MITCHELL : DRIFT DEPOSITS OF THE VALE OF MOWBRAY. 
181 
one side of the river sometimes on the other. The gravel in these, 
the lowest part of the brick clay, is laminated, being interstratified 
with fine sand. There is no doubt that the chief agent by which the 
drift, which I have described, was brought into this valley was an 
enormous sheet of ice which advanced from the north-west, but I also 
think that there must have been a later and smaller ice-drift which 
came round the north end of the Hambleton Hills, and kept to the 
east side of the valley. This glacier would, no doubt, have its origin 
in the Scandinavian Mountains. But water, as well as ice, has done 
a great deal towards bringing about the existing state of things. 
The whole tract of land from the estuary of the Tees through the 
vales of Mowbray and York to Holderness, is not much elevated, and 
a very moderate depression would convert it into an arm of the sea. 
Besides this tJie thawing of such piled-up accumulations of ice as had 
gathered through ages of frost, when every atom of vapour was retained 
as ice in the frozen north, the thawing of these accumulations must 
have produced torrents of water rushing southward to find again 
its level in the ocean. By these currents from the north this 
valley must have been swept again and again, and by them denuded, 
to what extent the resulting alluvial deposits of Holderness bear 
witness. The stones of the drift in this part testify in several ways 
to having been subject to submergence and the action of water; they 
are, even in the case of very large ones, considerably rounded. Ice 
markings, though very evident in some cases, are decidedly rare 
among them ; this, I think, is due to their having been so rubbed 
and rounded by water action that their surface marks have been 
obliterated. When these " cobble" stones, as they are called, are 
used in building, the walls made of them turn wet in ^damp weather, 
I beheve that this is due to the same cause which makes seaweed 
turn damp under similar circumstances, that is to say, that they still 
contain some of the salt which penetrated into them when they were 
covered by the sea, salt having the power of attracting moisture 
from the air. I have here very strong evidence of one of these 
stones having been under the sea ; it is a piece of sandstone 
which, from its colour, has laid long in the soil. It contains several 
cells, which have been made in it by some rock-boring shell-fish. 
