Really ancient history
Aspen Times Weekly
Imagine, from your perch at the top of the Ute Trail, watching icebergs float across a lake at your feet as you gaze across the Roaring Fork Valley. Had you visited the area during the most recent glacial period you would have witnessed the final sculpting of Aspen’s landscape.
Several glaciations shaped the Rockies. During continental glaciation, thousands of feet of ice covered and moved across the Aspen area. From atop Smuggler and Aspen Mountains you can see that the mountain tops are generally flat and of the same elevation. Glaciers planed them to a relatively uniform height.
Those continental ice sheets receded into a network of interconnected mountain glaciers that carved Aspen’s major valleys: Maroon, Castle, Conundrum and the Roaring Fork. As you peer down these valleys (especially Maroon or Castle) from on high, you will see the telltale U-shape hewn by masses of slow-moving ice. Rivers subsequently eroded the glacier’s carvings, cutting the finishing touches and trenches into local terrain.
Glaciers easily plowed and gouged the valleys, especially in areas of soft sedimentary rock. At higher elevations they often sliced through granite. You can find glacial polish on granite surfaces in the Grottos area on Independence Pass. Thick layers of ice and sand moved slowly over stationary rock there, grinding and polishing for more years than you can imagine. However, the combined strength of several mountain glaciers did not push through the block of basalt that forms the mountain just west of Aspen Village. When driving down the Roaring Fork valley, you may notice that the valley significantly narrows at that point. The glacial valley ends and it turns into a stream-cut canyon.
Glaciers resemble giant snowplows as they push aside and leave mounds of material, called moraine, to their sides and where they stop. Two moraines dominate the valley. One crosses near the airport, and you drive over the other as you ascend from town level at the Cooper Street Bridge to the higher valley in the Stillwater area. These mounds of sand and boulders have been altered by water, but maintain the same appearance they had when the glaciers shrank in size. At the end of glaciation, a small lake formed behind the latter moraine. That moraine trapped tons of sand and gravel, stretching all the way to the Difficult Creek campsite.
The most interesting glacial event occurred when ice piled up near Aspen Village, forming a temporary dam. Water backed up, covering Aspen. Next, a hanging glacier flowed down Hunter Creek Valley and broke off into the lake. Icebergs floated across the lake and down the valley. The glacier dropped granite boulders that it had carried from the upper end of the valley. That is why you find so many boulders along Hunter Creek as you hike upstream from town.
Had pioneers prospected the area during the end of glaciation instead of 10,000 years later, the Roaring Fork would have looked to them more like the high elevations of the Canadian Rockies. Mounds of gravel, rocks and boulders haphazardly strewn across valleys by glaciers would remain too coarse to support vegetation. Instead of naming their settlement “Aspen” after the trees, they might have named it “Boulder.”
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