THE BUILDING OF THE COLORADO ROCKIES 243 



flanks of the upfolded belts. The granite mountain belts are more 

 deeply rooted than the less uplifted strips adjacent. They con- 

 stitute the true Rocky Mountains of this latitude. 



According to the testimony of the Lyons sheet the folded shell 

 of the Front Range thins rapidly as it approaches the Great Plains. 

 This repeats in type, though less strikingly, the rise of the boundary 

 shearing plane on the western border of the Appalachian Mountains 

 near Tyrone, Pennsylvania.^ Neither the Lyons section nor the 

 Tyrone section developed an actual fracture, though they suggest 

 an approach to it. But on the western border of the Appalachians 

 farther south the boundary shearing plane has actually emerged 

 from the depths as a strong thrust fault which separates the 

 disturbed mountainous belt on the east from the region of hori- 

 zontal strata on the west. This is especially conspicuous in Ten- 

 nessee. In close analogy to this, in Montana and Alberta, the 

 Rockies are separated from the Great Plains by an overthrust 

 fault, or faults, of great displacement. These instances pointedly 

 suggest the generalization that bordering thrust faults, particu- 

 larly on the inland margin of a strongly deformed mountain belt 

 away from the assumed active oceanic segment, are a common, 

 and if so significant, phenomenon, especially as they accord with 

 the theory of mechanics. With greater intensity of thrusting and 

 the appropriate rock materials, the great upfold in the foothill 

 belt of the Colorado Rockies would presumably have passed into 

 a bordering thrust fault.^ Such a bordering thrust fault appears 

 to have developed on the east flank of the Wet Mountains, where 

 that range continues the Rocky Front en echelon, south of the 

 Arkansas River.^ The pre-Cambrian- granite has been thrust 

 eastward over the Cretaceous strata of the plains and lower foot- 

 hills, and also in one place over a conglomerate which has been 

 classed tentatively as Arapahoe. 



The westernmost four divisions of the plotted section, repre- 

 senting the region of but slightly disturbed strata between State 



^ Jour. Geol., XVIII (1910), 245, Fig. 6. 



^ Some local fracturing did develop at Boulder and at Golden. See Ziegler, 

 Jour. Geol., XXV (1917), 728-40. 



3 C. W. Washbume, "The Canon City Coal Field, Colorado," U.S. Geol. Surv., 

 Bull. 381 (1910), p. 342 and PL XVIII. 



