534 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



might be frozen into a solid mass, and the snows gathering on this, it might 

 with Uttle change become the base of the glacier and be moved on bodily- 

 Much of tlie same material Avas puslied along beneath the ice, its parts 

 crashed and ground against each other, whereby all the softer rocks were 

 soon reduced to an impalpable mud, the largei- and the harder ones 

 enduring longer, but all at last y^ielding to the same fate, unless, worked 

 up into the ice itself or gathered in thick accumulations beneath it, they 

 were shielded from the more violent action of its mass. The ice was, 

 however, constantly providing itself with new material, and soon wore the 

 fragments into the peculiar shapes so characteristic of glacial accumula- 

 tions, three- and four-sided forms, with irregular ends more or less elongate 

 as the rock was more or less schistose, the sides flat or broadly convex, 

 joined by rounded edges and scratched in various directions. 



These peculiar forms, called by the Germans " dreikantner," are as 

 characteristic of the till as graptolites of the Silmnan. Thus the ice 

 elaborated in immense quantity a peculiar subglacial material of varying 

 but always characteristic composition, and spread it with unequal and 

 sometimes with very considerable thickness upon the rocky surface. For 

 the ice did not everywhere and always rest with its rasping surface upon 

 the rock and grind into it without interanission. Over a given surface it 

 might wear for a long time continuousl)', bixt by this means a new surface 

 would be gradually produced, partly by^ the unequal force of the ice, partly 

 from the varying hardness of the rock, and this would react upon the 

 ice, producing slight variations in its subordinate cuiTents, transferring its 

 intenser action to another area and allowing it to deposit material over 

 the first area. At a later time the maximum of eroding power might be 

 tr;insferred back to its former position and the accumulation so laboriously 

 brought together would be again swept away. In this way one may 

 explain some of the cases where the rock surface- shows strife in two 

 directions, for the local movement of the ice might be somewliat diffex'eut 

 at widely separated times. 



Very commonly the ice heaped up its accumulations in the rear of some 

 obstruction in a long- ridge projecting from the obstructing rock in the direc- 

 tion in which the ice was moving, as the water arranges sands. At other 

 places, especially in broad open portions of a valley, the ice molded its fine 

 clayey moraine material into massive hills, called drumlins, rounded and 



