Vol. 70.] GLACIAL GEOLOGY OF EAST LANCASHIRE. 213 



Origin of the Local Drift. 



Owing to its limited extent, and to the fact that its boulders 

 consist of rocks similar to those upon which it rests, the Local 

 Drift furnishes far less evidence of the direction of ice-movement 

 than the other two types. That it is due to the action of the 

 same ice as that which deposited the foreign Drift is evident 

 from the following considerations : — 



(1) At the junction between the two, the foreign Drift passes insensibly into 

 Local Drift similar to it in every way, but for the absence of far-travelled 

 stones. Thus the Local Drift forms a belt intervening between the foreign 

 Drift and the unglaciated area. 



(2) Wherever any evidence is obtainable from its boulders, as. for example, 

 where boulders of Millstone Grit are found in Drift resting upon Coal-Measure 

 shale, the direction in which they have travelled is always in agreement with 

 the direction of ice-movement deduced from evidence obtained within the area 

 covered by the foreign Drift. 



(3) There is a similar agreement where the direction of ice-movement is 

 indicated by the disturbance of the underlying solid rock, of which good 

 examples occur within the area covered by Local Drift. 



These phenomena are attributable to the action of the clean ice l 

 which no doubt existed at the surface of the ice-sheet (see section, 

 p. 214). This upper layer of clean ice would become charged, where 

 in contact with the ground, with local rock-debris ; and. as the ice- 

 sheet gradually spread farther, more debris would be picked up by 

 a new portion of clean ice, the previous local debris being mixed 

 with erratics by the further extension of the lower layers of ice. 

 This would continue until the ice-sheet attained its maximum 

 development, the Local Drift now remaining having been produced, 

 at that stage only, by the clean ice which extended beyond the 

 limits of the underlying ice containing foreign Drift. Moreover, 

 the Local Drift would, on this assumption, be best developed near 

 the margin of the foreign Drift, for here the ice-bearing Local Drift 

 would be thickest, and therefore the boulders within it would be 

 subjected to more severe glaciation for a longer time than those 

 nearer the edge of the ice-sheet. In no case would the Local Drift 

 be transported far ; the materials of which it is composed would 

 be originally much weathered, as the thin ice would not have much 

 erosive action on the rocks below, 2 and consequently the boulders 

 are not likelv to be striated. All this is in agreement with the 

 facts actually observed. 



1 P. F. Kendall, in G. F. Wright's ' Man & the Glacial Period ' 1892. p. 166 j 

 and G. W. Lamplugh, ' Geology of the Isle of Man ' Mem. Geol. Surv. 1903, 

 pp. 393 & 394. 



- An interesting example illustrating this point may be cited from the 

 eastern slopes of Black Hameldon. Undisturbed subsoil and more finely- 

 divided material, apparently produced by ordinary weathering from the grit 

 and shale upon which they rest, are covered by from 6 inches to a foot of 

 Drift with rounded stones (in one case with chert also). Here not only was 

 the ice thin and near its extreme limit, but the soil and subsoil over which 

 it passed were probably frozen hard and so preserved intact. 



