MORTIMER : SECTIONS OF DRIFT. 
377 
passing over might drop boulder clay and cover the patches of sand 
and laminated marl as we now find them. 
These sand, gravel, and clay mounds distributed over most of 
Holderness in great numbers, denote then the frequent grounding 
of rock-loaded masses ot floating ice. Their north- western limit 
seems to have been determined by the approach of shallow water, 
on nearing the edge of the chalk hills ; there they came in contact 
with the edge of the land ice which capping the chalk hills and 
pressing eastwards down the dip slope to the sea, crushing and 
grinding the chalk into the form of gravel, and on the inner 
margin of the wolds, much obliterating the hills and dales. To 
these opposing forces — land ice and bergs — I attribute all the 
peculiarities of deposition observed in the sections, as well as the 
transport of foreign material. The edges of the land ice-cap would 
break off into pieces on reaching the shore line and soon become 
stranded, or for a time float hither and thither, as they melted 
dropping their loads of chalk gravel into the hollows of the clay 
which had been ploughed up by the sea-borne bergs. 
But as before stated, the land -borne chalk gravel is mainly 
confined to an area included between the inner edge of the chalk 
hills, and a line of sand, gravel, and clay mounds and ridges, 
formed by the packing and stranding of icebergs, which had checked, 
except in a few cases, the flow of land-ice eastwards. Hence the 
less frequent occurrence of chalk gravel in the interior of Holder- 
ness, until the present coast-line is reached, where it is seen in 
lenticular sheets at the top of the cliffs. Probably it has for the 
most part been brought from the promontory of Flamborough 
head. It is worthy of note that this line of mounds and ridges is, 
in the neighbourhood of Hull, as previously mentioned, much 
further from the edge of the chalk hills than it is towards Beverley, 
Driffield, and Bridlington. This may have been caused by a 
western flow of ice through the gorge of the Humber, accompanied 
by an under current of water, which would keep back in its front, 
the bergs from the Scandinavian ice-sheet. And it is very probable 
