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BLM>Colorado>Field Offices>Royal Gorge Field Office>Geology and Paleontology>Upper Arkansas River Valley
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Upper Arkansas River Valley


Geology

The upper Arkansas Valley, through which the Arkansas River flows, is a north-tapering, sharply defined valley that extends from Salida to the continental divide near Leadville and is located between the Sawatch and Mosquito Mountain Ranges. The area has witnessed three periods of mountain building, subsidence, and regional uplift to the west and to the east - as exhibited in the Sawatch Range and the Mosquito Mountain Range respectively. The Ranges consist of pre – Cambrian crystalline rock overlain by sedimentary rock with many of the peaks in these Ranges exceeding heights of 14,000 feet.   The upper Arkansas River Valley floor extends in width from 3 to 10 miles and lies approximately 4,600 feet below the tops of the Sawatch and Mosquito ranges. During the Pleistocene age, glaciers covered the uppermost areas of the mountains and throughout the upper valley examples of glacial valleys, morraines, and outwash are exhibited along the highway and river. The valley is the northernmost valley in the chain of valleys that comprise the Rio Grande Rift system. This rift extends from Texas through New Mexico and appears to terminate near the town of Leadville and the Continental Divide. 

Paleo Highlights: Miocene Fauna - Dry Union Formation

Mineral Resources: Gold, Silver, Lead, Zinc, Sand and Gravel, Molybdenum, gems