168 W. N. THAYER 
probably continued beyond them through the Copper River Basin, 
Sushitna Basin and Cook Inlet, and possibly includes the Shelikof 
Straits. 
The divisions represented by the Lower Colorado Basin, the 
Great Valley of California, the Willamette and Cowlitz valleys, 
the Copper River Basin, and the Sushitna Basin are land surfaces; 
the remaining divisions are at present submerged. Within the 
United States the eastern and western boundaries of the subaérial 
divisions may be definitely drawn. The floor of the Great Valley 
is, for example, practically coextensive with the area of Quaternary 
deposits shown on geologic maps, and in Oregon and Washington 
the greater part of the depression is filled with glacial or fluvio- 
glacial deposits and alluvium formed during or since the glacial 
period. Brooks" uses the same criterion to delimit the terrane of — 
the Copper River Basin, which he describes as a broad floor of. 
Pleistocene gravel and silt deposits extending from the inland slope 
of the Chugach Mountains to the foothills of the Alaska Range. 
The boundaries of the submerged divisions may be regarded as 
practically coincident with the limiting shore lines. 
The topography of the subaérial divisions within the United 
States may be characterized as a “floor.” The broad, sloping 
alluvial plains of these sections are generally featureless. The slope 
of the valley floor generally increases toward the foothills, finally 
merging with the alluvial fans in the foothill gulches. In some 
places this merging is so gradual that it is impossible to say where 
plain ends and foothills begin. Brooks has applied the term “‘floor”’ 
to the Alaskan section and emphasizes its monotonous lack of relief. 
“The Pacific Coast downfold has been a feature of the western 
coast since the Cretaceous period, and during several geologic 
periods was so deeply depressed as to lie beneath sea-level and 
receive a considerable body of sediments.’ These marine sedi- 
ments, however important geologically, do not contribute in any 
way to the present topography. The physiographer is concerned 
chiefly with the origin of the later alluvium and gravel deposits, 
which were produced by stream action after certain parts of the 
trough had been cut off from the sea. These deposits were of 
tA. H. Brooks, op. cit., p. 54. 2 Isaiah Bowman, Forest Physiography, p. 177. 
