348 B. WILLIS — LEWIS AND LIVINGSTON RANGES, MONTANA 



In strong contrast, the Great Plains exhibit features of erosion entirely 

 independent of structure. Galton range, though as a mass bounded 

 by structural limits, is within itself apparently a simple uplifted block. 

 Whatever minor flexures or faults may exist, near the 49th parallel 

 they are not sufficiently pronounced to interrupt the. unity of the moun- 

 tain mass. While the general altitude of 7,500 feet is due to uplift, 

 details of heights express effects of earlier or later erosion only. In this 

 respect Galton range is like the Plains and unlike the Front ranges. 



Over the Plains and over Galton range a peneplain was developed. 

 On the soft rock of the Plains it was planed flat. On the harder rocks of 

 the Galton mass it was probably not so completely smoothed. Observa- 

 tions of 1901 were neither so extensive nor so precise as to distinguish 

 monadnocks from features of later carving, but the general relation of 

 height to an old lowland is as distinct as it is on the Schooley plain, in 

 the Highlands of the Hudson, New York. The peneplain on the Great 

 plains, the Blackfoot plain, is neither incidental nor local. It is the re- 

 sult of a long cycle of erosion, which affected a wide territory, and its 

 representative must occur in the nearby mountains among the oldest 

 features, if not as the oldest, unless it has been obliterated by later 

 activities. A tentative correlation of the Blackfoot plain with the pene- 

 plain over Galton range is a reasonable inference from these facts. 

 Nevertheless, in the intervening Front ranges the observer seeks in vain 

 for that general uniformity of altitudes or that breadth of contour which 

 might represent the Blackfoot plain. 



Recognition of peneplain in the Front ranges. — The peculiarly bold sculp- 

 ture of the Front ranges is explicable, offhand, as an effect of great ele- 

 vation, from which there resulted special conditions of glaciation and 

 erosion. It resembles the sculpture of the Cascade range, Washington, 

 as nearly as is consistent with diversity of rock types. But unlike the 

 Cascades, whose summits inherit common altitudes from a broad pene- 

 plain, the Front ranges exhibit no general upper limit of heights com- 

 mon to many widely distributed peaks. Instead, they present an ex- 

 treme case of localized deformation, accentuated by intense corrasion. 

 Realizing this, one may still recognize the position of the oldest topo- 

 graphic surface of the province near the summits of the ranges. It is 

 notable that each peak approaches in height those of its neighbors which 

 stand in similar structural positions — that is, along the strike. A surface 

 restored over the peaks, or over their wider shoulders, should represent 

 that from which they are carved, plus or minus the effects of warping 

 and minus the effects of later erosion. Detailed observations of structure 

 will determine the former; studies of stratigraphy in relation to sculp- 



