CANADIAN AND MONTANA ROCKIES 



305 



DIVISIONS OF CANADIAN AND MONTANA ROCKIES 



The two divisions of the Canadian and Montana Rockies generally 

 recognized are the mountain belt and the foothill belt. The latter is com- 

 monly referred to simply as the foothills. The two divisions are shown on 

 the map of Fig. 20.2. The western limit of the mountain belt of Laramide 

 age in Canada is recognized by some as the remarkably regular depression 

 called the Rocky Mountain trench. It extends from Liard River southeast 

 for 800 miles to Flathead Valley in Montana. This serves as a convenient 

 physiographic boundary of the Canadian Rockies, but as a structural 

 boundary it is not secure. The Rocky Mountain trench may be traced 

 southward to Kalispell in Montana, but from this point southward three 

 great valleys exist, any one of which might be chosen for the trench. 

 Clapp ( 1932 ) believes all the ranges of western Montana should be in- 

 cluded in the Rocky Mountain system because they are alike structurally 

 and stratigraphically. There, even more so than in Canada, the relation of 

 the Nevadan and Laramide orogenic belts and the position of their bound- 

 ary, if a common one, are little known and still speculative. 



MOUNTAIN BELT 



The mountain belt is made up of imposing front ranges such as the 

 Lewis Range (Glacier Park) and the Canadian Rockies of the Ranff and 

 Jasper areas, as well as many large ranges to the west. All ranges trend 

 approximately parallel with each other in a north-northwest direction, 

 except from the Idaho batholith southward, where later deformation has 

 imposed a topography discordantly in places across the Laramide struc- 

 tural trends. Compare Fig. 20.2 of this book with Raisz' Landforms Map 

 (1939). 



The mountain belt may be divided into two parts according to the 



Fig. 20.2. Tectonic map of the Canadian and Montana Rockies showing their major divisions, 

 the chief faults, the intrusions, and the lines of cross sections. Cross-ruled area is, with minor 

 exceptions, folded and faulted Beltian (Proterozoic) rocks. Vertically ruled orea is folded and 

 faulted Paleozoic and Mesozoic rocks. The horizontally dashed area is the folded and faulted 

 Mesozoic foothills belt. Horizontally ruled area is Beltian or underlain by Beltian but not part 

 of the folded and thrust belt. Structures associated with the intrusions not shown. The arrows 

 through the faults indicate the direction of movement of the overriding sheets. 



