TOPOGRAPHY OF THE PACIFIC COAST. 33 



by which the level is determined, and gradually spreads inland 

 toward the principal divides. Under similar conditions the 

 shales and limestones wear away more rapidly than the coarser 

 sediments and crystalline rocks, and local baselevels appear for 

 a time determined by the harder rocks. But these are all oblit- 

 erated in a general baselevel when it is completely developed. 

 The land is so unsteady that it rarely, if ever, remains without 

 elevation or depression long enough for the complete develop- 

 ment of a baselevel of erosion. It commonly happens, however, 

 that the large masses of harder rocks upon the slopes of the 

 principal divides form independent elevations in the plain which 

 may be more or less distinctly defined upon the softer rocks. 

 The topography of the region is then essentially a peneplain. 



It is evident that a general baselevel of erosion must have 

 originated approximately at sea level. This is the onl)^ position 

 in which a very extensive baselevel of erosion can originate. If 

 we now find such a baselevel at considerable elevation above the 

 'Sea, its position furnishes evidence that since the baselevel was 

 formed the country has been uplifted in the process of mountain 

 building. 



Upon our Atlantic slope, ancient baselevels of erosion are 

 well developed in the Piedmont region and elsewhere at consid- 

 erable altitudes above the sea, as shown by Davis, McGee, Wil- 

 lis, Hayes, and Campbell. The ancient mountains have been 

 swept away, and the modern mountains, at least in large part, 

 are the result of later upheavals. Similar changes have taken 

 place on the Pacific slope. Russell found in the St. Elias range, 

 at an elevation of over 5,000 feet, shells of marine mollusks still 

 living along the Pacific coast, showing that the great mountain 

 range had been uplifted in very late geologic time. So, also, the 

 Sierra Nevada and Coast ranges, and to some extent the Cascade 

 range, now such prominent features of the Pacific coast, have 

 been upheaved to their present great height, and deep canons cut 

 upon their slopes in the later geologic ages. At an earlier epoch 

 the whole country was comparatively low and near sea level, or, 

 •in other words, near its baselevel of erosion. The mountain 



