380 
MORTIMER : SECTIONS OF DRIFT. 
and other drift deposits are found to almost completely fill up the 
old preglacial chalk valleys, and in the form of a ridge near the latter 
place to cap the summit of the wolds up to an elevation of 400 ft., 
with a thickness of from 50 to 60 ft. In no other place, along the 
whole length of the elevated outcrop from Speeton to the Humber, 
do I know of any trace of a valley having ever been completely 
filled up with drift, in the same manner as those in the neighbour- 
hood of Mamboro'. All we find is a few small patches of sand and 
gravel, and occasionally clay, of a somewhat doubtful age, in 
shallow depressions on the summits of the hills, but rarely in the 
bottoms of the valleys. Had the valleys on this large area of the 
wolds ever contained the drift to the extent it is now found at 
Flamboro' and Speeton, we should, I think, find numerous traces of 
it. It is diflicult to conceive how it could have been swept so 
completely from the whole of the valleys, except in the above 
named small area along the coast, where to all appearance its 
removal would have been equally easy. The great probability is, 
that where little or no trace of the drift is now found on the chalk 
area, very little ever existed. The fact that the preglacial valleys 
are filled in, and the hills capped with boulder clay along the 
east coast to an extent of one mile inland only, seems to mark 
in this neighbourhood, the western limits of the Scandinavian and 
northern ice-sheets, which pushed before them great quantities of 
earthy matter plowed from the bed of the North Sea, and rode over 
the chalk area until arrested by the land-ice, which had already 
filled the valleys and capped the hills, and was creeping eastwards. 
This capping of land-ice would seal up the dales, and even if 
submerged for a time, would only admit drifted material to reach 
the chalk through rents and breaks in its mass, hence the almost 
entire absence now of boulder clay, and the comparatively small 
quantity of transported sand and gravel. 1 repeat that the sharp 
outlines of many of the dales are mainly due to having been thus 
preserved by this lower stratum of ice. 
After the disappearance of the ice, the last touch given to the 
