PHYSIOGRAPHIC EXTENSION OF UNITED STATES 173 
This fact has not been fully established, however, and since there 
is apparently no accordance of summit levels, the origin of these 
mountains may still be open to question. The Aleutian Range 
with its long line of typical cones built along an anticlinal axis 
closely resembles the Cascades. As a matter of correlation it 
may be remarked that intermittent vulcanism has continued in 
both sections to the present time." : 
The physiographic history of this province properly begins with 
the emergence of the area from the sea at or near the close of the 
Mesozoic era. Deformation either accompanied the emergence 
or closely followed it, and then from a point as far north as the 
sixtieth parallel to the southern end of the Sierra Nevada folding 
on a large scale came to a close. Since Cretaceous time the system 
has been comparatively rigid, and its various units in so far as they 
have moved at all have moved en masse or in large fault-blocks.? 
The present mountains, however, were not produced by deforma- 
tion only. Several forces have worked together from the close of 
the Mesozoic era to the present time to produce the topography as 
we now see it. This will be shown in a brief summary sketch of the 
history of the various units. 
In the Sierra Nevada the present simple orographic form is 
strikingly contrasted with an older and quite complex structure 
which was developed during the Mesozoic deformation, and which 
consisted of strong folds intruded by extensive granodiorite batho- 
liths. In late Cretaceous time these folds were truncated by ero- 
sion and the surface reduced to one of low relief.3 The Eocene 
epoch was probably a time of mild deformation and uplift. This 
began an erosion cycle that culminated in the Miocene epoch by 
reducing the area to a peneplain.4 Another uplift late in the Plio- 
cene epoch raised this peneplain to the level of the plateau, now 
dissected, which lies between the crest of the Sierras and the Great 
Valley. The crest of the Sierras, or, as it is irequently called, the 
High Sierras, stands several thousand feet above the general level 
t A. H. Brooks, op. cit., p. 275. 
2 F. L. Ransome, in Problems of American Geology, pp. 358, 359. 
3 F. L. Ransome, oP. cit., p. 351. 
4J.S. Diller, U.S. Geol. Survey, r4th Ann. Rept., Part II, pp. 404-11. 
