486 Fisher — Glacial Origin of Denudation. 



In the first place, then, the smallness of the inclination of a 

 surface would not prevent motion, and consequent friction and denu- 

 dation. Secondly, there would be no selection made of the materials 

 carried away, while a certain quantity of till containing all the 

 ingredients of the surface derived from an area in rear, would be 

 interposed between the ice and the land. The remains of this I 

 suppose to constitute what I have ventured to call the trail. The 

 softer strata would be worn away most quicldy, and, in general, each 

 successive outcrop would be maintained at a uniform level depend- 

 ing upon its hardness.^ I should expect that, when a soft rock had 

 been denuded to a certain depth in proportion to the thickness of 

 the ice-sheet, a fissure would be formed, causing an ice-fall, which 

 would excavate a steep escarpment at the junction of a harder 

 with a softer stratum. We know that it is the tendency of an ice- 

 sheet to " descend towards the sea in successive steps, leading up to 

 as many icy-platforms, the ridges and valleys (of the land) being 

 levelled up to one u.niform plane, and concealed by these tabular 

 masses of ice." (See Sir C. Lyell's account of the continental ice of 

 Greenland : " Antiquity of Man," p. 235, ed. 1863.) The thicker 

 the ice-sheet the more uniform its action upon the surface would be, 

 and the less it would tend to form minor features. But as the 

 climate gradually became milder, and the ice-sheet thinner, features 

 would be formed on a smaller scale, and it would be these which we 

 should now see. 



As far as I have had opportunities of examining the furrows that 

 I have described, I have noticed that they are largest and most 

 numerous in the neighbourhood of valleys, and are either parallel 

 or inclined at a small angle to them.^ This is what we should 

 expect : for, according to the theory, the valleys represent the 

 directions in which the ice flowed off dui-ing the later periods of its 

 existence ; and since the sheet would partake of a similar motion for 

 some distance on both sides of the line of maximum flow, we should 

 find the surface scored, in a minor degTee, nearly in the same direction 

 as the valley. 



Finally, the sheet would be confined to the higher grounds 

 with glaciers descending towards the plains. 



To seek for the beds of extinct glaciers among Chalk downs might 

 be thought fruitless. Nevertheless, I have noticed combs in the 

 Wiltshire downs which have much that appearance. There is one 

 in particular above the village of Heddington, near Calne, up which 

 the footpath to Devizes runs. Its sides are steep walls of Lower 

 Chalk, and ia general form and inclination it most closely resembles 



1 An excellent illustration of the effect of the comparative hardness of material, in 

 determining the form of the ground, may be noticed in the neighbourhood of Cromer. 

 The landscape suddenly changes from one of a rather monotonous character to one of 

 rapidly varjdng hiU and dale, on account of the extremely irregular collocation of 

 masses of clay, chalk, and gravel, among the glacial deposits of that district. 



- The workmen at Ilford told me that the furro'svs all ran nearly in the same 

 direction, which was one inclined at an angle of about 45° to the Kiver-valley 

 adjoining. 



