178 QEOLOGY: ATWOOD AND MATHER 
raphy must have been produced, and those mountains may be thought 
of as the first generation of the San Juan range (see fig. 1). During the 
period of mountain growth there was some volcanism. Many por- 
phyritic intrusions and the deposition of the great volcanic tuffs which 
made contributions to the Ridgway till date back to this period. The 
Eocene till indicates that during the dissection of these early San Juan 
Mountains ice formed in the range and descended to the bordering 
lowlands. Possibly ice formed in neighboring ranges and approached 
the San Juan Mountains, and possibly there were distinct glacial epochs 
in that period of glaciation. 
After the retreat and disappearance of the early Tertiary ice, stream 
erosion continued, and the western portion of the San Juan Mountain 
area was reduced to a surface of slight relief which may be thought of 
as a peneplain.^ This peneplain bordered on the west a higher area of 
mountainous character, which supplied the material for the Telluride 
FIG. 1. DIAGRAMMATIC CROSS SECTION SHOWING THE PHYSIOGRAPHIC EVOLUTION 
OF THE SAN JUAN REGION SINCE THE BEGINNING OF TERTIARY TIME, c, PRE-CAMBRIAN 
SCHIST AND GNEISS; b, ALGONKIAN QUARTZITES; c, PRE-CAMBRIAN GRANITE INSTRUSION; 
d, PALEOZOIC AND MESOZOIC ROCKS; e, RIDGWAY TILL AND TELLURIDE CONGLOMERATE; 
/, MIDDLE AND LATE TERTIARY VOLCANICS; g, TERTIARY INTRUSIVES. 
conglomerate. The deposition of gravels upon this peneplain surface 
was probably due to some uplift and rejuvenation of the streams in the 
eastern portion of the range. After the deposition of the Telluride con- 
glomerate there was further erosion in the range, and then came the three 
great epochs of volcanism, the San Juan, the Silverton, and the Potosi. 
During these epochs of volcanism a great volcanic plateau was developed. 
By this time the Miocene epoch had been reached and possibly passed, 
and with the quieting down of volcanic activity began the erosion 
and dissection of the volcanic plateau. During this period of dissection 
another generation of San Juan Mountains was carved, this time out of 
volcanic debris and great lava flows (see fig. 1). 
Recent physiographic studies of the range have led to the recognition 
of a summit peneplain in the San Juan region (see fig. 1), so it would 
appear that the first dissection stopped at a plain which is now repre- 
sented by many of the summit areas within the region. The San Juan 
Mountains that were first carved out of this great volcanic plateau 
should then be thought of as surmounting those of today. Perhaps, if 
