PLEISTOCENE HISTORY OF SAN JUAN MOUNTAINS 407 
side of the range. In outline these stages are as follows: At some 
time subsequent to the deposition of strata of Wasatch (Lower 
Eocene) age, and probably near or at the close of the Tertiary period, 
the San Juan region was reduced to an almost base-leveled con- 
dition. Only a few elevations remained as monadnocks above the 
peneplain upon which streams with gentle gradients were depositing 
at places a thin mantle of remarkably well-rounded and water- 
worn quartzite and jasper pebbles. The cycle of erosion which 
developed this extremely old-age topography was closed by a general 
uplift in the district, which was emphasized in the San Juan dome. 
Dissection of the peneplain began in the uplifted dome area and 
resulted in the formation of great alluvial fans, composed of 
bowlders as well as gravels, upon the peneplain fringing the dome. 
At the same time rejuvenation was working upstream across the 
broad plateau and as soon as this headward erosion had reached 
the margin of the dome the growth of the great alluvial fans ceased 
and their dissection began. Remnants of this gravel- and bowlder- 
capped peneplain are now found as the summit elevations in the 
mountains and upon the neighboring plateaus. 
Below the peneplain level there are other broad bowlder-capped 
mesa-like forms which appear to represent the base to which the 
streams worked when the peneplain was first dissected. The 
bowlder-capped mesas noted in the paragraph descriptive of the 
Uncompahgre interglacial epoch are typical of this bowlder-mesa 
stage in the dissection of the area. Another uplift associated with 
the more or less continuous growth of the mountains deformed the 
graded surfaces of the bowlder-mesa stage, again rejuvenated the 
streams, and opened another -cycle of erosion. The surfaces to 
which the streams then worked have been referred to in the studies 
on the south side of the range as the graded surfaces of the Oxford 
stage. Beneath these surfaces the streams have cut comparatively 
narrow valleys. 
The problem of the relationship of the epochs of glaciation to 
the stages in the erosion history of the range is of special interest. 
The available data make it possible to present a tentative or work- 
ing hypothesis for the solution of this problem. 
On the northern slopes of the range between Cow Creek and 
