642 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



The work of reducing the original chain of early-Eocene mountains to the 

 present more subdued relief is of the same order as that accomplished by the 

 erosion which was active through the entire Tertiary period in the equally 

 resistant terranes of the Appalachians and of other mountain-chains. The 

 opening of Waterton lake valley, the Rocky Mountain and Purcell trenches, the 

 Selkirk Valley, and the Okanagan valley forms a series of tasks comparable to 

 those of opening the Great Valley, or the Hudson, Connecticut, or Berkshire 

 valleys of the east. The many narrower valleys of the Cordillera are analogues 

 of the young to mature Tertiary valleys cut in the Cretaceotis peneplain of the 

 Appalachians. 



Some individual canyons of the Cordillera are due to rearrangements of 

 drainage through glacial action or through river-capture, or through other 

 exceptional causes; but there is little doubt that there has been a general uplift 

 of the Cordillera in this latitude during the late Tertiary. The relief has con- 

 sequently been increased — perhaps by as much as that claimed by Dawson, 2,000 

 feet, for the Belt of Interior Plateaus.* Such uplift is an important incident 

 complicating but not radically changing the erosion conditions which already 

 existed before the elevation. Before it took place, we may believe that the moun- 

 tains all the way from the Gulf of Georgia to the Great Plains, ranged in height 

 from 3,000 to 8,000 feet or more. This late Tertiary uplift invigorated the rivers ; 

 it did not begin a new erosion cycle at the close of a completed former cycle. 



The view that the entire post-Laramie history belongs to one complex erosion- 

 cycle explains the apparent predominance of consequent drainage in all the 

 ranges here constituting the Cordillera. It also tends to explain the absence 

 of well-defined adjusted drainage which is so noticeable in the trans-Cordilleran 

 belt. Of course, we should not expect even a second erosion cycle to produce 

 in this mountain-chain the extraordinary amount of subsequent drainage which 

 characterizes the very heterogeneous terranes of Virginia and Pennsylvania. The 

 local Cordilleran rocks are too nearly uniform in ' hardness ' for that. 



This outlined history has the advantage of not overloading the Tertiary 

 with what seem to be impossible feats of erosion. The vast denudation proved 

 in the soft rocks of the High Plateaus of the United States is a quite different 

 phenomenon from that postulated by the advocates of Tertiary peneplanation 

 in the exceedingly strong rocks of the ranges crossed by the Forty-ninth Parallel. 

 The Tertiary period was long; the question is, how long? The attentive study 

 of erosion will help in answering that question, but there must be a lithological 

 control over speculation and, above all, a careful comparison of records from 

 all parts of the world. The hypothesis of late Tertiary peneplanation at the 

 Forty-ninth Parallel section of the Cordillera cannot be reconciled with the 

 facts showing the speed of erosion in eastern America or in Europe, nor with the 

 physiographic histories which seem so firmly established in those large areas of 

 the earth's surface. 



* G. M. Dawson, Bull. Geol. Soc. America, Vol. 12, 1901, p. 90. 



