SOME GLACIAL WASH-PLAINS. Ill 



lobation, l>y which the equalizatiou of pressures in the ice 

 uloiiu: the front miiintains a convex outward curve. 



From all these considerations it seems to me possible 

 to conclude that the ice-sheet retired from southern New 

 England at least as far north as the Cape Ann boulder 

 moraine while the main mass was still live ice. 



DECOMPOSITION IN WASH-PLAINS. 



The retreat of the ice from this field was so recent that 

 the general form of the deposits and most of their details 

 remain unaltered. Owing to the openwork structure of 

 the wash-plains, and to the fact that the clays made at the 

 same time were carried ofl' into deeper water, the sands 

 allow the rain water which falls upon the plains to sink 

 through instead of running over the surface and cutting 

 trenches. While the deposits are thus by their structure 

 protected from erosion, they are subjected to chemical 

 alterations b}' the action of the water which passes down- 

 ward through the soil. In this region, where the plains 

 are largely built of particles of feldspathic rocks, most 

 pebbles contain solvable nunerals which sooner or later go 

 to pieces. 



Croll' has pointed out the fate of glacial deposits strewn 

 over the land surface and so left for an indefinitely long 

 period without preservation by burial beneath overlying 

 strata. Glacial drift so left must gradually waste away, 

 going to the sea mainly in solution, while quartz vein 

 pebbles and the quartz of the granitic rocks alone will 

 remain to make pebbly beds, in which there may remain 

 no distinguishable feature of glacial origin. The begin- 

 i\ing of this change is already far advanced in the glacial 

 deposits even in the latest in the latitude of Boston. 



' Climate and Time, chap. xvii. 



