of clay made up chiefly of chalk debris and resting upon the Chalk in a glaciated condition in the Yare valley near Norwich (at Trowse Junction and at Thorpe Asylum) ; and we then interpreted this bed as being of valley-origin and posterior to the Upper Glacial ; but having subsequently met with so many instances of the Upper Glacial sweeping over the sides of the interglacially denuded valleys, wo were compelled to think that our original view of the valley- origin of the bed in question was erroneous, and that it could only be the Upper Glacial clay out of its usual place; and accordingly we so represented it in the map which accompanies the " Introduction" to the ' Crag Mollusca Supplement.' The crucial test applied, how- ever, by excavations at Cringleford, and continued reflection upon the observations which we have made during more than ten years on the geological features of East Anglia, have led us to the belief that our original view of this bed being of valley-origin and un- connected with the Upper Glacial was so far correct — but that, instead of its being posterior to the Upper Glacial, it is really of interglacial age, intermediate between the Contorted Drift and Middle Glacial, and a formation belonging to the interval which is indicated by the general unconformity between those deposits, and which has become exposed by the removal from it of the Middle and Upper Glacial during the Postglacial re-excavation of the valley. We have not reproduced here the woodcuts used in the paper in the