76 SPENCER: GLACIAL BOULDERS IN CALDERDALE. 
1,200 feet to 1,450 feet in height on its route, with the same ease as 
a storm-swollen brook envelopes and overtops the boulders in its bed, 
it finally impinged against the steep flanks of Blackstone Edge and 
the western end of the Walsden Pass. When the glacier reached 
Blackstone Edge it had become greatly reduced in thickness, which 
was at that place estimated by the late Captain Aitken at about 
1,150 feet. So that only a limited portion of the ice-sheet, with its 
burden of far-travelled rocks, was pushed over the Pennine Chain 
and found its way into Calderdale, the great bulk of the glacier being 
turned down the western flanks of the Pennine Chain into the plain 
of South Lancashire. This range of high ground is in the neigh- 
bourhood of Littleborough and Burnley cut through by two deep 
passes—namely, those of Walsden and Cliviger. Now, as the height 
of the Walsden Pass is only 627 feet and that of Cliviger 768 feet 
above sea-level, it must be evident that, unless these passes were 
either non-existent at that time or they were filled up by some 
means or other, the ice-sheet must have been pushed through them, 
down to Todmorden and thence down the valley of the Calder. 
The general opinion of those most familiar with the facts of the case, 
is that the level of these passes has not materially altered since 
pre-Glacial times. Hence, in order to keep back the ice-sheet, they 
must have been filled up by some means or other, otherwise the 
passes would have been strewn with boulders dropped from the 
glacier. Now, one of the most strange and puzzling facts in con- 
nection with the glacial phenomena of our district is that there are 
absolutely none of those foreign boulders to be met with in either 
of these passes. 
Let the diligent student search these passes as long and as 
carefully as he may, not a single erratic of either granite or any 
other far-travelled rock will reward his search. But if he choose to 
ascend the hill on either side of the Walsden Pass, especially that 
on the south, he will walk over a great thickness of them for a long 
distance up the hillside and close to the edge of the pass, and up 
to the height of 1,150 feet or more above the sea-level, and in 
walking over the hill he may meet with some of them here and there 
until he reaches Walsden, which lies on the eastern side of the chain. 
How to account for this strange fact was for a long time a puzzle to 
local geologists. At length my friend, the late Captain Aitken, in 
order to account for the absence of glacial boulders from these 
passes, propounded the theory that these passes had become jammed 
full of local ice from the surrounding hills during the earlier portion 
of the glacial period, so that the great Lancashire ice-sheet when it 
reached the locality, found those passes full of compact ice which 
Naturalist, 
