206 DE. A. JOWETT ON THE [June 1914, 



— Along the south side of the Irwell Valley, from Rawtenstall to 

 Bacup, the limit of the North- Western Drift can be traced con- 

 tinuously ; and, precisely as on the north of the valley, after falling 

 at first, it again reaches higher altitudes farther east. 



Near Rawtenstall, the Irwell bends south-westwards and then 

 southwards, flowing through a deep narrow valley that opens out 

 upon the South Lancashire plain at Bury. In this valley the 

 North-Western Drift is very abundant, reaching a limit which, on 

 the whole, becomes gradually lower southwards until it makes a 

 sudden bend towards the east, passing along the southern slopes of 

 the Rossendale highland, where it attains a much higher altitude. 

 Lake-District rocks occur near the top of Knowl Hill (over 

 1350 feet), and reach a limit on the moorland farther north at 

 about 1325 feet above O.D. On the next southward projecting 

 spur farther east, two boulders of andesite were found at an 

 altitude of about 1385 feet. From this point the North- Western 

 Drift-limit turns north-eastwards and then northwards to rejoin 

 that in the Irwell Valley south of Bacup, gradually diminishing in 

 altitude northwards. A similarly northward decrease in altitude 

 of the limit of the North- Western Drift may be traced on the 

 high moorland north-east of the Whitworth valley. Reaching a 

 maximum of over 1400 feet north-east of Whitworth, it falls to 

 1240 feet on the watershed between the Whitworth and the Upper 

 Irwell valleys, and to 1160 feet east of Bacup. It is noteworthy 

 that boulders of igneous rocks (particularly Buttermere granophyre) 

 are very abundant and of large size in the Wliitworth valley 1 

 up to and just over the watershed to the north, and also occur high 

 up towards the Drift-boundary in the Upper Irwell Valley, whereas 

 they are smaller and less numerous in the Drift on the lower slopes 

 of the Upper Irwell Valley. On the other hand, there is a greater 

 admixture of boulders of the Ribblesdale type in the Drift of the 

 Irwell Valley and a paucity of such boulders in the upper part of 

 the Whitworth valley. Thus one can hardly escape the conclusion 

 that the North- Western Drift of the Whitworth valley and of the 

 higher parts of the Upper Irwell Valley differs markedly in its 

 boulder contents from the North- Western Drift of the Upper Irwell 

 Valley at lower levels, the latter being similar to that in the Irwell 

 Valley farther w:st. In fact, a band of mixed North- Western and 

 Ribblesdale Drift might be mapped in the valley of the Irwell and 

 its tributaries from near Accrington to Bacup and Bury, but for 

 the impossibility of drawing a definite boundary on the west and 

 south, in which directions the mixed type, so rich in Ribblesdale 

 Drift, passes insensibly into the true North- Western Drift. 



After crossing the watershed east of the Whitworth valley at 

 about 1400 feet above O.D., the North- Western Drift-limit falls 

 eastwards towards the southern entrance of the Walsden gorge, 

 terminating on the gentler slopes at the brink of the gorge, a 



1 S. S. Piatt, ' Some of the Recent Results of the Investigations into Local 

 Erratic Blocks ' Trans. Rochdale Lit. Sci. Soc. 1892. 



