NEOCENE DEPOSITS OF CALIFORNIA 389 
Pliocene of the Sierra Nevada region was developed in this 
region, it has been completely destroyed by erosion, and is not 
identifiable, while, as already intimated, the Middle Pliocene 
baselevel did not penetrate far from the border of the present 
Sacramento valley. In general, we may say, the present topo- 
graphic features are older than those which dominate the Sierra 
Nevada country. 
The nearest approach to the development of a late Neocene 
peneplain in Trinity county was effected at the close of deposition 
of the Neocene gravels. The surface of the alluvial deposits 
rose by aggradation and the neighboring rock surface sank by 
degradation, until at the close of the epoch the two planes met. 
This is precisely the same relation that exists between the 
auriferous gravels and the peneplain of the Sierra Nevada region, 
indicating that we are treating equivalent and contemporaneous 
events. So confident am I that this is true that I am inclined 
strongly to accept the Neocene baselevel of the region under 
discussion as a datum plane of the same value as the Sierra 
Nevada peneplain, consider its maximum development of rather 
late Pliocene age, and base upon it speculations as to the ages 
of all other physiographic features in the province. 
The Sierra Costa Mountains rose above the Neocene base- 
level to elevations of three to five thousand feet, and had a 
topography similar in its larger features to that of today. The 
higher valleys of these mountains are essentially the valleys of the 
Neocene. They are commonly called ‘‘cafions’’ by the miners, 
because they have relatively broad floors and very steep walls. 
Part of this cafion-like form is due to glaciation, but more is 
preglacial in its origin. The work of the glaciers was short and 
confined to a removal of talus from the foot of the precipices, 
to the smoothing of inequalities, and to the filling of the deeper 
portions of the valleys by drift débris.1 Beyond the ends of the 
glacier sites, the same broad valleys continue at the same level, 
but this fact is obscured by the Pleistocene erosion of deep 
gulches in the bottoms of the older valleys, the slopes of which 
tyJour. GEOL., Vol. VII, 1899. 
