734 ALLEN DAVID HOLE 
may prove true that the deposits classed as (2) above should be 
regarded as merely early phases of the more recent epoch; but 
additional evidence from adjacent territory is needed before the 
matter can be placed beyond question. 
The age of the oldest of the earlier drift as indicated by the 
amount of erosion which has taken place since the deposit was 
made is probably best shown on the mesa on which Diamond Hill 
is located. The vertical range of the earlier drift in this area is 
from 9,300 to 10,100 feet in elevation; the pre-glacial surface of the 
mesa must, therefore, have had a relief of about 800 feet; for the 
underlying rocks are here chiefly sandstones or igneous rocks, so 
that any considerable lowering of the surface due to solution as 
might have been the case in a limestone region cannot have occurred. 
Between areas of drift now at approximately the same elevation, 
valleys half a mile broad and 100 to 150 feet deep exist. The 
slopes of these valleys are gentle, the tops of the hills and ridges 
rounded or flattened, and undrained depressions are practically — 
unknown. When it is remembered that most of the valleys 
separating the isolated patches of earlier drift are occupied by 
temporary streams only, and that for a considerable period the 
climate of the region has been semiarid, it is evident that the time 
represented by post-glacial erosion on this mesa is very long. 
In considering the relation of the oldest of the earlier drift to 
that of the more recent epoch, one feature seemed especially 
prominent as the fieldwork was in progress; that is, the presence 
of large San Juan bowlders as the most conspicuous constituent of 
the more recent drift from the main valley of the San Miguel, and 
similarly, equally large Potosi rhyolite bowlders, characteristic of 
the oldest of the earlier drift on the mesas and ridges adjacent to 
the same stream. The presence of large bowlders of a given 
formation in abundance in drift of a certain period at once raises 
questions as to the conditions under which glaciers get possession 
of an abundance of large rock fragments. Judging from the 
composition of the drift brought down the main valley of the San 
Miguel to Keystone, and from the relation of the exposures of the 
various formations in the upper valleys tributary to the San Miguel, 
it would seem that an abundance of large bowlders in drift is to be 
