BOWLDERS OF THE GLACIAL GRAVELS. 335 



quite certainly slid down into the channel of the glacial river from the ice 

 overhead or the walls at the sides. Such are the bowlders overlying the 

 osar at the south end of the Grrand Lake of the St. Croix. (See p. 75.) 

 In the wilderness a few miles southeast of Aurora the gravel of the great 

 Katahdin osar is found in an interesting relation to a train of granite 

 bowlders. The place is situated in the valley of Leighton Brook, a tribu- 

 tary of the Middle Branch of the Union River. The course of the train 

 is nearly north and south, and parallel with the ice flow. The train con- 

 sists of bowlders piled one above another so as to make a moraine-like 

 ridge 10 to 30 feet high, and some of the bowlders are 10 to 20 feet in 

 diameter. The osar here forms a broad ridge of sand, gravel, and cobbles 

 transverse to the bowlder train. The train comes up to the edge of the 

 osar, and several of its bowlders overlie the gravel. Near the same place 

 the osar crosses another similar ridge of till, and its flanks are overlain by 

 the bowlders. 



An important and difficult question arises concerning the proper inter- 

 pretation of the facts as to the presence of large water-rolled bowlders as 

 an integral part of the kame or osar gravel. Several facts should be noted. 



1. In many places, especially in western Maine, the large bowlders are 

 more abundant in the kames and osars than in the same amount of the 

 average till of the region. (See PI. XXVII, A.) This is due to the finer 

 part of the till having been washed away, leaving the coarser residue. 



2. Almost universally the largest bowlders of the till are most abun- 

 dant at the surface. In the glacial gravels the larger bowlders are as 

 often, perhaps more often, contained in the lower part of the gravel. The 

 two arrangements so alternate in the glacial gravels as to make the inter- 

 pretation doubtful. Most of the large bowlders in the glacial gravel are 

 found in the granite areas, sometimes underlying and sometimes overlying 

 finer sediments. 



In this connection we must also consider what has become of the 

 bowlders that were contained in the ice that was itielted and eroded during 

 the formation and maintenance of the channel in which the glacial sediment 

 was deposited. This ice must have contained its average proportion of 

 bowlders, yet over large areas only fine matter appears on the surface of 

 the osars and osar-plains. As elsewhere noted, we have reason to believe 

 that the osars are, on the average, areas of accumulation. With material 



