GEOLOGY 



(2) the Irish Sea Glacier, (3) the North Sea Glacier, while the one from 

 the Derbyshire hills may be termed (4) the Pennine Glacier. Their 

 history has not been completely made out, and the order in which they 

 invaded the district is uncertain, but the local glaciers had probably 

 reached a considerable size before the foreign ice penetrated into the 

 heart of the country. 



We will now briefly describe the phenomena presented by the 

 different ice masses, mentioning neighbouring areas where necessary for 

 a complete comprehension of the subject : 



Arenig Glacier. Descending from the Arenig Hills (2,817 ^ eet ) 

 this glacier passed down the Vale of Llangollen and then debouched on 

 to the Shropshire plain, where it threw down the masses of morainic 

 material at Ruabon and Ellesmere. It would be natural to suppose that 

 it would then have passed northward down the Dee valley with over- 

 flows to the south along the Severn valley. The northern path however 

 was blocked with ice coming from the Irish Sea and the southern course 

 barred with ice from Plinlimmon. It was therefore compelled to assume 

 a south-easterly course, impinging upon Staffordshire, round Wolverhamp- 

 ton and the ground to the south, where occasional boulders of Welsh 

 rocks, but mixed with others brought by the Irish Sea ice, are met 

 with. Around the southern margins of the South Staffordshire Coal- 

 field boulders from Wales become common, but the greatest number and 

 the best sections in the drift lie beyond the county border. The Rowley 

 Hills lie in the direct path of the Arenig glacier. Mr. Jerome Harrison 1 

 finds no foreign drift on their summit, but on the contrary a train of basalt 

 boulders has been traced from them for some distance to the south and 

 east. On the rock being bared in quarrying operations, clearly striated 

 rock surfaces, with the stria? pointing N.W. to S.E., have been laid bare, 

 and the general contour of the hills Mr. Harrison regards as that of a great 

 roche moutonnee. 



Carried along by the great moving mass of the Irish sea ice which 

 also probably helped to push the Arenig glacier up the south-western 

 flanks of the South Staffordshire Coalfield the glacier from Wales may 

 have impinged on the northern coalfield, as along its western margin 

 some boulders are met with which correspond very well with the 

 rhyolitic lavas of Arenig. 



Irish Sea Glacier. This was the dominant and all-powerful mass of 

 ice of which the presence can be traced over the greater part of the 

 county. Its great thickness and power was derived from the glaciers of 

 the south of Scotland, Ireland and the Lake district, which during glacial 

 times descended into the Irish Sea basin, and uniting there with the glaciers 

 resulting from the accumulated snowfall became ultimately piled up until 

 the ice overrode the summit of Snaefell (2,024 feet) in the Isle of Man. 

 Advancing southward it met with the resistance of the Welsh hills, and 

 consequently split into one lobe which passed down St. George's Channel, 



1 ' Glacial Geology of the Birmingham District,' op. cit. 

 27 



