444 GLACIAL GEAVELS OF MAINE. 



and fine gravel of a broad osar. They are exceptional, and the question of 

 their origin is discussed elsewhere. 



In general we find in the bi'oad osar terraces no unpolished stones or 

 bowlders that can be regarded as till dropped from the roof of an ice arch, 

 though near the borders of these plains the stones have received much less 

 attrition from water rolling than have the stones of the osars or central 

 parts of the broad osars. 



2. The great breadth of the terraces. 



If the broad osar channels were roofed with ice, the size of the ter- 

 races demands ice arches of great lengths of span, numbers of them up to 

 one-fourth of a mile, several one-half of a mile, and a few three-fourths of 

 a mile. These would be very long spans for bridges of high-grade iron 

 and steel. If the arches sagged and were supported on the gravels or on 

 abutments of ice, we ought to find the terrace uneven on its surface, with 

 kettleholes and reticulated ridges. The sizes of the subglacial channels 

 would be so restricted, too, that only coarse sediment would be deposited 

 all the way out from the central ridge to the margins. We now and then 

 find reticulations and hummocky ridges in the midst of osar terraces that 

 may have had such a history, but the broad osars proper are very level in 

 cross section and contain such fine sediments that they must have been 

 deposited in large channels where the flow of the water was moderate. 



The interpretation of these facts is further discussed below. 



FORMATION OF THE BROAD OSAR CHANNELS. 



Among the methods whereby the ordinary osar channel might become 

 broadened, we may mention the following, premising that these • channels 

 had somewhat parallel sides: 



1. Subglacial channels were enlarged laterally and subglacially to the 

 full breadth of the channel. 



This theory would require us to assume that the larger bowlders, at 

 least over most of the State, were contained in the basal ice. For the broad 

 osar is composed, as a rule, of rather fine material, and does not carry 

 bowlders such as ought to have fallen from the roofs of so broad tunnels if 

 the englacial bowlders were high in the ice. 



The difficulties of this hypothesis are very great if not insuperable. 



