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 



