122 COLE : PHYSICAL GEOGRAPHY OF THE EAST PJDING. 
the district drained by the river Hull. Consequently in former times 
large portions of ITolderness were under water, and even now are 
liable to be extensively flooded. 
Beds of extinct fresh-water lakes are by no means uncommon, 
and are easily recognisable by the whiteness of the clay. 
Hills of sand and gravel, some containing crushed arctic shells, 
others, shells now confined to warm climates, as Cyrena fluminaliSj 
are also frequent. They are almost invariabl}^ covered with a 
coating of Hessle Boulder Clay. 
Huge trunks of trees, occasionally found embedded in peat, 
show that the district was once covered with forest growth, 
subsequently to the disappearance of the ice, and the submerged 
forests on the shore at Hornsea, and at Hull tell the same tale. 
On the side of the Yale of York there is comparatively little 
Boulder Clay. What there is does not extend much south of Escrick. 
There are, however, some extensive sandy tracts, known by the name 
commons, as Skipwith, Allerthorpe, etc., dating from the same age. 
In the Yale of Pickering, near Ganton, there is a wide district of 
sand, apparently Oolitic in its origin, derived from the Tabular Hills 
and Moorlands to the north, having been brought down Forge 
Yalley by the River Derwent, and spread over the surface. 
We must not omit to mention that a large part of the East- 
Riding, in the neighbourhood of Spalding Moor, drained by the 
river Foulness, was once a morass, and that extensive accumulations 
of peat took place, which, from the absence of lime in them, were, 
and are, unsuitable for the growth of mollusca. 
On the west side of the Wolds, especially in the neighbourhood 
of Pocklington, a quantity of chalk gravel forms the subsoil, though 
removed a considerable distance from the present Chalk Escarpment, 
and conveyed over beds of older date. 
On the low gTounds near the Humber and Ouse the tidal wave 
deposited great quantities of warp, especially before the rivers were 
embanked. This warp forms a fertile soil. It appears to be fine 
mud washed out of the Boulder Clay on the coast, and carried with 
the tide up the rivers. It differs materially from the sediment 
brought down by the rivers themselves. 
