Vol. 70.] GLACIAL GEOLOGY OF EAST LANCASHIRE. 225' 



diversified than that of the pre-Grlacial surface, buried valleys 

 being met with in boring and in excavations, some of which extend 

 considerably below sea-level. 



Immense quantities of the rock-debris resulting from the 

 excavation of overflow-channels, mixed with rearranged Drift, 

 were deposited along the southern margin of the Kossendale 

 highland, and the effects produced by strong currents in the 

 shallow lakes which latterly occupied the South Lancashire plain 

 make it easy to understand how these superficial deposits were 

 long regarded as of marine origin. 



Although its consideration opens up another chapter in the 

 Glacial history of the Western Pen nines, it may be stated here that,, 

 on the abandonment of the Walsden channel, the drainage of the 

 Irwell-Mersey basin for some time escaped into the Trent basin 

 and possibly into that of the Severn, until the ice-barrier between 

 North Wales and the Kossendale highland was removed. 



VI. Conclusions. 



Three types of Drift have been recognized in East Lancashire,, 

 namely : — (1) North- Western Drift, which contains, among other 

 materials, igneous rocks from the Lake District and the South of 

 Scotland ; (2) Kibblesdale Drift, containing Carboniferous Lime- 

 stone and chert and Silurian grit ; and (3) Local Drift, consisting 

 of materials that can be found in situ in the neighbourhood. 



The distribution of these types of Drift, together with the evi- 

 dence of striated and of disturbed rock-surfaces beneath the 

 Drift, suggests that East and South Lancashire were invaded by 

 the composite ice-sheet from North Lancashire and North- West 

 Yorkshire postulated by Mr. Tiddeman. This ice-sheet reached up 

 to the Pennine watershed, and lobes of ice crossed the watershed by 

 way of the gaps at Widdop, Grorple, Cliviger (between Burnle} T and 

 Todmorden), and Walsden (between Rochdale and Todmorden). 



While the main ice-movement was from north- north-west to 

 south-south-east, on the north and north-east of Manchester there 

 was a curious movement of the ice partaking of the nature of an 

 eddy, whereby the ice-flow was directed first eastwards, then north- 

 eastwards, and ultimately northwards, the main current of ice thus 

 bending completely back upon itself in the neighbourhood of 

 Rochdale. 



The Local Drift is believed to have been produced by the over- 

 lapping of 200 feet or so of clear ice, forming the upper portion of 

 the ice-sheet, beyond the limits reached by the ice containing erratics. 

 No evidence has been found in favour of any local glaciation in this 

 area. A conservative estimate of the gradient of the surface of 

 the ice, based upon the mapping of the limit of the North-Western 

 Drift, gives a rise of about 4 feet per mile from the Pennines 



