180 W. N. THAYER 
occurred at the same time on the Interior Plateaus and on the 
Yukon Plateau." 
Throughout all Tertiary time the belt of intermontane plateaus 
north of the Great Basin was fairly rigid.2 However, in late 
Miocene time slight orogenic movements warped the early Miocene 
lavas into broad synclinal basins and anticlinal domes. These 
movements were probably coincident with the profound faulting 
movements which affected the Great Basin. Lava flows which 
continued until very recent time have largely obscured the evidence 
of such movements on the Columbia Plateau, but they are plainly 
to be seen in the northern divisions. 
- Late in the Pliocene epoch there was a general but differential 
uplift of the entire Cordilleran region. This prepared for the devel- 
opment of the present upland surface. Dissection following this 
uplift marked out the main features of the present topography. | 
The erosion cycle thus begun was halted, however, north of the 
Columbia Plateau by the formation of the Pleistocene ice cap. 
Normal processes of erosion which were renewed after the with- 
drawal of the ice have continued the dissection, but the topography 
bears the characteristic marks of glacial modification. 
THE ROCKY MOUNTAIN SYSTEM 
East of the Intermontane Plateaus and bordering this region 
throughout almost its entire length is another major division of 
general Alpine habit in strong topographic contrast with the 
plateaus. It is the most difficult of all the Cordilleran regions 
to name or define, because of the indefinite and varied manner 
in which names have been applied to it and its subdivisions. 
A ruling of the United States Geographic Board makes the term 
“Rocky Mountain System”’ embrace the whole of the mountainous 
region between the forty-ninth parallel and the Rio Grande River. 
However, this definition does not harmonize with recent studies 
in physiographic boundaries, as it includes a large area in Texas 
and New Mexico of a character essentially different from that of 
the mountains, and excludes a large and closely related region north 
™L. Reineke, op. cit., p. 38. 
2 F. L. Ransome, oP. cit., p. 338. 3C. W. Drysdale, op. cit., pp. 235, 236. 
