448 GLACIAL GKAVELS OF MAINE. 



considerable enlargement as taking- ^olace at the base of the crevasse where a 

 superficial stream pours beneath the ice ; but I do not see how we can admit 

 local supplies of ordinary superficial waters in such quantities as would 

 account for the disappearance of the roofs. The overflow theory postulates 

 known processes, and seems to be sufficient for the work accomplished. 

 Local stoppages of the timnels here and there would cause the local disap- 

 pearance of the roofs, with the consequent broadening of the channels. 



When we come to apply the hypothesis to the enlargement of the 

 narrow marine delta channels and those of the border-clay channels that 

 were beneath the sea, we find special difficulties. The ebb and flow of the 

 tide and the temperature of the sea would introduce new elements into the 

 analysis, but their quantitative significance is uncertain. 



Applying these principles to both the up and the down slopes of the 

 land as we go south along the courses of the osar rivers, I have failed to 

 find any constant relation between the land slopes and the enlargements of 

 broad osar channels, at least such as would warrant the prediction of their 

 occurrence at particular places or slopes. If there is such a rule it is that 

 in most cases a broad osar extends for some distance north of the tops of 

 hills crossed by the osars. On the steeper down slopes there may have 

 been the same broad channels, but quite often no gravel was left for 1 to 3 

 miles south of the hilltops, and we have only inferential evidence of the 

 breadth of the channel. Also the alternation of broad-chamrel deposits 

 having a horizontal surface in cross section with the area of reticulated 

 ridges will require more detailed study before correlation of these deposits 

 with topographical features can be asserted. Indeed, they may often have 

 had no connection with the land surface and have depended on ice condi- 

 tions alone. 



RETICULATED ESKERS OR ICAMES. 



In external appearance these uneven and hummocky complexes, which 

 show an endless variety of ridge and hollow, are perhaps the most remark- 

 able of all the deposits left by the glacial rivers. They afford all grada- 

 tions of complexity from the simple branching of a ridge into two ridges 

 which soon come together again, up to the great plexus 3 or 4 miles broad, 

 its surface covered with a jumble of heaps, mounds, cones, and ridges, 

 inclosing all forms of hollows, fumiels, hopperholes, kettleholes, basins, and 



