Mine Ranch

This Is Colorado Living


Pentiente Canyon Is 3 Miles North of GMR.

Space-shuttle photograph of San Luis Valley and surroundings, southern Colorado. 1 - Rio Grande, 2 - San Luis Lakes, 3 - Great Sand Dunes, 4 - Sierra Blanca Range of the Sangre de Cristo Mountains, 5— Mount Blanca, 6—Ancient course of Arkansas River. JSC Digital Image Collection; photo ID: STS040-151-126

The Summer Coon Volcano (SCV), located in the San Juan Volcanic Field about 10 km north of Del Norte in southwest Colorado, is a heavily eroded composite volcano.






 Spanning 8,000 square miles, the San Luis Valley is the world's largest alpine valley. The average altitude is 7,500 feet. The valley sits atop the Rio Grande Rift, a split in the crust of the Earth where the sides are pulling away from each other. (The Rio Grande Rift system extends from central Colorado southward through New Mexico and West Texas into northern Mexico.) If the Valley floor were excavated, bedrock would be down about 30,000 feet, making the bottom of the Valley close to 4 miles below sea level. Over millennia, the mountains surrounding the Valley have eroded away, filling in the hole with rock, sand and earth. Quite a few streams flow into the Valley, only to sink into the ground before they go very far. The only surface water to leave the Valley is the Rio Grande itself, flowing to the south along the crack named for it. There is a huge water aquifer under the valley and, below a certain depth, all of it is hot.

Pre-Valley History—570 million years ago (MYA) the northern end of what will become the San Luis Valley is located in  the Colorado Sag.  This lowland basin was occupied by seas that deposited marine sediments. Later the area went through a series of uplift/subsidence cycles  where  erosion occurred during uplift, and sediments from the Highlands and local seas  were deposited during  subsidence. The Sierra Grande Highlands occupied the southern portion of the Valley and maintained a high topography throughout this time period.

310 MYA the eastern Valley was part of the Central Colorado Trough and the western Valley was part of the Uncompaghre Highlands. The highlands provided sediments which were deposited in the lowlands.

136 MYA the Sangre de Cristo mountains began to form, as well as the Saguache Arch in the northwest Valley area and the Wet Mountains to the east of the SLV.

53 MYA, most of Colorado was an area of low hills and shallow depressions which resulted from erosion and uplifting whcih seem to be occurring at the same rate. The San Luis Valley still does not yet exist as a geographic feature.

Early Valley Formation—35 MYA during the Oligocene the Rio Grande Rift was activated. This fracture of the earth's crust runs from Leadville Colorado to El Paso Texas. As the rift became active, a series of large faults move which created a raised section called the Alamosa Horst and two lower areas called the Monte Vista Graben to the west and the Blanca Graben to the east. Overall the area subsided as a result of spreading and fault movement. 

At the same time, extensive volcanism begins in the west in what becomes the San Juan Mountains. This volcanic activity deposits thick lava flows, ash flows (tuffs) and volcaniclastic detritus in the west side of the valley. The Platora and Creede Calderas and the Summer Coon Volcano are important volcanic features which arose during this time. The two calderas have been the site of extensive mining. The Summer Coon Volcano is of great interest to geologists as it has eroded almost entirely down, exposing the inner core to study.

23 MYA the Arkansas River ran down a trough formed to the west of the Sangre de Cristo range along the Blanca Graben. The valley was subsiding due to spreading of the Rio Grande Rift and sediments from the Arkansas River Valley were deposited in the east side of the valley creating the Santa Fe formation.

Fan deposits from the San Juan Mountains were deposited in the Monte Vista Graben and on top of the Alamosa Horst.

Late Valley History—Throughout the Miocene and Pliocene (23 - 7 MYA), the Sangre de Cristo Mountains continued to rise along the faults making up the east edge of the Rio Grande Rift. There is 30,000 feet of relief along the Sangre de Cristo fault, although only about 6-8,000 feet are exposed.

In the late Pliocene and early Pleistocene (2 MYA), movement of the Salida-Maysville Fault at the north end of the Valley, cuts off flow from the Arkansas River Valley, ending deposits in the Santa Fe Formation.

Throughout the Pleistocene, erosion of the San Juan and Sangre de Cristo Mountain ranges deposits alluvial fans along valley margins.  The central area of the basin is characterized by lake sediments.  These lake deposits are know as the Alamosa Formation.  The San Luis Lakes and Dry Lakes are the remnants of the larger  lake system, and are surrounded by playa lakes.

The late Quaternary Period has the Sangre de Cristo Mountains with up to 14,000 feet of relief and still uplifting.  The valley floor shows very little relief as a result of continual reworking of the sediments by the Rio Grande, the Conejos River and adjacent streams.  The San Juan Mountains  are rounded showing their age and current quiescence. The Rio Grande Rift is subject to only minor (imperceptible) tremors and major movement is not expected for 3,000 years.


Natural Arch Through A Summer Coon Volcano Dyke. (2 miles west of GMR.)