When my family wants a ski vacation, we visit Aspen and Snowmass Village, Colorado. Ready to head to the slopes?
Sharon Lyon
Editor, GeoLifestyle
The Geology of Aspen and Snowmass Village
Powder skiing on Big Burn, Snowmass Ski Resort.
The Aspen area is located in the Roaring Fork drainage basin of the Colorado River, west of the Continental Divide, along the eastern part of the Elk Mountains.
The area’s geology is a complex story of Precambrian granitic basement rocks, orogenies, erosion and deposition cycles, intrusions, metasomatism, and glaciation.
From the Late Cambrian through the Mississippian, the Aspen region was part of a broader area alternately covered by shallow seas or occupied by low-lying land. Shelf quartzites and carbonate formations were deposited during this time, overlying a variety of Precambrian granitic and metamorphic rocks.
In late Paleozoic and early Mesozoic times, the ancestral Rockies ascended, comprising two north-south trending ranges: the eastern branch located near today’s Front Range, and the western branch through modern-day Grand Junction.
As these twin ranges rose, erosion created thick aprons of sediment encircling their flanks. Gravel, sand, and mud more than 10,000 feet thick accumulated in the intermontane basin known as the Central Colorado Trough, between the two ancestral ranges.
Over millions of years, tremendous pressure and fluids circulating through these sediments cemented the grains into coarser rocks near the mountain front and finer-grained mudstone farther out in the basin. Iron-rich minerals rusted, creating rocks with a distinctive red hue.
Between 72 and 40 million years ago, uplift, folding, and faulting during the Laramide orogeny built the modern-day Rockies, as the Farallon Plate collided in a low-angle subduction with the North American Plate. In the Aspen area, the rise of the Elk Mountains uplifted the sedimentary rocks of the Central Colorado Trough, producing most of the range’s spectacular peaks.
Intrusions of porphyries and granodiorites, with the emplacement of associated silver, lead, and zinc mineralization occurred between late Cretaceous to Oligocene times. These are part of a belt of Laramide-age plutons known as the Colorado Mineral Belt.
Pleistocene glaciations have sculpted the landscape. The major mountain valleys are glacial troughs modified by postglacial alluvial fans, talus, and landslides. Streams occupy hanging valleys. Moraines and outwash dominate the major stream valleys of the Roaring Fork and Snowmass Creek.
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The Maroon Bells in early summer. Trees from a winter avalanche clog Maroon Lake behind me.
My version of skiing can be described as “throwing my body down the mountain,” but the rest of my family tackles the black diamonds. At Snowmass Ski Area, they recommend Big Burn on powder days and the exquisite view of the Maroon Bells from the top of Elk Camp.
Geo facts:
The Maroon and North Maroon peaks are recognized by their heights (they are two of Colorado’s famous fourteeners), as well as their distinctive color and layered appearance.
Comprised of red terrigenous clastic rocks, with minor limestones and evaporites, the Maroon Formation was deposited in a fluvial floodplain environment in the Central Colorado Trough during the late Pennsylvanian and early Permian.
Tips for visiting: In warmer seasons, visit the Maroon Bells scenic area and hike around Maroon Lake. If you visit in the spring, make sure the road has been cleared of avalanches. During the summer, buses depart from Aspen Highlands every twenty minutes.
Stew stop: After your day of trekking or skiing, refuel at The Stew Pot in Snowmass Village. There’s nothing like a warm bowl of stew or soup after a cold day on the slopes.
In 2010, construction workers expanding the Ziegler Reservoir near Snowmass Village encountered buried bones and tusks. Scientists from the Denver Museum of Nature & Science consulted on the finds, and a dig commenced.
What they found:
Bones and teeth from 50 different Pleistocene species, including 35 American mastodons, four Columbian mammoths, four ice-age bison, a ground sloth, a camel, beavers, frogs, lizards, birds, and rodents.
Snails, insects, leaves, and tree trunks. Some of the insects are still iridescent and the leaves green.
Why it matters: Snowmass Village sits at 8,300 feet, making this site one of the highest Pleistocene fossil localities ever discovered.
Mastodons and mammoths did not live in the same environment. Dating explains why they were both found here:
The oldest layers were dated at about 140,000 years old, when much of Earth was glaciated. Between 130,000 and 115,000 years ago, temperatures had warmed during an interglacial event. Forests dominated the area—the preferred environment for browsing mastodons and sloths.
Between 100,000 and 87,000 years ago, the area was cold: No fossils were found from this period.
Grasslands spread across the region 87,000–60,000 years ago, along with grazing mammoths and camels.
A new glacial period began 55,000 years ago, at the end of the sequence.
Future work: The dig concluded when the reservoir needed to be refilled. The fossils unearthed represent only about 10 percent of the total assemblage. Perhaps, future paleontologists will get a chance to expand their knowledge from this site.
Visiting the Snowmastodon site:
Ziegler Reservoir today appears an undisturbed alpine pond; however, visitors can view the lake as it appeared in the Pleistocene through the Ice Age Discovery Exhibit.
The trails are open to hikers and uphill mountain bikers in summer, and to snowshoers in the winter.
Get your free passport at guest services to mark off the stops in Snowmass Mall and Base Village, then collect your free mastodon stuffed animal.
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