135 
DERIVATION AND MODE OF ACCUMULATION OF THE DRIFT 
DEPOSITS OF THE WEST RIDING. 
1. The Greyish-hlue Clay. — The uniforin character of this 
clay, and the fact that in the valleys traversed by the Aire, 
and in the plain of Craven (where the clay is principally 
developed), at least seven-eighths of the stones it contains are 
limestone, while the underlying rock and the rock forming 
the slopes and summits of the hills above it for great dis- 
tances is often millstone grit,* renders it evident that this 
greyish-blue clay could not have been distributed either as 
suhglacial debris moved forward by a great sheet of land ice, 
or the supra-glacial moraine of a valley glacier, for in both 
cases the local rocks as well as the rocks the ice must previ- 
ously have passed over, would have been worked up into the 
clay and into the boulders. A rather shallow sea, as the land 
was gradually sinking, and a sea acting principally between 
about 300 and 600 feet above the present sea-level, would 
appear to have been the main cause of the distribution of the 
blue clay.f This sea could not have been much indebted to 
land-ice sliding doTO the hill sides or along the valleys, for 
such ice would have brought down a supply of materials 
different from those composing the blue clay. We are, there- 
fore, driven to the conclusion that the ice which undoubtedly 
had a share in shaping the stones and boulders found in the 
blue clay was sea-ice, coast-ice, or ground-ice. Most of the 
stones found in this clay are much rounded, as if they had 
* The occurrence of blue clay with a great preponderance of limestone boulders 
on millstone grit (as at Keighley) may be referred to an ice-laden current carry- 
ing materials, which, for great distances from the sources of supply, did not become 
much mixed up with local detritus. 
f The shale and limestone which formed the principal sources of supply for 
the blue clay and its boulders do not rise higher than about 600 feet, and the 
surface of the sea which floated the boulder-laden ice could not have stood much 
higher. 
