532 PRINCIPLES OF STRATIGRAPHY 



produced by mechanical crushing of fresh rocks. Such sand, from 

 the Mer-de-Glace, Switzerland, shows sharp, fresh, angular frag- 

 ments with the barest traces of weathering, and very little wear. 

 The composition is varied, and the assortment very poor. The 

 quartz grains of glacial sands show the characteristic conchoidal 

 fracture, sharp edges and keen points. Cleavable minerals show 

 fresh, clean surfaces and little, if any, trace of internal decomposi- 

 tion. In composition the variation is very great, dependent, of 

 course, upon the original character of the rock ground up by the 

 ice. The assortment according to size is also very marked. 



Till or boulder clay is an intimate mixture of rock flour and 

 glacial sand, together with a varying per cent, of clay derived 

 from rocks decomposed prior to glaciation. Crosby holds that very 

 little of the original residuary soil is incorporated in the till cover- 

 ing New England, for tlie color of this till is bluish and shows no 

 traces of oxidized material below a certain level, where recent 

 oxidation has been active. "Experiments show that an admixture 

 of a very small proportion of highly oxidized residuary clay, like 

 that of the south, with a typical till, is readily detected in the 

 change of color." (Croshy-y:2^2.) In the most typical till, i. e., 

 that due wholly to glacial erosion of a country from which all 

 decomposed material is absent, the amount of kaolinite is ex- 

 tremely slight, all the finer material being rock flour due to me- 

 chanical crushing. An analysis of till from drumlins of the Boston 

 Basin, from Somerville to Nantasket, gave Crosby the following 

 results expressed in percentages of the total material exclusive of 

 the boulders and other stones over 2 inches in diameter 

 (Crosby-6) : 



I . Coarse 1 7 . 08 



Gravel { 2. Medium 2 . 99 ^ 24 . 90 



3- Fine 4 -83 



4. Coarse 3 . 33 



Sand { 5. Medium 9-25^ I9-5I 



6. Fine 6 . 93 



f 7. Coarse 12.80] 



Rock Flour ! 8. Medium 6 . 52 I 43-86 



) 9. Fine 24.14 ( 



[ 10. Superfine o ■ 30 J 



1 1 . First decanting ... o . 86 1 



Clay { 12. Second 913 [ 11.67 



13. Third 1.78 J 



99 94 



