ORIGm OF LARGER COMPLEXES. 465 



ing'. The floods must find instant escape in some way, and the natural 

 result would be a complicated system of surface channels. These supposed 

 surface channels probably served for the escape of the waters only for a 

 short time each j^ear — during the time of highest floods — yet they would 

 contain some sediment. TJuis probabl}^ streams both above and beneath 

 the ice contributed to the formation of such ridges as were formed in chan- 

 nels between walls of ice. 



While it is ti'ue that the situation of the large complexes of western 

 Maine is in general favorable to a free flow of ice and the production of 

 crevasses, yet the reticulated ridges often do not expand to fill a whole val- 

 ley, as they would if the ice were so shattered that the subglacial waters 

 could freely pass along crevasses in any direction. They cross valleys 

 and go over cols in a way impossible unless the ice at the sides of the 

 system formed solid barriers. 



These considerations bear on the question whether the ovei-flow chan- 

 nels Avere all subglacial Admitting that the huge central ridges were 

 deposited in subglacial tunnels, the question recurs whether so many addi- 

 tional channels could be formed subglacially. The answer depends on the 

 number and arrangement of the crevasses. If the ice was solid at the sides 

 of a clogged subglacial channel, I see no physical process whereby the 

 stream could form new subglacial outlets. The facts showing considerable 

 solidity of the ice at the sides of the glacial rivers are many. These facts 

 favor the hypothesis that a portion of the overflow channels were super- 

 ficial. Probably the water would rise through crevasses onto the surface 

 only after the ice had become rather thin. It has been noted before that 

 the transverse ridges are sometimes composed of finer matter than the main 

 longitudinal ridges. This is consistent with the hypothesis that the former 

 were deposited in superficial channels, but the question can only be settled 

 by examining their stratification. 



During tlie time of formation of the kames the ice must in many 

 places have been in motion. Many of the plains have no transverse hills 

 in front of them and the ice motion could continue up to the last. If at 

 this time the ice had power to push forward subglacial sediments, the trans- 

 verse ridges which had to bear from their sides the force of the sea, ought 

 to be of great breadth and of gentle slopes, like the lenticular ridge of till. 



MON XXXIV 30 



