GLACIAL DEPOSITS 533 



Thus, after exclusion of the larger stones the till consists of 

 about 25% of gravel, 20% of sand, 40 to 45% of extremely fine 

 sand or rock flour and only about 12% of clay. (See also 

 Ries-19.) 



The intimate admixture of coarse and fine material in the un- 

 modified glacial drift or till renders its pore space of the slightest 

 and so makes it an admirable impervious layer, through which 

 ground water cannot penetrate. This is well shown in the unoxi- 

 dized character of much of the till, its color when freshly ex- 

 cavated being of a bluish slate. Only the surface layers, or some 

 included layers of modified sand or gravel, may show such oxida- 

 tion. Modified till dift'ers mainly in the somewhat worn character 

 of the granules, the progress of weathering in the decomposable 

 minerals and the better assortment of the material according to 

 size. With this comes increased pore space and consequently pene- 

 tration of ground water accompanied by oxidation, and a change to 

 ochery or brown colors. All gradations may be observed, from 

 the slightly modified drift barely moved by the waters from the 

 melting ice sheet, to the much worn and fairly well-assorted ma- 

 terial of the glacial streams, which quickly assumes all the charac- 

 teristics of water-laid deposits and must be classed with the hydro- 

 elastics. 



The pebbles and boulders of the glacial drift also show dis- 

 tinctive peculiarities. One essential i^ their imperfectly worn char- 

 acter. Most commonly they are in the form of flattened masses 

 highly polished and striated on the flat faces, but angular or but 

 slightly worn on the other sides. Rounded boulders are the ex- 

 ception. In size they vary enormously, and this is often true for 

 the lithologic character as well. Crosby has found that much of 

 the coarse material of some New England deposits was of compara- 

 tively local origin, these deposits, moreover, having been subject 

 to a certain amount of transportation by water. Of some tons of 

 material coarse enough for ready identification about 50 per cent, 

 came from a belt of rock extending for 13 miles north of the 

 deposit, and 40 per cent, from the next 8 to 10 miles, while only 

 10 per cent, came from distances greater than 23 miles (7:2?/-.?). 



Transportation of the coarser material by glacial streams results 

 in more or less perfect rounding of the boulders, and the efifacement 

 of the striations on the surface. Moreover, the attendant assorting 

 of the material according to size will result in the greater concen- 

 tration of such bouldery deposits, whereas in the true till the 

 boulders are scattered and embedded in the finer material. 



Boulders of ice as well as ice sand derived from the gliding ice 



