296 W. J. Miller — Ice Movement and Erosion. 



would have been too low to be 'compatible with much cutting 

 power. No doubt there was some movement of water along 

 the edge of the waning Black river ice lobe, but the only cur- 

 rent of any importance was a northerly one between the 

 eastern edge of the limestone terrace and the ice margin. 

 The limestones here are somewhat waterworn, but this 

 stream was about 200 feet below the top of the terrace and 

 thus clearly could not have done the work of erosion over 

 the whole terrace. Also the presence of the glacial strise high 

 up on the terrace shows that no great amount of water erosion 

 could have taken place there since the ice retreat. 



The Black river valley certainly existed, in its broader out- 

 line, in preglacial times. The Paleozoic-Precambrian bound- 

 ary line had for a long time been gradually moving westward 

 by wearing away of the Paleozoics. It seems certain that the 

 lowermost Paleozoic layers must have extended farther east- 

 ward, by overlap on the Precambrians, immediately preceding 

 the glacial period. This means that Black river was some dis- 

 tance farther eastward and that the western tributaries, from 

 Tug Hill, entered it with lower gradients. As above shown, 

 the lowest sedimentary layers could not have been cut. back in 

 pre- or post-glacial times nor were they cut back by glacial waters. 

 Evidently they were cut back by the ice to develop the steep 

 slope now shown. This allowed Black river to shift westward to 

 its present position. Thus the slight trench of the Precambrians 

 here shown could not have been preglacial. As already stated, 

 it is clearly not post-glacial and apparently it was formed by ice 

 cutting. The concave character of this inner portion of the 

 valley is well shown in the figure and strongly suggests ice 

 work. 



Also we should consider the fact that we are here dealing 

 with unaltered sedimentaries with slightly upturned edges 

 resting upon a rather smooth surface of igneous and metamor- 

 phic rocks, and that the lowest sediments are w T eak sandstones 

 and sandy limestones, which greatly favored the stripping off 

 power of the ice. Robert Bell* has noted the same thing in 

 Canada, and he says : " When unaltered strata lie at low angles 

 upon a nucleus of crystalline rocks, there is a marked differ- 

 ence in the effects produced by the action of the passing ice- 

 sheet according as the latter moved from the overlapping 

 strata onto the solid nucleus or off the latter against the 

 upturned edges of the stratified rocks, . , . in the latter (case), 

 great erosion has always taken place and valleys and basins are 

 formed whose width depends upon the angle of dip and the 

 softness of the strata which have been scooped out. The strata 



*Bull. Geol. Soc. Amer., vol. i, p. 296, 1890. 



