148 DAVIS : SOURCE OF BOULDERS IN CALDER VALLEY. 
fragments of granite or trap whilst the railway line was in 
process of construction, and a few well-rounded boulders 
are occasionally found at Mytholmroyd, two miles further 
down the valley, but they do not occur in any considerable 
quantity until North Dean is reached. 
Turning next to the second source whence the boulders 
may have been derived, it is well known that the valley of 
the River Ouse contains immense quantities of boulder 
clay, brick-earths, sands, and gravels, derived from the 
glaciers which it has been shown descended Swaledale and 
Wensleydale. The boulder clay may be considered as the 
foundation on which the others are deposited, and where it 
has not been subjected to the action of water, still extends 
across the whole breadth of the valley from the elevated 
Permian limestone plateau on the west to the liassic and 
oolitic wolds on the east. Sections exposing the relative 
position of the series in the valley are not frequently 
exposed. The boulder clay occurs extensively east of 
Knaresborough, and southwards ; and on the opposite side 
of the valley it is 60 or 80 feet thick near Easingwold. At 
York the boulder clay is 70 feet thick, and the new railway 
station works are all built on this material. South of the 
River Wharfs the clay has a thickness of 60 feet. It contains 
immense quantities of scratched stones; boulders of moun- 
tain limestone are numerous ; pink granite, trap, syenite 
and others, derived from the mountainous districts, are 
common, and mixed with these there are also boulders of 
sandstone, chert, and chalk. The centre of the valley is 
covered with deposits of a more recent origin ; sands and 
gravel occur plentifully, and appear to have been derived 
from the glacial clays beneath by the disintegrating action 
of water. The stones are generally more rounded, and the 
strise, or ice- scratches, have been removed by attrition. In 
a few cases the boulders still retain scratches. " These 
