The San Juan Mountains are the most extensive range in Colorado, and
also the most heterogeneous. Covering more than 10,000 square miles of
the southwestern part of the state, these mountains are formed mostly of
Tertiary volcanic rocks, the result of repeated outpourings of lava and
ash from a cluster of volcanoes. Water-laid gravels composed of volcanic
sand and pebbles are interlayered with basalts and ash beds; the total
thickness of these beds reaches many thousands of feet.
[Illustration: The mining town of Ouray, now also a tourist haven and
summer resort, nestles below Pennsylvanian sedimentary rocks of Ouray
Canyon. At the north end at town can be seen the Ouray Hot Springs
swimming pool. Gold, silver, lead, and zinc are still mined in this
area. (Jack Rathbone photo)]
The widespread volcanic activity which formed most of the range began in
mid-Tertiary time and continued for several million years. A few
Quaternary volcanic flows are known in the region, but there is no
active volcanism there at present.
The western side of the main range, including some of the highest peaks,
consists primarily of uplifted and faulted Paleozoic sedimentary layers.
These layers, highly dissected by erosion, can be seen near Ouray, at
Molas Lake, and at Durango. Large patches of Precambrian granite and
metamorphic rocks protrude through the sediments, as in the Needle
Mountains; they indicate that this part of the range is a faulted
anticline like many other Colorado ranges.
Early Cenozoic glacial deposits occur in some parts of the San Juans.
These are unusual features, as glaciation of this age is unknown
elsewhere in Colorado.
Three small ranges rise just west of the San Juans: the San Miguel,
Rico, and La Plata Mountains. Each consists of several small masses of
Tertiary igneous rock intruded into Paleozoic conglomerates, shales, and
limestones.
Mineralization has been intense in the San Juans; most of it took place
during the Late Tertiary volcanic period. Rich veins penetrate
Precambrian gneiss and granite, and Paleozoic limestones are often
enriched also. Several mines are still active near Ouray, Silverton,
Telluride, and Rico.