BEOAD OSARS OR OSAE TEEEACES. 443 



above the terraces. But the broad osar can easily be differentiated into 

 the following- tracts : (1) An axial belt of coarse composition and with the 

 material very much waterworn. (2) Bordering terraces, sometimes not so 

 high as the central ridge, composed of finer materials and often not so much 

 waterworn. 



These facts point to the following interpretation: that first there was 

 an ordinary narrow osar channel in which more or less coarse gravel 

 accumulated, and that as the channel subsequently broadened the original 

 osar was more or less washed away and incorporated with the growing 

 marginal terraces. These conclusions are strengthened by the fact that in 

 several cases an osar expands for several miles into a broad osar and sub- 

 sequently narrows again into the ordinary osar type of ridge. The broad 

 osar or osar terrace is thus seen to differ in no essential character from the 

 narrow osar except that it has advanced a stage farther in its development. 

 The history of this development was in part as follows: 



First there was an ordinary narrow osar river. Whether this had 

 begun to deposit gravels within its channel previous to the great enlarge- 

 ment of the channel is to be determined in each case separately, for these 

 rivers appear to have had different histories. We need not here inquire 

 whether the narrow rivers flowed in subglacial vaults or in superficial can- 

 yons open to the air. The flow of water increased, and so gradually that 

 the ice at the sides of the original channel could be melted and eroded 

 at a corresponding rate. I defer the question whether the enlargement 

 took place retreatally. While the channels remained comparatively narrow 

 only coarse matter could be dropped in them, and as the channel widened 

 the central osar was more or less washed away aiid spread laterally into the 

 sides of the broadening channel. The broader the channel the finer the 

 sediments that were deposited in it, unless there was a corresponding increase 

 in the supply of water. It is diie to a gradual broadening, accompanied by 

 rapid currents, that the central osar is not abruptly differentiated from the 

 bordering terraces of finer sediments. 



Were the broad osar channels roofed with ice? Two facts can be 

 named as especially bearing on this question. 



1. The rarity of till on or within the broad osars. 



In the northern part of Baldwin are a number of bowlders very little 

 if in any degree polished by water, yet situated upon and within the sand 



