174 W. N. THAYER 
and is not a part of the Miocene peneplain. It is the residual part 
of the surface that was reduced in late Mesozoic time.* 
A similar record of events is preserved in the Cascade Moun- 
tains. A range corresponding to the present Cascades was prob- 
ably formed by folding during the Mesozoic era accompanied by 
intrusions of igneous rock, but the configuration of the present 
range is ascribed to later events and processes. In early Tertiary 
time the region was comparatively rugged. During the éarlier 
epochs of the Tertiary period there was an alternation of basaltic 
lava flows and shallow water deposition. The Miocene epoch was 
a time of further mild deformation, followed by an erosion interval 
that subdued the whole region. This subdued surface was uplifted 
during the Pliocene epoch to form the mass of the present Cascade 
Range.? Erosion and vulcanism have since combined to produce 
the present topography from this uplifted peneplain. | 
On a previous page an attempt was made to justify the classi- 
fication of the Coast Range of British Columbia with the Sierras 
and Cascades on a basis of crest-continuity. A further justification 
is found in the record of the physiographic history of the Coast 
Range. ‘The later part of the Mesozoic era was in this region also 
a time of deformation and granitoid batholithic intrusion. Erosion 
followed this deformation and produced a peneplain, or at least a 
subdued surface. Milder deformation, probably in the Miocene 
epoch,’ uplifted this subdued surface, and another erosion cycle 
was started, which before the close of the Pliocene epoch had pro- 
duced a second peneplain. Late Pliocene time witnessed another 
uplift and the beginning of the erosion cycle that produced the 
topography as it now appears. 
Writers do not all agree regarding the Tertiary peneplains of 
the Cascades and the Coast Range of British Columbia, the exist- 
ence of which is inferred from the accordance of summit levels. 
Russell? and Willis and Smith’ agree on a late Tertiary base level 
t Isaiah Bowman, op. cit., p. 170. 
2G. O. Smith, U.S. Geol. Survey, Folio 86. 
3 A. C. Spencer, Bull. Geol. Soc. Am., XIV, 117-32. 
41. C. Russell, U.S. Geol. Survey, 20th Ann. Rept., Part II, pp. 140-44. 
5 Bailey Willis and G. O. Smith, U.S. Geol. Survey, Prof. Paper ro. 
