250 HISTORICAL GEOLOGY 



may be truly said to have had their beginning at that time, al- 

 though their existing altitude and relief features are largely due 

 to later movements and erosion. Instead of folds,, great thrust 

 faults were sometimes developed, a fine example being near the 

 Canadian boundary, where Algonkian rocks were pushed about 

 7 miles over Cretaceous rocks. 1 



Another very important physical disturbance, accompanying 

 the "Rocky Mountain Revolution" in the northwestern United 

 States and southern British Columbia, was the inauguration of 

 vast igneous (chiefly volcanic) activity, which continued almost 

 unabated through the immediately succeeding Tertiary period. 



»PLOn 



Fig. 154 

 Structure section in the Rocky Mountains of western Montana showing moder- 

 ate folding of Cretaceous and older rocks. Argn, Archean; Cg, Cf, Cam- 

 brian; Dt, Dj, Devonian; Cq, Cn, Carboniferous; Kl, Kmc, Kd, Cretaceous. 

 (After Peale, U. S. Geological Survey, Folio 24.) 



Volcanic and fissure eruptions took place on a grand scale, and 

 lava-flows accumulated to a thickness of several thousand feet 

 over an area of about 200,000 square miles. This was the time 

 of greatest igneous activity in North America since pre-Cambrian 

 days. 



Close of the Period in the East. The Cretaceous Peneplain 

 and its Uplift, — Turning our attention to the eastern part of the 

 continent, we find that significant changes took place there also. 

 In fact the area of the eastern United States was subjected to the 

 greatest crustal disturbance since the "Appalachian Revolution" 

 toward the close of the Paleozoic. 



During all of the Mesozoic era most of the eastern portion of 

 the United States, except the Coastal Plains during part of the 

 time, was above sea water and undergoing erosion, so that, as a 

 result of this very long time of wear, the region was reduced to the 

 condition of a more or less perfect peneplain. It is known as the 



1 It is possible that this fault was developed a little later, that is in early 

 or middle Tertiary time. 



