DAVIS : SOURCE OF BOULDERS IN CALDER VALLEY. 
147 
angular, and have all been derived from the neighbouring 
hills. Granites, limestones, or other travelled boulders, do 
not appear to be present. Some distance lower down the 
valley, as Burnley is approached, however, the travelled 
boulders become common. 
If the erratics in the bed of the Yorkshire Calder were 
derived from the boulder clays east and north-east of 
Burnley, it is quite evident that they could not have been 
transported by river action. The great rise to the summit 
of drainage disposes of that theory. The only agent equal 
to the task appears to be drifting icebergs. It is possible 
that large masses of ice may have been in existence in the 
Burnley valley, and these, becoming loose and floating 
away, would carry with them any stones or boulders with 
which they happened to be in contact. In order that these 
icebergs should be able to float over into the valley of the 
Yorkshire Calder, it would be necessary that the land 
should be lowered to the extent of between 750 and 800 
feet, so that the sea might overflow the summit to a suffi- 
cient depth to afford a passage for the ice-floes and their 
contents. There are also other objections to this method of 
their entrance into the valley; amongst others, that the 
valley is entirely devoid of erratic blocks or boulders for 
many miles from its source, and it is only when the lower 
parts are reached that they occur, and the nearer the mouth 
of the valley the more numerous are the boulders. It 
might also be expected that some of the boulders would lie 
strewn on the sides of the hills bounding the valley ; but 
hitherto both the hill-sides and their tops have proved 
entirely devoid of such evidence. For eight or ten miles 
the deep valley does not present any sections exposing 
erratic boulders. At some distance below Hebden Bridge, 
Dr. Alexander* quotes Mr. Gibson as having found a few 
* Proc. Geol. and Polyt. Soc. of the W. Biding of Yorkshire, vol. i., p. 148. 
