1872.] TIDDEMAN ICE-SHEET m NORTH LANCASHIEE ETC. 485 



thrust forward its " moraine profonde." This would accumulate be- 

 tween the ice and the rock in inverse proportion to the weight and 

 power of the ice. This accumulation of the Till would be an effect 

 constantly following the edge of the ice-sheet in its retreat. 



If this were so, we should get very much the kind of section shown 

 in the diagram. The edge of the ice would be working up red drift 

 on to the grey I'ocks south when it had no longer strength to enable 

 it to abrade the grey rocks themselves, and so in retiring succes- 

 sion. And this is just what I have found in this district in the dis- 

 tribution of colour through the Till. 



Of course the Till coloured by the red rocks is not entirely made 

 up of their waste. It contains nearly the same boulders as the grey 

 Till ; it is only the matrix which has been coloured by an intimate 

 mixture of red sand or clay. 



It is obvious that, although strictly speaking these variously 

 coloured beds have a certain sucession about them, it is a succession 

 so rapid and so brought about by the same causes under the same 

 conditions, that to refer them geologically to different ages would be 

 wholly wrong. Any geologist not acquainted with this changeable 

 character of the Till might think when he found drift of one colour 

 resting with a hard line upon drift of another colour, that he had 

 discovered an unconformability in the Boulder-clays, and classify 

 them accordingly. For this reason I cannot but think that such 

 terms as " the Blue Clay, the Grey Clay, the Yellow Clay, the Clay 

 wdth Chalk and without," when applied to glacial deposits of parti- 

 cular ages, are cumbersome and misleading. Such characteristics 

 can only hold good as a test of time over a very limited area ; and 

 even that is doubtful. It is quite possible, nay, it is certain, that 

 in some areas you may have Till of totally different appearance, 

 colour, and material deposited side by side, by the same agents, and 

 nnder the same conditions, at the same time. 



As regards the transport of material, it is an interesting fact that 

 over the greater part of this district there are none but local rocks 

 in the Boulder-clay. In the term local I include any rocks which 

 are derived from any part of the drainage-system in which they are 

 found as boulders. Thus you may find Silurian rocks in any part 

 of the Kibble valley, though they become more scarce in proportion 

 to the distance from the parent rock, which lies chiefly in the neigh- 

 bourhood of Ingieborough and along a line thence to Malham Tarn. 



The ground from which the ice travelled in this district was 

 almost entirely east of the Lake country ; consequently you get in 

 the Till none of those granites, porphyries, traps, and ashes w^hich 

 might, did they occur, be so easily traced to their origin. The country 

 from which it passed hither is chiefly a Carboniferous tract ; therefore 

 many of the boulders may have travelled a long way overland into the 

 Lune and Eibble valleys and yet appear to be derived from local rocks. 



But there is a part of the district which is an exception to this 

 rule. A line might be drawn parallel to the glacial curves along the 

 valley of the Lune and down along the eastern border of the 

 western seaside plain which would roughly represent the boundary of 

 the Lake-country drift. 



