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R. D. SALISBURY 



fully charged with debris, becomes more rigid, while the more 

 mobile part above moves on over it. If in Fig. 2 the several curves 

 a, b, c, d represent the profiles of the ice in successive stages of 

 advance, the ice which is at the bottom, and which is eroding 

 formation 4, may not be the ice which was at the bottom over 2, 

 but instead ice which was well up from the bottom over this 

 formation. In this case it is clear that the ice working on 4 has 

 relatively little material derived from 2. At first thought it 

 might seem that it should have nothing, but this conclusion 

 does not follow. As the relatively debris-free ice above moves 



Fig. 2. 



over the relatively heavily debris-charged ice below, it drags 

 along some of the material lying in the upper part of the debris 

 zone. Thus as the ice moves over 4, the upper part, being in 

 faster motion, will be carrying along some of that derived from 

 2 and and even from I, so that at least a small amount of the 

 material from these formations is to be expected in the bottom 

 of the ice over 4. Nevertheless, the tendency is all along to 

 leave the material already carried by the ice in the rear, and to 

 let the advancing edge acquire its own load, primarily from the 

 successive formations invaded. The result of differential move- 

 ment, the faster movement being above, is to diminish still fur- 

 ther the proportion of the material in the drift at any given point 

 which was derived from distant sources. Because of the effects 

 of differential movement therefore, the last series of fractions 

 would need to be still further changed, so that the first member 

 would be smaller and the last larger. 



The topography over which the ice had moved would also 

 influence the proportions of material of near and distant origin. 



