82 MAN AND THE GLACIAL PERIOD. 



portant in its bearings upon the question of glacial erosion 

 to merit notice at this point.* The object of his investi- 

 gations was to determine how much of the so-called 

 ground moraine, or till, consisted of material disintegrated 

 by mechanical action, and how much by chemical action. 

 The " residuary clay," which has arisen from chemical de- 

 composition, would properly be attributed to the disinte- 

 grating agencies of preglacial times, while the clay, which 

 is strictly mechanical in its origin, remains to represent 

 the true " grist " or " rock flour " of the Glacial period. 



The results of Mr. Crosby's investigations show that 

 " not more than one-third of the detrihis composing the 

 till of the Boston Basin was in existence before the Ice 

 age, and that the remaining two-thirds must be attributed 

 to the mechanical action of the ice-sheet and its accom- 

 panying torrents of water. In other words, if we assume 

 the average thickness of the drift as thirty feet, the 

 amount of glacial erosion can scarcely fall below twenty 

 feet. After scraping away the residuary clays and half- 

 decomposed material, the ice-sheet has cut more than an 

 equal depth into the solid rocks." 



Mr. Crosby's investigations also convinced him that the 

 movement of the till, or ground moraine, underneath the ice 

 was not en masse, but that " it must have experienced dif- 

 ferential horizontal movements or flowing, in which, nor- 

 mally, every particle or fragment slipped or was squeezed 

 forward with reference to those immediately below it, the 

 velocity diminishing downward through the friction of 

 the underlying ledges. . . . The glaciation was not limited 

 to masses which were firmly caught between the ice and 

 the solid ledges, and it was in every case essentially a slip- 

 ping and not a rolling movement. . . . These differential 

 horizontal movements mean that the till acted as a lubri- 



* Proceedings of the Boston Society of Natural History, vol. 

 xxv (1890), pp. 115-140. 



