Willoughby: Water overhead and fossils underfoot
August 18, 2018
I taught a local geology class for Colorado Mountain College ages ago. One tour looked at remnants of glaciation, the final chapter in Aspen's geologic history. As we traveled down the Roaring Fork Valley, the obvious evidence of the last ice age gripped our imagination.
I enjoyed revealing that at one time we could have looked skyward from Aspen to see only water. The story of the source of that water spanned tens of thousands of years. Continental glaciation had buried the Rockies so deep that sheets of ice planed the points off mountains. This shearing left flat mountaintops such as the top of Aspen Mountain. When the ice sheet melted, local mountain glaciers prevailed. Castle Creek and Maroon Creek valley ice joined a larger glacier in the Roaring Fork Valley. The main glacier pushed and carved until harder basalt rock, just west of Aspen Village, blocked it. Then, as the glacier melted, the blocked end dammed the water. And that water formed the lake that submerged Aspen.
Another glacier slowly slipped down Hunter Creek Valley and dropped into the lake over Aspen. Icebergs calved from that hanging glacier and floated into the lake. The glacier also pushed boulders along the valley and toppled them, along with the icebergs, into the lake. This explains the hefty rocks found in the stretch of river from town to the beginning of the valley.
When a glacier pauses long enough to melt, it leaves a rock pile called a moraine. This pile resembles the bank left by a snowplow: an accumulation of sand, gravel and boulders packed into ice that gradually melts. Moraines act as temporary dams. You can see evidence of several of these pauses in the receding glacier, when you look out from the top of Red Butte, or from a car when you drive along McClain Flats Road. One moraine, one of the largest, stretches across the valley by the airport.
As glaciers melted and receded, water formed streams that eroded the valley bottoms. From the vantage point of the top of Red Butte, or from an airplane window, you may notice the characteristic cross section of Aspen's valleys. Slow glacial carving shaped these valleys like a "U." In contrast, you may see a sharp "V" cross-section in places where fast stream erosion cut a groove in a glacial valley.
If you drive downvalley on the original road to Aspen — the section that is Upper River Road, through the Woody Creek area — you will see the floor of the glacial carving. Walk around where the roadcuts expose the bottom of the glacial material. Look for sand, gravel and boulders that ice deposited over the original sedimentary shale. Not a very hard rock, shale was no match for glacial carving.
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For my students, the concept of iceberg-studded water a thousand feet overhead was a hard act to follow. But I capped that story with an even more engaging activity, a fossil hunt. Much of Aspen's exposed rock is fossil-free igneous. Fossils are not common in red sandstone, either. But the shale formation harbors many of time's secrets.
Highway construction has modified the area since I taught the class. Back then, within a few minutes, every student found a fossil at Shale Bluffs. Next time you drive through the bluffs, note the rock color. Then seek a place where a rock hound may pursue happiness without getting run over.
A "glacial pace" used to mean "slow." Glacial receding at the end of the last ice age has taken place over thousands of years. But within the past 100 years, global warming has greatly reduced the glaciers of the continental U.S. You must use your imagination to picture mountainous sheets of slow moving ice as it shapes the Aspen area. Nevertheless, there is nothing more tangible than a fossil in your hand to connect you to geologic time.
Tim Willoughby's family story parallels Aspen's. He began sharing folklore while teaching Aspen Country Day School and Colorado Mountain College. Now a tourist in his native town, he views it with historical perspective. Reach him at firstname.lastname@example.org.
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