THE MIOCENE PERIOD. 275 



In addition to the more distinctly deformative movements, body 

 movements and block movements resulting in the increased altitude 

 of the land throughout much of the western half of the continent 

 were in progress at this time. It appears to have been at about this 

 time that the plateau region of Arizona and southern Utah, a region 

 which prolonged erosion had reduced to a peneplain, was uplifted so 

 as to permit the beginning of the excavation of the Grand Canyon of 

 the Colorado. 1 Other regions were depressed relative to their sur- 

 roundings, and the differentiation of levels was often by faulting along 

 planes of earlier displacement. It appears that the later part of the 

 Miocene was the time when the greater relief features of the rugged 

 west, as they now exist, were initiated. The great relief features 

 of earlier times, for such there had been, appear to have lost their 

 greatness before the end of the Miocene. 



After the movements of the late Miocene had been accomplished, 

 it is probable that the western part of the continent had a topography 

 comparable, in its relief, to that of the present, though by no means 

 in correspondence with it. The details, and even many of the larger 

 features, of the present topography are of still later origin. Subsequent 

 changes have been the result of (1) deformation, largely without 

 notable folding, (2) faulting, (3) the extrusion of lava, and (4) exten- 

 sive degradation and aggradation, by running water, by ice, and by 

 wind. 



Volcanic activity and faulting, both on a great scale, seem to have 

 attended the deformative movements of the closing stages of the Mio- 

 cene. The lavas on the plateaus north of the Grand Canyon have 

 been referred to the close of the Miocene, and the Tertiary volcanic 

 activity of the Basin region reached its maximum at this time. 2 

 Though direct connection between intensity of movement and vigor 

 of volcanic activity has not been established, the connection of the 

 extensive igneous eruptions with the crustal warping and breaking, 

 can hardly be fortuitous. How far the one was cause and the other 

 effect, how far they were mutually cause and effect, and how far they 

 were effects of a common cause, are questions to which no decisive 

 answer can now be given. 



1 Dutton, Mono. II, U. S. Geol. Surv.; see also Davis, Am. Jour. Sci., 4th series, 

 Vol. X, p. 250. 



: King, op. cit. , pp. 414-415. 



