PHYSIOGRAPHIC EXTENSION OF UNITED STATES 169 
course derived from the adjacent highlands, both to the east and 
west, and therefore the history of the growth and dissection of the 
mountains is in a large measure the history of the filling of the 
trough. é 
The mountainous province which borders the trough on the east 
has been contributing deposits since Cretaceous time. The Coast 
Ranges were probably sufficiently elevated in early Pliocene time 
to commence contributing to the filling of the trough. In late 
Pliocene time both mountain systems were further uplifted, increas- 
ing the carrying power of the streams and resulting in a more rapid 
filling of the depression. By the close of the Pleistocene epoch the 
deposits had accumulated to such a depth in certain places as to 
fill the trough to sea-level and to cut off parts of it from the sea. 
From that time to the present the history of the subaérial divi- 
sions has been that of flood-plain formation.’ These facts apply 
chiefly to the divisions within the United States, but the deposits of 
the Copper River basin also bear evidence of a similar sequence 
of events in that division.” 
The history of the submerged parts of the trough is important 
only as it contributes to the discussion of the land surface. The 
history of the Gulf of California has been recounted in the writer’s 
paper on Mexico previously cited,3 and Willis and Smith‘ have given 
a good summary of the history of Puget Sound. A review of the 
literature on the subject would lead one to believe that an identical 
series of events took place within the submerged parts of the trough 
in Canada and Alaska. From the data available at this time, how- 
ever, it is impossible to make a positive statement. 
THE SIERRA-CASCADE PROVINCE OF THE PACIFIC MOUNTAIN SYSTEM 
This province comprises a very persistent mountainous feature 
of Western North America in which folding came to a close in 
Mesozoic time and which has since been comparatively rigid.s The 
1F. L. Ransome, Univ. Cal. Bull. (Dept. Geol.), I, 387. 
2 A. H. Brooks, op. cit., p. 54; also W. C. Mendenhal, U.S. Geol. Survey, Prof. 
Paper 41, p. 84. 
3 W. N. Thayer, oP. cit. 4 Bailey Willis and G. O. Smith, op. cit. 
5 F. L. Ransome in Problems of American Geology, pp. 358, 359- 
