336 GLACIAL GliAVELS OF MAINE. 



that may De termed indigenous — because, in order to become a part of 

 the glacial gravel, it only needed to be laid bare by the melting and 

 erosion of the ice around it — there is mixed much matter derived from the 

 subglacial till or the adjacent regions. The iridigenous matter, as it was 

 released from the ice, necessarily fell into the channel and became mixed 

 with the foreign drift. The details of osar transportation are very complex, 

 and the intei'pretation in case of individual bowlders is doubtful, in view of 

 the many alternative fates that might happen to any particular stone or 

 bowlder. When a polished bowlder overlies finer sediment, we know the 

 order of deposition, biit we do not know whether the bowlder fell from the 

 roof of an ice vault, or slid down from the overhanging walls of a canyon, 

 or was transported to the place by moving water, or by floating ice, or by 

 an ice gorge. Unfortunately the presence of large water-rolled bowlders 

 does not give us a conclusive answer to the question whether they were 

 transported by subglacial or superficial streams. Yet where they overlie 

 finer sediments, the larger the number of such bowlders the greater is the 

 probability that they wer^ dropped from the roof of a subglacial vault. 

 This is a sufficient dynamical cause for large bowlders being lifted above 

 finer sediments, and it is a constant and inevitable feature of the marginal 

 part of an ice-sheet. Certainly a less number of bowlders will be con- 

 tained in the ice of the overhanging walls of an open channel than in the 

 whole roof of an arch. Besides, the other modes of transportation named 

 are agencies that would naturally be occasional rather than constant methods 

 of the glacial river. 



Summary. — In casc of a supci^ficial river, the melting and erosion of the 

 ice in the channel would proceed from above downward to the ground, and 

 then laterally outward. The disposition of the larger bowlders of the till 

 indicates that, on the average, the large bowlders were as high in the ice 

 as the finer materials were, or probably higher than they were. If so, 

 these large bowlders would first be laid bare in the bottom of the deepen- 

 ing superficial channel. Subsequently deposited sediments would be laid 

 on top of them or at their sides. The only large bowlders dropped from 

 the ice into the osar channel would be from the overhanging lateral walls. 

 We thus see that in case of a superficial canyon most of the bowlders of 

 the upper till contained within the ice melted or eroded to form the chan- 

 nel ought to be beneath the gravel. The argument is complicated by the 



