DAVTS: SOURCE OF BOULDERS IN CALDER VALLEY. 151 
floating icebergs and the tides would be amply sufficient 
to account for the presence of the boulders which have 
already been described. They are most abundant in the 
lower and wider parts of the valley, and the boulders of 
distant origin preponderate in number, in every section, in 
the lowest beds, those higher in the series being for the 
most part, and near the surface entirely, composed of sand 
and boulders of local origin. The icebergs would in the 
first place supply the erratics in the lower beds, and as the 
ice-sheet still receded, and a warmer climate prevailed, the 
rolling action of the tides reduced the boulders higher in 
the series from the rocks of Carboniferous age which sur- 
rounded the valley, and were constantly being broken loose 
by the action of the water wearing away the shales support- 
ing them. 
The fact is, that in all the sections which have been noted 
in the lower part of the valley, the erratic boulders are 
always most numerous, and that they are much larger in 
size, frequently a foot or more in diameter, at the base of the 
section, diminishing in size and frequency higher up until 
they are lost completely, and layers with only stones of local 
derivation occur. In the higher parts of the valley, as at 
Elland and North Dean, where the beds are half the thick- 
ness of those at Wakefield or Dewsbury, no large boulders 
have been found, and they are rarely seen to exceed two or 
three inches in diameter. There is also a great decrease in 
the proportionate number of foreign and local stones even in 
the lowest beds. This arrangement of the heavier boulders 
at the base of the sections, and especially their localization 
in the lower parts of the valley, points most clearly to their 
eastward, origin and the sorting action of the sea. Had they 
come westward by river action, the largest boulders would 
have been left in that part of the valley nearest its source, 
Or, if carried to their present positions, would have been 
