424 CAETER : GLACIATION OF DON AND DEARNE VALLEYS. 
the wide embay ment of the Aire and Calder, bending back the 
Airedale glacier and moving southwards through Frickley gorge 
into the Dearne valley. As the lateral pressure of the Stainmoor 
ice increased, the Pennine stream would spread out south and 
west, wearing back the Magnesian Limestone escarpment, eroding 
the soft Middle Coal Measure shales between Frickley and Conis- 
brough, and pressing against the higher land on the southern 
side of the Don, probably sent a tongue of ice through Conis- 
brough gorge into the plain at Doncaster. Meanwhile westwards 
the low watershed between Clayton and Xotton Avould be sur- 
mounted, the ice pressing on to Staincross and Ardsley, and 
southwards to Rotherham. This would constitute the first 
glacial invasion of the Don and Dearne, and its moraine would 
be largely composed of local Coal Measure detritus with some 
Magnesian Limestone, and with an admixture of Carboniferous 
Limestone and chert boulders. Subsequent denudation would 
leave mere fragments of these glacial deposits. Then there 
would appear to have been a partial retreat of the Pennine ice 
northwards, but with no corresponding diminution of the Stain- 
moor glacier, but rather an increase, which enabled it to press 
over the Permian escarpment, probably over-riding the lobe of 
Pennine ice in the Conisborough gorge, and successively taking 
the place of the Pennine glacier as it retreated, until the Stain- 
moor ice apparently reached its maximum extension in this 
area along the line of the Dearne from Barnsley to Adwick and 
on to Conisbrough. 
In order to explain this retreat I would suggest that as 
the Irish Sea became choked with ice the line of maximum 
snow-precipitation would slowly be shifted westwards from the 
axis of the Pennine Chain, and therefore that there would 
gradually be a lessening of the snowfall that fed the glaciers 
in the Pennine valleys sufficient to cause an arrest of the 
advance of the Pennine ice, and ultimately to necessitate its 
slow retreat. The pressure of the Scandinavian ice, however, 
would be in no wise relaxed, and the continued crowding of 
the Scottish ice westwards by its strenuous thrust would 
render the Irish Sea more and more thoroughly choked, and 
hence the necessity of an escape over Stainmoor more con- 
