1872.] TIDDEMAIT ICE-SHEET IN NORTH LANCASHIEE ETC. 483 



brook-, and road-section. The least-favourable position for their 

 preservation is that at a in the diagram. It is only natural that it 

 should be so ; for denudation is greater along escarpments than else- 

 where, and the effect of subaerial drag would be to destroy them. 

 A very good example occurs in Twiston Brook (just along the line 

 of the scratches on Twiston Moor), where the overturned beds may 

 be seen for a distance of 150 yards along the brook. It is covered 

 and preserved by the overlying Till ; and that shows some evidence 

 here, from its materials, of its being merely the waste product of the 

 rocks passed over, and of the direction in which it was being pushed. 

 Another occurs in shale above Moorside, on the north side of Pendle. 

 My friend and colleague, Mr. W. Gunn, has seen similar overturn- 

 ings of the siu'face-edges at different points along the chain between 

 Pendle and Skipton. Perhaps the most interesting example of all 

 is one occurring at Blackburn, a representation of which I have 

 embodied in the diagram at the point marked with an asterisk. My 

 friend Mr. James Eccles, E.G.S., called attention to it in a paper 

 read to the Manchester Geological Society ; but as it illustrates so 

 well the subject before us I cannot forbear to mention it. It is 

 seen in a road-cutting in a lane near the public park. Beds of 

 shale, sandy flags, and gritstone are dipping southerly at high angles. 

 In one part a bed of soft shale, resting on some harder rock, has 

 been pushed away from it at the surface; and in the angle between 

 the two lies some Till. It seems pretty clear that ice has pushed 

 away the softer bed to a gTcater extent than the harder ; conse- 

 quently a vacant space was left; and into that a portion of the 

 " moraine profonde " was forced. I know not any phenomena in 

 connexion with the Glacial period which give so vivid a picture of 

 a resistless force working in an undeviating course over hill and 

 dale across the ordinary drainage- channels of the country as this 

 wreck and ruin of all opposing obstacles. 



III. The Evidence of the Till. 



In speaking of the Till I have no . intention of entering upon 

 its well-known characteristics, its local composition, its boulders 

 scratched but not usually rounded, its absence of bedding, &c. 

 I would merely wish to point to those appearances which seem to 

 indicate the direction in which the ice-sheet which formed it tra- 

 velled ; and when speaking of the Till as the product of the ice- 

 sheet, I must remark that much of what is called Till must have 

 been remodelled by the glaciers which were the direct successors, 

 without interruption, of the ice-sheet ; and no distinct line can be 

 drawn in this district between the one and the other. 



It is a common practice to speak of the Till as being coloured by 

 the rocks beneath ; and to a certain extent this is true ; but when it 

 is stated that it is always of the same colour as the rock on which 

 it lies, such a remark must either be founded on insufficiency of 

 observation, or is due to the observations having been made in dis- 

 tricts where the differences in the rocks passed over are not suffi- 

 ciently marked to impress their distinctive colouring &c. on the Till. 



