Ransom k.] 



The Great Valley, 



tion to the fact that the vertical curves of the Neocene rivers do 

 not correspond with the curves of erosion of the present streams, 

 the latter showing a greater concavity. Becker considered this 

 as due to a more prolonged erosion of their middle courses by the 

 present streams, while the upper portions of the Sierra were pro- 

 tected by a Pleistocene ice-cap. But the work of Browne and 

 Lindgren has shown that the cause of the difference is to be found 

 in a deformation of the western slope of the range during an uplift 

 " extending over a long period since Neocene times." Speaking of 

 the Middle Yuba River, Lindgren says : " If the modern river curve 

 shows such regularity, it would be natural to expect that that of 

 the Neocene river, which represents a more advanced stage of base- 

 leveling, should be still more so. But the plotted curve of the 

 Neocene Middle Yuba River does not correspond to the normal 

 curve of erosion. Instead, it appears to be composed of two curves 

 with the convex side upward. I think this convexity, which can 

 not be explained by differences in the resistance of the rock masses 

 over which the river flows, must be due to a deformation of the 

 surface during the uplift of the Sierra. The most prono'unced 

 departure from the normal curve of erosion results from the present 

 steep grades of the Neocene channels near the valley. This is 

 marked in both of the profiles given, and must, I think, be regarded 

 as indicating a subsidence of the portion adjoining the sediment- 

 filled trough of the Great Valley relatively to the middle part of the 

 range, or a rise of the latter relatively to the former." 



At the first glance, this result appears to be a rather striking 

 confirmation of the isostatic hypothesis; but second thought shows 

 that it is susceptible of a different interpretation. For if we regard 

 the whole western slope of the Sierra and the floor of the Great 

 Valley as a single orographic block, partly tilted, and very slightly 

 flexed by tangential pressure, then the arching of the Sierra slope 

 may be regarded as the geo-anticline corresponding to the geo- 

 syncline of the valley floor. In such a case it would be natural to 

 find an increased grade of the old stream beds near the edge of the 

 valley, where the anticlinal passes into the synclinal curve, and the 

 result would be in harmony with the continued activity of the oro- 

 genic forces during Pleistocene times. It is fairly certain that a 



