OURAY COUNTY – Two dozen geology buffs from around the region journeyed well over a billion years back in time last Wednesday, Aug. 6, on the second annual Geology Tour offered by the Ouray County Historical Society.
The guided day-long tour, led by Ouray geologists Larry Meckel and Robert Stoufer, took participants on a geologic trail along the northwestern San Juan Mountains that started at the glacial moraines and fault zones of the Ridgway area, traveled through the glacier-carved remnants of the heavily mineralized Silverton Caldera and concluded on Coal Bank Pass, covering 1.7 billion years of geologic events along the way.
“I’m still dazed by the sweep of geological time we witnessed on the tour, as well as the grandeur of the San Juan Mountains,” said Anne Eggebroten, who offered these “surprising facts” she learned along the way:
- At Bear Creek Falls and other spots along Highway 550, there’s a 1.3 billion year gap between one rock layer and the next. The Precambrian rock there is roughly 1.4 billion years old, and on top of it lies the San Juan Tuff, only 27 million years old. Because the Uncompahgre River cuts through the ancient rock, it’s known as the Uncompahgre Formation.
- There were 15 stratovolcanoes in the San Juan area between 33 and 23 million years ago, and they erupted with pyroclastic debris flows extending to New Mexico, central Colorado, and near the Utah border.
- Telluride is on the eastern edge of the Paradox Basin, geologically speaking. When the Uncompahgre Plateau rose up during the late Paleozoic era, the basin to its west went down. Red and white rocks of Paleo- and Mesozoic eras were deposited in it.
- The Telluride Conglomerate is hard to follow from place to place because it was deposited by a system of river channels flowing out of mountains to the east. Rocks from the Grenadier Mountains are among the many pebbles and large rocks found in it.
- The whole erosional surface that extended over much of Colorado about 50 million years ago is sometimes called the Telluride Unconformity – also the Eocene Unconformity and the Great Unconformity. Volcanic rocks were laid down on it, and then glaciers arrived.
- There are sea shell fossils on Molas Pass near Andrews Lake in the Leadville Limestone formation from the Mississippian era, 350 million years ago.
- The Ridgway Fault runs east-west along the foot of Log Hill and marks the southern end of the Uncompahgre uplift, known locally as Dallas Divide. “Uncompahgria” was one of two island ranges of the ancestral Rockies.
- Red Mountain is part of the debris that fell back into the 10-mile-wide hole made when the Silverton supervolcano erupted about 27.6 million years ago. It’s red because later hydrothermal pressures were able to intrude the crushed rock with minerals that weathered to iron oxides. Layers of volcanic rock outside the caldera resisted this intrusion.
The geology tour was a fundraiser for OCHS, and was made possible with contributions of local sponsors including Gregg Pieper of San Juan Scenic Jeep Tours; Larry and Barbara Meckel; Michael Underwood of Better Real Estate Services Inc.; Gary Lindberg of Colorado West Rentals and Jeep Tours; Mickie and Joe Dziubasik of the Matterhorn Motel; Lora Slawitschka of Ouray Chalet Inn; Robert Stoufer of Buckskin Book Sellers and Colorado Minerals.com; and Matt and Jackie Genuit of Ouray Liquors.
San Juan Mountains Minerals Symposium Offered In September…
Up to 150 geology and mineralogy buffs will be gathering again in Ouray again on Sept. 5-8 for the Ouray-Silverton San Juan Mountains Minerals Symposium. The weekend-long event, sponsored by the Friends of Mineralogy of Colorado and the Friends of the Colorado School of Mines Geology Museum, will include presentations and field trips featuring the mining history, geology and minerals of one of America’s most spectacular mountain areas.
Ouray-based Certified Professional Geologist Bob Larson is helping to head up the event, and will be offering a lecture titled “The Vein Systems of the Silverton Caldera and Their Minerals (Idarado Mine, Camp Bird Mine and Revenue-Virginius Mine)”. Other speakers include Duane A. Smith, John Dreier, Bill Jones, Don Paulson, David Smith and Ken Sloan, speaking on a range of topics having to do with the geology and mining history of the region, on Friday evening and throughout Saturday.
One highlight of the symposium will be an evening presentation on Saturday, Sept. 6 at which “Thomas Walsh” (impersonated in costume by his biographer John Stewart, a Denver attorney and author) will talk about his mining activities at Ouray’s Camp Bird Mine during the 1890s.
Symposium events on Friday and Saturday will take place at the Ouray Elks Lodge, while guided and self-guided field trips on Sunday and Monday will take participants into the surrounding mountains near Silverton and Ouray.
For more information, including registration materials, visit http://friendsofmineralogycolorado.org/index.php/events/13-meetings/39-ouray-silverton-san-juan-mountains-mineral-symposium2.