464 GLACIAL GEAVELS OF MAINE. 



such as were formed at the sides of swift streams entering a body of rather 

 still water, and where the hollows between the ridges represent portions of 

 the channels in which the rivers flowed or unfilled parts of the surface 

 which was then covered by open water. Here the ridges were in greater 

 part caused by the filling up of channels formed between ice walls, and the 

 hollows and basins represent ice which separated the different channels or 

 lay beneath the sediments as they were dropped. The following discussion 

 assumes that these plains of steep-sided reticulated ridges, except as they 

 pass into marine or lake deltas, were formed between ice walls in most 

 cases. The gist of the problem lies in accounting for the formation of so 

 many new longitudinal and transverse channels. Some cause must be 

 adduced for the streams acting in this manner here at the reticulated kame 

 tracts, while elsewhere they got along with only a single channel. 



All the field phenomena, as we have seen, favor the hypothesis that 

 there were rapid streams and consequently great transportation from the 

 regions lying north of the great complexes of reticulated eskers. So also 

 all the causes of sedimentation combine to retard the streams and cause 

 deposition at the areas of reticulated ridges. Many of them were in the 

 region of backwater north of the hills. The slopes were less steep than 

 farther north. The subglacial drainage had been extended north over the 

 area in question, and many of the subglacial channels had come to be very 

 large. During each fall and winter the existing channels would become 

 more or less clogged with sediment brought down from the steeper slopes. 

 A time would come when the stream would no longer be able each summer 

 to sweep away the ddbris accumulated during the preceding cold season. 

 At the time of the spring floods the water, under great pressure from 

 behind, would collect in the tunnels. If it found transverse and longitudi- 

 nal crevasses reaching deep down in the ice, it would follow them laterally, 

 and thus in course of time a new subglacial channel would be formed par- 

 allel to the old. Where the new subglacial outlets proved insufficient to 

 carry off the waters, they would rise through crevasses and escape over 

 the surface. The situation of many of the larger plains of reticulated 

 kames is rather favorable to the formation of crevasses, and a large part of 

 these overflow channels were probably subglacial. But when the summer 

 floods came and found the old channels clogged the emergency was press- 



