TOPOGRAPHIC DEVELOPMENT 533 



and faulting which borders the mountain front. Taken together, they 

 represent Willis's Blackfoot peneplain, the development of which he re- 

 garded as of early Tertiary age and one of the primary antecedents of 

 the Lewis overthrust. 3 The question whether or not the peneplanation 

 represented by these remnants of high-level plains occurred prior or sub- 

 sequent to the time of the Lewis overthrust need not be considered in 

 this connection. A noteworthy and fortunate circumstance is the re- 

 markable preservation of these remnants in this part of the region. 



Reason of Difference in topographic Development 



The original extent of the Blackfoot plain to the northwest, north, and 

 northeast is not known, but it may well be supposed to have extended to 

 and over the site of the Porcupine Hills. Most of the region north of 

 the 49th parallel is tributary to Saskatchewan River and Hudson Bay 

 by way of Bow and Belly rivers and their tributaries. The distance to 

 tidewater by way of these streams is much less than by way of Missouri 

 River, whose tributaries drain the areas of the Browning and adjacent 

 quadrangles. Though the streams were working in the same kinds of 

 rocks, the area north of the boundary was thus the more rapidly denuded. 

 Moreover, owing to this ascendancy, or to some other contributory cause, 

 the waters from that part of the east slope of the mountains between the 

 boundary and North Fork of Cutbank Creek were diverted northward 

 via Saint Mary River to the Hudson Bay system, thus almost totally 

 depriving the branches of Milk River of receiving any of the mountain 

 waters. In consequence of this, while the adjacent region in southern 

 Alberta was denuded of hundreds of feet of rock by vigorous streams 

 heading in the mountains, totally obliterating the Blackfoot peneplain 

 and developing in its stead a lower hilly surface, the comparatively feeble 

 streams on the headwaters of Milk River cut only broad valleys 100 to 

 100 feet in depth, with tributary coulees dissecting, but not obliterating, 

 i be high-level plain. 



So, also, farther south, where the tributaries of Cutbank Creek and 

 Two Medicine River head in the mountain valleys and flow eastward 

 across the adjacent plain to "Missouri River by a more direct route than 

 that followed by Milk River, the extent of denudation of the plain is 

 more nearly comparable to thai in southern Alberta, and no uneroded 

 remnants of the peneplain are found until one reaches (he divide between 



8 Bailey Willis: Stratigraphy and structure <>r Lewis and Livingston ranges, Montana. 

 Bull. Geol. Soc. Am., vol. 18, pp. 810, 886, 840, 



