428 GLACIAL GRAYELS OF MAINE. 



remark that tlie plains of reticulated ridges are often found in very level 

 regions not faA'^orable to the production of crevasses, except perhaps those 

 of tension near the ice margin. So far the probabilities favor the theory 

 of superficial streams. On the whole, the reticulated ridges can not be 

 admitted as affording a crucial test. 



PROBABLE VELOCITIES OF THE TWO KINDS OP STREAM. 



In many places in the osars we find rounded bowlderets and bowlders 

 in the midst of much finer material. To account for these bowlders we 

 may postulate moderate currents for most of the time, with now and then 

 a sudden flood; or, more often, such bowlders probably fell from the ice 

 onto the gravel in the bed of a glacial stream and were rounded, not so 

 much by being themselves rolled forward as by the attrition of smaller 

 stones pushed past them. Such bowlders are very common in the reticu- 

 lated ridges of western Maine. In these cases we need not postulate more 

 rapid currents than would be necessary to move the finer matter. If we 

 make proper allowance for such adventitious bowlders, obviously the size 

 of the transported rocks and stones will measure the velocities of the 

 currents. 



If most or all of the morainal ddbris was contained in the lowest part 

 of the ice, as is generally believed, then the superficial streams that are 

 found near the nev(i line, or anywhere high upon the ice, would be glacial 

 torrents, but not osai--forming debris. Obviously, only those portions of 

 superglacial channels that are in ice containing d(ibris can be of significance 

 in osar formation. The theory that such streams could form osars Avhere 

 the ice was deep must stand or fall with the theory that the debris reached 

 high elevations within the ice. 



We need not, then, in estimating the velocities of the superficial 

 streams, consider the general surface gradient of the ice, but only that of 

 the marginal portion rising to the height of the englacial matter, perhaps a 

 few hundred feet above the ground. Here for a few miles, say 2 to 5 

 miles, we can grant to the superficial streams waterfalls, rapids, pools, and 

 all other accidents of open surface channels, and velocities both greater and 

 less than those due to the surface gradient of the ice. 



On the other hand, the velocities of subglacial streams are only in 

 part determined by the slopes of the laud. When the capacity of the 



