Volcanic Fury, Glacial Patience: How the San Juans Were Built
Near Ridgway, Colorado
The mountains visible from Ridgway are not merely impressive. They are the product of one of the largest volcanic events in the history of North America. Between roughly thirty five million and twenty three million years ago, the region now called the San Juan Mountains experienced a period of catastrophic volcanism.
A series of calderas, the large volcanic depressions that form when magma chambers collapse after eruptions, blasted enormous volumes of ash and volcanic rock across hundreds of square miles. The San Juan Volcanic Field is the largest remnant of a once-vast volcanic field that covered much of the southern Rocky Mountains. After the volcanic episodes subsided, successive periods of glaciation went to work on the landscape.
Glaciers flowing down from the high country ground out valleys, carved cliff faces, and sculpted the jagged ridgelines visible today. The broad valley in which Ridgway sits was shaped in part by glacial ice that filled it and then retreated at the end of the last ice age. The Uncompahgre River now follows the path the glacier carved.
The peaks to the south, including the Cimarron Ridge and the summits around Mount Sneffels, rise from that volcanic and glacial foundation. What looks from the highway like a static, ancient landscape is in geological terms the aftermath of tremendous violence, the record of millions of years of eruption and burial and slow grinding erosion written in every cliff face and every valley wall around Ridgway.
