Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/151

Rh LATER TERTIARY GEOLOGY Of' EAST AXGLIA. 113 Looking at the subject in the light of the evidence at present available, the probabilities appear to be that such conversion ol* the sea-bottom into land did take place, and that first by tidal erosion during its emergence, and afterwards by subaerial agencies, the de- nudation which we have been describing was accomplished. The presence of a bed of clay full of chalk debris exactly resembling the Upper Glacial, and apparently formed by similar agency, beneath the Middle Glacial in the Yaro valley, and probably in other East- Anglian valleys also, seems to us to indicate that the valleys thus interglacially denuded became, prior to their submergence, filled with ice. The interval marked by the formation of the interglacial land-surface and valley-excavation may, and indeed, if our sugges- tion of the interglacial age of the Kessingland bed should prove to have good foundation, must have been accompanied by a climate as temperate as that of the Preglacial Forest-bod of the North-Norfolk coast. The return of glacial climate would probably have first filled these valleys with small glaciers, and thereby for the most part caused the destruction of any river or terrestrial deposits which had been formed in them, the Kessingland bed (supposing it to belong to this interval) being one which escaped this destruction. As sub- mergence set in, these glaciers would retreat before the sea, which would first occupy their valleys as fiords, and in so retreating would leave behind the moraine-material they produced and extruded at their terminations. Inasmuch as all the valleys in which we find a bed of clay with chalk debris that is presumptively identical with the one in the Yare valley, shown in sections Y. and VI., are, in the upper portions of their courses, excavated interglacially down to the Chalk (though this is in some instances concealed), the debris of that formation would necessarily constitute a large part of the moraine of their glaciers ; and we have already mentioned, as bearing upon this hypothesis, that wherever this bed of valley-clay with chalk- debris rests on the Chalk, the surface of that formation, for a few feet depth, is in a highly glaciated condition, forming a soft greasy marl, very different from the condition which it presents beneath the Crag or Lower Glacial sands through which the valleys contain- ing this moraine bed are cut. The gradual change of clay (which, except for its bluer colour, is in all respects similar to this of the Norfolk valleys) upwards into stratified brick-earth at Appleford Bridge, in the Black water valle)^, seems to indicate the deposition of a sedimentary deposit in one of these fiords following immediately on the recession of the glacier ; and, as we have mentioned, one of the sections in the Yare valley, that at Trowse Junction, shows something similar. 3. Consideration of the Mode in which the Middle Glacial was accu- mulated, and of the Way in which the Sequence of the Beds posterior to the Lower Glacial is to be traced. The origin of the formation of sand and gravel which we have called the Middle Glacial, and its succession by the wide-spread