THE LOCAL ORIGIN OF GLACIAL DRIFT 



Normal till is made up predominantly of materials which 

 "have not been transported many miles, though some of the 

 minor constituents have often come greater distances, Roughly 

 •speaking, the more distant the contributing rock formation, the 

 less its contribution to the till at any given point. There are appar- 

 ent exceptions to this generalization, but they are chiefly the 

 result either of the unequal exposures of the several formations 

 •contributing to the drift, or to their unequal resistance to abra- 

 sion. 



Locally, the constituents of the drift of distant origin, drop 

 almost to zero. This is. true both of drift composed chiefly of 

 clay, and of th»at which contains abundant coarse material. In 

 -extreme cases, stony till is chiefly composed of blocks of rocks 

 that have been moved but little from their original positions. 

 They have been displaced and the clay or sand (the finer 

 portions of the till) have been mixed with them, so that the two 

 sorts of material appear to have been kneaded together. In 

 :such cases the rock masses are angular and rough, and fre- 

 quently increase in size and number from the top of the till to 

 its base. At the base they may be so abundant as to nearly 

 •exclude all other constituents. If the surface of the rock be 

 much broken, and its uppermost layers disrupted and crumpled, 

 as is sometimes the case, it may be difficult to say where the 

 line between the bottom of the till and the surface of the under- 

 lying rock is to be drawn. Where the rock which gave origin 

 to the drift was not well suited to making bowlders, the com- 

 iminution of the material gave rise to clayey or earthy matter, as 

 really though less obviously local in origin, as if it had 

 (remained in larger pieces. Considering the area of the continen- 

 tal ice-sheets, and the distance which much of the ice traveled, 

 the small proportion of the drift which has come from distant 



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