46 TOPOGKAPHIC DEVELOPMENT OF KLAMATH MOUNTAINS, [bull. 196. 
pared with the narrow belts of the marine terraces along the coast, 
strongly emphasizes the view that the peneplain is of subaerial origin. 
This view is supported also by the character of the marine deposits 
along its border. During a later portion of the Empire epoch the 
sediments all along the coast from the Columbia River to Humboldt 
Bay were fine, such as are derived from beaches on the edge of broad 
stretches of lowland so reduced by erosion that the streams carried 
only fine sediments to the sea. Such conditions prevailed also when 
the Wymer beds were deposited. They mark the time of greatest 
development of the peneplain as a land surface in the Klamath 
Mountains, and being undisturbed in relative position as far as the 
peneplain is concerned, they mark approximately the position of the 
Miocene base-level controlling its development. 
The approximate plain which gives to the Klamath Mountains their 
plateau character may be explained by subaerial erosion with the j 
sea marking its border, but the Bellspring peneplain of the coast 
range may owe some of its development to submarine erosion, 
although the greater part, if not the whole, is due to land streams. 
No evidence has yet been found to show that the northern portion of 
the Coast Range has been beneath the sea since the tilting of the 
Miocene beds at the close of the Klamath peneplain stage. The 
topography of this portion of the range is gently sloped above, 
descending to steep canyons along the present large streams. Some , 
of its evenness of crests may be due to subequality of interstream 
spacing, as Shaler and Smith have explained for other regions, but 
this could not have produced the flat tracts, remnants of the pene- 
plain, which are found on some of the ridges. The rocks here are 
softer and on the whcle more uniform than those of the Klamath 
Mountains, and as they erode more easily a somewhat more advanced 
stage of topographic cycle might be expected when compared with 
the Klamath Mountains, notwithstanding the fact that the latter are 
a much older land surface. 
The Klamath peneplain may have originally been covered by residl 
ual deposits of considerable thickness, but if so they have been j 
largely removed, for the character of its surface exposures as com- i 
pared with that of the earlier valleys to be noted presently is essen- 1 
tially the same, but is strongly contrasted with that of the later 
valleys. 
Stream gravels have not been found so closely associated with the 
Klamath peneplain as to give strength to the argument regarding its 
subaerial origin, but they have been found in the earlier valleys cor- j 
responding to the Sherwood peneplain at an elevation but little below 
the Klamath peneplain in the Coast Range. By far the most impor- j 
tant deposits of this character belong to the ancient Klamath River; 
and will be noticed more particularly under the heading "Earlier 
valleys." 
I 
