Mt. Sopris, south of Glenwood Springs, is an igneous intrusion. (Jack Rathbone photo)
Crested Butte, at the south end of the Elk Mountains, is a small intrusive igneous mass called a [laccolith]. Hard and resistant to erosion, it stands over 2,000 feet above the adjacent valley floor.
San Juan Mountains
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.
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.