144 JOSEPH B. UMPLEBY 
Northwest and the parallel lines of the same figures show the 
position of the areas described as representing the plateau type. 
The distribution of Eocene sediments around these areas strongly 
suggests a relation between the two. It seems that the sediments 
could not have been derived from the region after its last elevation 
for two reasons: (1) It is very doubtful if the plateau is sufficiently 
dissected to afford the volume of material represented by the Eocene 
beds, and (2) All the more important valleys of the area drain 
westward, and in all probability have done so throughout their 
entire history. This is true of the Rocky Mountain trough, the 
Purcell trough, and the Snake, Salmon, and Columbia river chan- 
nels. These, together with their tributaries, represent perhaps 
go per cent of the present dissection of the plateau region. If we 
assume that the old erosion surface is pre-Eocene the material 
derived from these several valleys may be thought to account for 
the narrow fringe of Eocene sediments on the west, but cannot 
account for the incomparably more extensive Eocene beds which 
lie to the east of the present plateau region. 
Conclusion.—From this line of evidence it is concluded that the 
Eocene sediments were derived from the plateau area during that 
great cycle of erosion which resulted in a comparatively level 
surface, and therefore that the plateau region of the present day 
was characterized by gentle topographic features at about the 
close of the Eocene period. Whether or not that former great 
cycle of erosion began with the Eocene may be an open question, 
but that it closed with the Eocene, and therefore that the present 
plateau surface is of Eocene age, there seems to be little room for 
doubt. 
THE EOCENE SURFACE AS A DATUM PLANE 
Over much of Idaho no satisfactory datum plane has been 
recognized between the Algonkian and the Pleistocene. In the 
southeastern part of the State formations of Paleozoic age are 
present and along the western side are the great sheets of Miocene 
basalt. Other datum planes are recognized, but they are all well 
removed from the plateau area. Thus where a datum plane is 
most needed the Eocene erosion surface is best preserved. 
