Conference Schedule >

Evolving Shape of the Rockies

Robert S. Anderson
Professor, Earth Sciences
University of California, Santa Cruz

Smooth peaks, glacially ornamented valleys and deep river gorges in the Front Range: Late Cenozoic evolution of the Rocky Mountains

The Laramide ranges, of which the Front Range of Colorado is an excellent example, formed by thrust faulting some 80-50 million years ago. Others are the Wind River, Bighorn, Laramie, and Medicine Bow ranges of Wyoming, and the Black Hills of South Dakota. This deep-seated faulting brought old, hard crystalline rock into contact with the sedimentary cover, as the crystalline rock upwarped or overrode the sedimentary cover emplaced by Mesozoic seas. It is the subsequent evolution of the range that I will focus on. I will first summarize the broad character of the topography of the Front Range, emphasizing those features that are in common with other such ranges. I will then suggest a scenario by which this topography has evolved, and discuss how we are going about studying this.

The Front Range is roughly 50 km from front to crest, from the base of the Flat Irons to the continental divide. The range is bordered by a very flat basin floor with a number of flat smooth surfaces marking former levels of the plains, among them the prominent Rocky Flats surface between Boulder and Golden. We are all familiar with the abrupt front, where the range steps upward by 800 m from the adjacent plains. If one avoids the canyon floors, a walk from plains to crest would then encounter a rolling surface at 8000-10000 feet (2500-3000 m) elevation that stretches toward the spine of the range. This has been called the sub-summit surface. The topography then steps up by another kilometer within the last few kilometers to the narrow summit spine that forms the drainage divide between Colorado and Mississippi rivers. On the range crest itself, many of the peaks are very smooth tundra-covered surfaces, with occasional bedrock knobs poking through shallow regolith. It is easy walking; the work of the climb to the summits is largely done getting to the ridge. A walk up the river canyons would be significantly different. From the gently sloped river of the plains, one would ascend through a steepening deep bedrock gorge whose walls get less and less deep as one gets further into the mountains. Many of these canyons emerge into reaches of river that are much more gentle, and less deeply incised into the subsummit surface. Further upstream one encounters the end moraines from the glaciers that have periodically occupied the headwaters. Thereafter, the character of the valley profile alters significantly. Major steps and flats dominate the profile up to the tip of the drainage, where the channel form halts and one is left standing in a cirque basin or a bench just below the divide.

While the details vary from range to range, and from drainage to drainage, these broad characteristics are our target. I argue that the broad smooth summit surfaces are very slowly eroding landforms whose parabolic form is adjusted to the rates of weathering of bedrock, and the rate of downslope motion of the soil produced by this weathering. We have used cosmogenic radionuclides to document the rates: a mere 5-10 meters per million years (microns per year). These surfaces are presently decoupled from the adjacent glacial canyons. They go about their slow business of erosion independent of the valley below, not changing shape significantly over very long times. It is our view that this smooth low relief character dominated the topography of the crystalline core of the range prior to say 4-8 million years ago. That this smooth surface and the lazy streams draining it merged with the depositional surface of the Great Plains can be imagined easily when looking back at the mountain front from well out on the plains. Some ill-understood event then occurred to allow the basins to erode. The Platte, the Bighorn, the Yellowstone, the Wind, the Green, all these have eroded to yield a staircase of abandoned surfaces out in the basins. We are attempting to constrain this history of basin lowering by dating the terraces that bound the range front, again using cosmogenic radionuclides. We then explore how the ranges have responded to this lowering.

The easily eroded nature of the sedimentary rock within these basins allows them to erode much more rapidly than the streams within the mountain front. This effectively causes a base level fall for the range-draining rivers. Given that all the rivers of a range have seen the same baselevel fall, the profiles within the mountain front become probes of the importance of drainage basin characteristics. These river profiles are in varying states of transience in responding to this base level fall. The lower reaches of the rivers have caught up, while the upper reaches have not yet 'felt' the baselevel fall. This has produced profiles with broad convexities, downstream from which the stream has responded to the baselevel fall, upstream from which it has not. The deep incision of the lower reaches has steepened the adjacent hillslopes, creating the canyon walls perfect for rock climbing.

The glacial troughs in the headwaters reflect the very efficient erosion of glaciers, which have grown and decayed dozens of times within the Quaternary (last couple million years). The legacy of this activity is a moraine record, and a strong signature of steps and flats within the valleys. It is likely that the erosion rates during glacial times reached one to several mm/year, hundreds of times greater than those on the high surfaces. Relief has therefore undoubtedly increased in the headwaters during the Quaternary. The major steps can be attributed to jumps in drainage area at tributary junctions. The glaciers impact the rivers downstream as well. So much sediment is produced during glacial times that it likely chokes the river system downstream, causing aggradation of coarse sediment within the canyons, and sluicing out of the range front as cobble-rich sediment.

Finally, we must acknowledge that the rearrangement of mass on the earth's surface, here by erosional removal, must drive rock uplift. Erosion of the broad basins drives isostatic uplift whose magnitude is dictated by the density of the crust and local upper mantle. The narrow ranges come along for the ride on the flexural shoulder of the basins. This drives uplift of the peaks, which we have argued are eroding very slowly. This in turn incites a strong feedback in which the higher topography, with a strong step at the range front, should alter the pattern of precipitation, and how much of it comes as snow, as the range topography evolves.