202 de. a. jowett on the [June 19 14, 



North- Western Drift may be seen to j>ass into Local Drift, the only 

 indication of change being the disappearance of fragments of chert 

 and Silurian grit, or of igneous rocks, as the case may be. The 

 boundary between the Kibblesdale and the North- Western Drifts 

 may be similarly traced by the absence or presence of boulders of 

 gneous rocks. 



Sometimes, instead of the proportion of erratics diminishing 

 gradually towards the limit of their distribution, they are parti- 

 cularly abundant immediately below it, and then suddenly disappear 

 from the Drift. 



It is obvious that whatever vicissitudes the transporting agents 

 of the Drift may have undergone, the distribution of its three types, 

 as ascertained by the method outlined above, will represent only the 

 last phase of their action. Certain anomalies, however, in the 

 facts of distribution provide us with an occasional glimpse of 

 changes which occurred during the deposition of the Drift prior to 

 the last phase. In order to avoid repetition these will be dealt 

 with in a later section of this paper. 1 



We are not, however, without direct evidence from the Drift 

 itself of alternation in the conditions under which it was distributed. 

 Prof. P. F. Kendall has described 2 sections at Whalley, in which 

 layers of Bibblesdale and North- Western Drift interdisritate one 

 with the other without any mingling of the contents of the 

 different layers. Moreover, in several parts of the Irwell basin, 

 thick masses of Eibblesdale Drift, sometimes containing an 

 occasional boulder of the north-western type, are overlain by typical 

 North- Western Drift with an abundance of fragments of igneous 

 rock. Such instances of the overlap of different types appear, 

 however, to be confined to comparatively low ground, where the 

 Drift has accumulated to a considerable thickness. On the higher 

 ground, where the Drift is thinner, mixing could more easily take 

 place. 



(i) The distribution of the Kibblesdale Drift. — North 

 of Boulsworth Hill, the extreme margin of the Kibblesdale Drift 

 crosses the Pennines into Airedale (Yorkshire). 3 The highest 

 altitude to which it has been traced is about 1550 feet above O.D. 

 on the south-west of Boulsworth Hill. Owing to lack of good 

 exposures, the limit has not been exactly determined, although 

 it may be somewhat higher in this locality. The summit of 

 Boulsworth (1700 feet) is driftless. 4 . 



South of Boulsworth Hill, the Bibblesdale-Drift limit falls, but 

 is traceable up to 1425 feet on the north-east of the Widdop gap, 



1 § IV, pp. 209-12. 



2 See H. Carvill Lewis, ' Glacial Geology of Great Britain, &c.' 1894, 

 pp. 415-16. 



3 A. Jowett & H. B. Mauf e, ' Glaciation of the Bradford & Keighley District ' 

 Proc. Yorks. Geol. Soc. n. s. vol. xv (1904-1905) p. 212. 



4 W. Gunn, ' Geology of the Burnley Coalfield ' Mem. Geol. Surv. 1875, 

 p. 137, & footnote 2, p.*128. 



