HARMER : THE PLEISTOCENE DEPOSITS OF EAST ANGLIA. 327 
Whetlier or not the origin of lake basins in hard rocks may 
be ascribed to the excavating power of ice, there can be httle 
doubt that the erosion of soft strata, such as the Oxford and 
Kimeridge Clays, might have been caused in such a way. It 
does not seem that the great basin of the Fens had been 
excavated to its present depth in pre-glacial times. 
The existence, moreover, of the immense sheet of boulder- 
cla}^ which covers East Anglia, composed to a great extent of 
Oolitic material, suggests the glacial deepening of this remarkable 
feature in the physiography of Great Britain. 
It is difficult to form an estimate as to the average thickness 
of the Chalky Boulder-clay, which varies very much in different 
districts, or as to the amount of Cretaceous and Jurassic material 
it contains, but it is clear that the area covered by it is very 
much greater than the extent of the region, other than that of 
the Fen depression, from which, locally, it could have derived, 
as will be seen at a glance at Fig. 4. Unless we may draw in 
imagination on the Fen area (which on the map practically 
coincides mth the part left unshaded in Sheets 64, 65, and 51), 
and that to a very large extent, for the source of the southern 
portion of the moraine of the " Great Eastern Glacier," it 
seems difficult to offer any reasonable explanation of the 
existence of the latter. 
The views formerly current as to the post-glacial origin of 
the valley system of East Anglia cannot, I think, be any longer 
maintained. In 1866, in a paper communicated to the Geological 
Society of London,* I called attention to the fact that Chalky 
Boulder-clay exists within the valley of the Yare, near Norwich, 
and I after^^ards discovered further evidence to show that this 
and some other East Anglian valleys were in existence in Glacial 
times.! Since then much further light has been thrown on the 
question, a similar state of things having been found to exist 
near the Ipswich Railway Station, for example, where an ice 
stream, travelling down the valley of the Gipping, has ploughed 
into the Red Crag, leaving upon its disturbed surface a mass of 
* A Third Boulder-clay in Norfolk. Q. J. G. S., Vol. xxiii., p. 87, 
1866. 
t Q. J. G. S., Vol. XXV., p. 445, 1869. 
M 
