Frisco’s geologic past written in stone
summit daily news
FRISCO ” At first glance, Main Street looks flat, but if you’ve ever coasted your bike, or watched skateboarders weave down the boulevard from west to east, you know it’s actually a long downhill run from the confluence of the North and West Tenmile drainages to the present-day marina.
It’s a little hard to picture from ground-level. But from a perch like Mount Royal, or better yet, Wichita or Chief, you can see it. The town is built on an apron of dense glacial debris that spilled out of the steep canyons as ice age glaciers retreated 10,000 years ago. In some town parks, you might find a huge granite boulder, plucked from near the very summit you’re sitting on by a 1,000-foot sheet of ice.
That ice age is just the latest blink of geological time. The landscape around Frisco was shaped by much older forces: Faulting, upheaval and intrusion, spanning unimaginable eons.
Stroll out to the junction of Main Street and I-70 to look at a jumble of primeval black rock. The pre-Cambrian gneiss is splayed with sparkling white veins that exploded as liquid from deep molten chambers to fissure the bedrock, in some places infusing it with precious ore.
Driving west on I-70 from Silverthorne to Frisco is like jumping backthrough time almost 1.7 billion years. Arcing up Lake Hill, you’re driving on the snout of a huge moraine, piles of earth and rock gouged by glaciers just a few thousand years ago.
All of a sudden, you enter Tenmile Canyon, surrounded by the crystalline gneiss, the most ancient rocks visible in this part of the Rockies. The domes and pinnacles of solid dark rock form the very basement of the range and were squeezed, folded and faulted again during the Laramide Orogeny. That epic mountain-building event 70 million years ago created the backbone of North America and left us with the basic shape of the mountains around us.
As the very crust of the planet heaved and buckled, more recent layers of marine sediments were pushed upward. Tenmile canyon runs along a a massive fault line and past uplift is shown by a 10-foot vertical displacement between layers of rock on either side of the freeway. The fault is also the most likely explanation for the course of Ten Mile Creek tumbling down from Copper Mountain. Without help from the crack in the Earth’s crust, it seems unlikely that the stream could have carved through the core of resistant rock.
Wrapping your mind around this geologic scale of time is not an easy thing, but if anyone can do it, it’s Karl Kellogg, of the U.S. Geological Survey, author of the agency’s official geological map for the Frisco quadrangle.
After many field visits, Kellogg can visualize how the ice flowed down from the crest and out of Tenmile Canyon.
“The biggest moraine goes nearly to Dillon, Kellogg said. “As the glaciers retreated there where huge flows and outwash of gravel,” he said, referring to the jumbled hills between I-70 and the Dam Road.
The exact origin of the Rockies is still part of a geologic puzzle, but the basics are understood. The convergence of the Pacific and North American plates about 70 million years ago created massive waves of mountain-building that swept inland from west to east, Kellogg said.
Previously, the area was under a broad, shallow sea, evidenced by layers of sedimentary ocean rock and red-bed sandstone, Kellogg said.
“The dinosaurs were around when Pierre shale was being deposited,” Kellogg
said of a marine layer full of fossils that is especially visible in crumbly road cuts along Highway 9, in Lower Blue valley, north of Silverthorne.
Kellogg will be returning to the field to work on updating the Vail geological quadrangle this summer, and the Summit Daily will join him and talk with other geologists and report on the geologic history of the local mountains in an occasional series. Send an email to firstname.lastname@example.org if you’d like to be notified when a new segment runs.
Go to http://pubs.usgs.gov/mf/2002/mf-2340/ for a pdf version of the Frisco geologic map.
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