150 DAVIS : SOURCE OF BOULDERS IN CALDER VALLEY. 
of an ice-sheet which probably had its termination in 
this district, and, being thin, could not exert a very great 
influence in grinding up the rocks over which it passed. 
The greater part of the boulder clay so deposited has since 
been removed by denudation. 
The glacier, whose existence is thus indicated, descended 
from the northwards, and consequently must have crossed 
the lower part of the valley of the Calder in the neighbour- 
hood of the place where Wakefield now stands, and several 
miles west of its confluence with the Aire at Castleford. 
The facts already stated, though very briefly, go to prove 
that on the recession of the glaciers which once enveloped 
the country north of the hills separating the Calder from 
the Aire, immense quantities of stiff glacial clay, filled with 
sub-angular, scratched boulders of both local and distant 
origin, were left filling up the valleys. The land appears at 
this period to have been submerged to the extent of some 
four or five hundred feet, and the glacial clays subject to 
the denuding and abrading action of water. The boulders 
released from the clay, and rolled hither and thither by the 
waves, were gradually reduced to a more rounded form, and 
by the same process the scratches were obliterated. There 
can be little doubt that to this action is due much of the 
sand and gravel existing in the valley of the Ouse, and also 
in the Aire valley, in the neighbourhood of Leeds. The 
climate appears to have been still cold, and icebergs, broken 
off from the receding ice-sheet, or masses of ground-ice 
bearing the boulders frozen from the bottom into their mass, 
drifted in every direction, and, melting, dropped their bur- 
den of boulders in new localities. 
Under such circumstances as these the valley of the 
Calder would be an estuary from the sea, of considerable 
width at its mouth, and gradually closing inland to a com- 
paratively narrow channel. The united action of the 
