472 GLACIAL GRAVELS OF MAINE. 



be left on the uplands, if there had been so great an erosion of the till after 

 it had become bare of ice that it furnished the material for such thick sheets 

 of finer sediment. The till contains material of all sizes from bowlders 

 down to rock flour. An)^ large erosion of the surface till, especially where 

 it would largely be localized in the smaller upland ravines and valleys, 

 ought to have left a mass of residual matter composed of the bowlders and 

 larger stones. This ought now to be either in the ravines of the hills where 

 it would be left when the finer matter was carried away, or to form alluvial 

 cones in the larger valleys near the mouths of the steeper hillside brooks. 

 In the mountains such cones are noticeable, but they at once show them- 

 selves to be composed of different material from most of the valley drift, 

 and they add very much to the difficulties of the hypothesis that the valley 

 drift is derived from fluviatile erosion products. Conclusion: Unless locally 

 in the mountains, there is no such body of residual coarse matter left on the 

 hillsides, or as alluvial cones at the mouths of brooks, as testifies to any 

 great erosion of the till since it was deposited by the ice, still less such a 

 vast quantity as would be required by the fluviatile hypothesis. Indeed, 

 the small sizes of the brook channels of the uplands is surprising. I 

 have known near the base of Pikes Peak a channel one-fourth of a mile 

 long eroded in a single storm to a larger size than many a large perennial 

 brook in Maine has been able to erode in all the time since the melting of 

 the ice. 



One class of fluviatile residual gravels here deserves further notice. 

 The larger streams and rivers have not infrequently excavated canyons in 

 the till or rock since the melting of the ice. This most often occurs in east- 

 and-west valleys, where the ice often left deep morainal sheets or ridges 

 across the valleys. Here rapids and waterfalls were formed. The rivers 

 excavated a channel in the till barrier and carried the coarser matter down a 

 short distance below the foot of the swift water, where it was left as terraces 

 of valley drift. The stones are usually subangular, and are easily traceable 

 in the midst of the original valley drift. Such a deposit at Kingman is 

 elsewhere described (see p. 98). Now if large rivers have left the residual 

 matter from channels formed in the till, much more ought the brooks to 

 show such proofs of any large erosion of the till. 



Another consideratiofi is this : Most of the east-and-west valleys contain 

 less valley drift than north-and-south valleys, and it is on the average of 



