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that it was nearly always in almost as close contact with the material 

 at the bottom of the furrow. The quantity of drift left in this way 

 was proportionate — \st, to the amount of material in the ice as 

 compared with the thickness of the ice itself; and, zndly, to the 

 distance of the point in question from the head of the valley. It 

 must be obvious that where the ice was thin, as it was on the 

 higher parts of the sides of valleys, there was less detritus in the ice 

 than there was over the middle of the valley. Where the ice was 

 2000 feet thick, there must have been more sediment left than 

 where the ice was thin ; just as we should get more sediment left 

 at the bottom of a vessel containing Thames water to a depth of 

 two feet than we should in another vessel where the depth of the 

 same dirty water was only an inch. 



Water-worn Detritus iti Drifts. — Then in regard to the second 

 point, it must be obvious that there must be less water flowing 

 beneath the ice in the higher part of each hydrographical basin 

 than there was nearer the sea, just as is the case with the volumes 

 of the rivers themselves. Consequently, much more of the finer 

 materials tended to be washed away to sea in the lower part of 

 each basin than was the case nearer the valley head. So we should 

 expect to find an increasingly large proportion of washed and 

 water-worn materials in the old subglacial sediments as we follow 

 them outward from the watersheds towards the sea; or, to put the 

 statement in another form, we should naturally expect to find an 

 increasing proportion of Till to water-worn material as we approach 

 the heads of the valleys. More than this. The ice melted at the 

 upper surface faster than it did at its lower, and detritus was liber- 

 ated there in proportion to the rate of melting. This is a very 

 important factor in the formation of the various kinds of drift. 

 Much of the material liberated at the surface of the ice would be 

 more or less rolled and washed by the action of the glacier streams, 

 the finer materials — the sand and mud — being washed down the 

 crevasses and moulins to be mixed with or interstratified with the 

 stuff accumulating beneath the ice; while much of the coarser 

 detritus would, by the action of the same streams, be rolled into 



