PHYSIOGRAPHIC EXTENSION OF UNITED STATES 167 
across the Chugach Mountains may also fall within this classifica- 
tion. 
The physiographic history of the Coast Ranges in the Unite 
States begins with the general uplift in the Pliocene period that 
started the erosion cycle which produced the first peneplain. Dur- 
ing this cycle the master-streams outlined their present courses. 
The date of the uplift that began the present erosion cycle has not 
been definitely determined, but it may be assigned tentatively 
to late Pliocene time. The present topography, excepting residual 
peaks, and all tributary stream courses are the results of dissec- 
tion during this later cycle. 
North of the International Boundary there are records of a 
corresponding sequence of events. Diastrophism on a large scale 
was the dominant process during the Tertiary period until the close 
of the Miocene epoch. Erosion following an uplift, probably in 
Pliocene time, subdued the mountains of Vancouver and Queen 
Charlotte islands* and also the mountains of Kodiak Island and the 
Kenai Peninsula,’ and it may fairly be assumed that it included the 
intermediate members of the Coast Ranges. 
THE PACIFIC TROUGH SECTION OF THE PACIFIC BORDER PROVINCE 
~ The great depression of the western coast of North America, 
known as the Pacific Trough, is a structural feature that extends, 
with minor interruptions, from Cape Corrientes, Mexico, northward 
to Alaska. It is made up of six natural divisions: the first and 
southernmost is the Gulf of California; the lowlands of the Lower 
Colorado Basin constitute the second; and the third is the Great 
Valley of California. The fourth comprises the Willamette and 
Cowlitz valleys and Puget Sound. The fifth or Canadian division 
extends northward from the Straits of San Juan de Fuca and 
includes the Straits of Georgia, Queen Charlotte Sound, and prob- 
ably Hecate Straits. The sixth or Alaskan division extends appar- 
ently through Clarence and Chatham straits to Lynn Canal. The 
mountains of the St. Elias group interrupt the extension of the 
trough northward from Lynn Canal in very much the same manner 
as do the Klamath Mountains of the United States, but it is 
«G. M. Dawson, op. cit., p. go. 2 A. H. Brooks, op. cit., pp. 292, 203. 
