TRANSACTIONS OF SECTION C. 567 
and Balby the ground is covered with drifted pebbles and fragments of limestone. 
The Magnesian limestone escarpment south of Conisborough is strewed for some 
miles with patches of drifted pebbles, of quartz, sandstone, and Trias. 
This evidence points to glaciation from the north and north-east by two move- 
ments of ice. ‘Two distinct tills, separated by warp and pockets of sand, are found 
at Staincross, the lower with Carboniferous boulders and the upper with Lake 
Country rocks. The drift patches are also of two kinds, one set being of a specially 
Carboniferous type, and the other rich in Lake Country rocks. It is the latter type 
that forms the Conisborough and Balby clays. In the Balby pits there is also 
found a large percentage of Middle Coal Measure material, which forms a perplex- 
ing mixture to explain. 
The author suggests that there was a double glaciation of this area early in the 
Glacial Period, first by Pennine ice, and secondly by the Tees glacier. 
It seems probable that at the commencement of the Glacial Period, before the 
Trish Sea was filled with ice, the Pennine Chain was an area of great snowfall, and 
extensive glaciers were formed in the valleys of Western Yorkshire. These 
glaciers would probably send down considerable streams of ice into the central 
plain, laden with Mountain and Yoredale limestones, cherts, ganisters, and 
Carboniferous sandstones. As the Glacial Period advanced the pressure of the 
Norwegian ice forced the Tees glacier into the Vale of York, and this in its turn 
would push back the Pennine ice into the lowlands of Airedale and over the low 
watershed between the Aire and Don, inside the Magnesian limestone escarp- 
ment, where it spreads out westwards and southwards as far as Staincross, 
Rotherham, and Conisborough. This seems to have been the line of farthest 
extent of this glacier, which, though it interfered for a time with the drainage of 
the Don, does not appear to have passed through the gorge at Conishorough. 
The country south of Frickley has undergone extensive denudation since the 
cutting of the Darfield gorge, and it seems probable that this was effected by this ice, 
and, on its northward retreat, by the deflected drainage of the Aire and Calder, 
which, as its course eastwards would still be blocked by the advancing Tees glacier, 
would find a ready route of flow through Frickley gorge. Thus a large quantity 
of Middle Coal Measures material must have been carried through the Conis- 
borough gorge into the plain at Doncaster, and would probably be suitably 
situated for the second glacier to carry forward to Balby. As it has been 
suggested that this material might be due to a glacier moving down the valley of 
the Sheaf from Dore and Totley, this question has been carefully considered. The 
geological surveyors do not record any drift in the valley of the Sheaf, and 
a careful search of the 6-inch contour maps has not disclosed any valleys which 
could have carried off the drainage of the upper Don if it had been obstructed by 
such a glacier at Sheflield. It is therefore concluded that no glacier capable of 
advancing to Conisborough was formed in the valley of the Sheaf. 
The retreat of the first glacier may have been due to a lessening of the 
snowfall on the Pennine watershed, owing to the shifting of the area of greatest 
precipitation to the west of the Pennine Chain as the Irish Sea became filled with 
ice. The evidence, then, points to a second invasion of the Don and Dearne Valleys 
by ice, the stream this time coming principally from the Tees. This glacier, 
which had advanced down the central plain, was now, by the retreat of the 
Pennine ice, enabled to push over the Aire-Don watershed and Magnesian lime- 
stoneescarpment. Westwards it abutted against the high land of Woolley Edge, 
and sent down a lobe of ice at Staincross and Monk Bretton into the valley of the 
Dearne. This second glacier does not, however, seem to have advanced far south 
of the Barnsley-Adwick-Conisborough curve, and laid down the upper clay of 
Staincross, the shap granites of Royston and Adwick, and the numerous Lake 
Country erratics of the district to the north and east of the Dearne. This glacier 
seems to haye advanced over the Magnesian limestone with a south-westerly 
movement, gradually closing the gorge of the Don and carrying the material of 
denuded Bunter and limestone beds over the escarpment to the south of Conis- 
borough, of which the pebble drifts are the relics. 
This movement does not appear to have extended much farther southwards, as 
