GLACIAL DEPOSITS 341 



though in a finely divided condition, only the harder and 

 tougher rocks retaining their lithological identity, the more 

 friable having been ground to the condition of clay and sand.^ 

 To attempt to give the composition of the till would necessitate 

 its study and analysis in innumerable localities, — an endless and 

 profitless task. It will be sufficient to here describe a few repre- 

 sentative occurrences. In nearly all till the boulders, consisting 

 of the harder and more resistant of the materials, are in a more 

 or less rhomboidal form, with their surfaces scarred and with 

 other marks of the rough treatment to which they have been 

 subjected. They are in fact the tools with which the glacier has 

 done its work, and the scars are but the signs of wear. Inter- 

 mingled with these boulders is an ever-variable amount of finer 

 detritus, largely a result of mechanical abrasion. Professor W. 

 0. Crosby has studied in great detail the physical properties of 

 the till about Boston, and states^ that, excluding the larger 

 stones, it consists of 25% of coarse material which may be classed 

 as gravel; 20% of sand; 40 to 45% of extremely fine sand, or 

 rock flour, and less than 12% of clay. The gravel in these cases 

 consists mainly of pebbles of the harder and more massive rocks 

 of the region, such as granite, diorite, diabase, quartzite, and 

 sandstone. In passing from gravel to sand, there is noted an 

 increase in the proportional amount of quartz, in clear and angu- 

 lar or subangular forms, due mainly to the disintegration of the 

 granite, quartzite and sandstone pebbles. The rock flour also 

 consists essentially of quartz. The most striking feature brought 

 out is the very small proportion of clay material, which varies 

 from one-tenth to one-eighth of the total bulk. 



The table on the next page, as given by F. Leverett, shows the 

 approximate physical condition of the till as represented by the 

 sub-soil in various parts of Illinois. 



The till is not, however, always spread out evenly over the 

 land, but though partaking in a general way of the topography 

 of the slopes which it covered, lies much deeper in certain 



^Alden (Professional Papers, JSTo. 24, U. S. Geol. Survey, 1904) found 

 that of the material in the glacial drifts of southeastern "Wisconsin, from 

 three to thirty-two per cent, was foreign in the sense that the formation 

 whence it was derived did not occur within the area surveyed. Eighty- 

 seven per cent, of the drifts, as a whole he regarded as local, the remaining 

 thirteen per cent, having come from distances as remote as one hundred 

 miles. 



* Proc, Boston Soe. of Natural History, 1890, p. 123. 



