Monument on the Move

By Professor Bill Hood

Colorado National Monument preserves a spectacular landscape of red rocks, steep-walled canyons, towering monoliths, and a wide variety of plant and animal life. The basis for all of this magnificent scenery is the geology: the underlying rocks and the processes that first formed, then deformed them, combined with the constant forces of water, freezing and thawing, and wind that continuously erode the rocks away.

The story of how the landscape of Colorado National Monument came into existence is at the same time both simple and complex. The simple side tells a story of extensive erosion over a long period of time. The complex side covers how events that occurred tens of millions of years ago set up the conditions that allowed the monument to be unique and distinct from the nearby landscapes of the Grand Valley and the Book Cliffs.

Erosion processes in the canyons can be described as "steady by jerks." If you look at the streams in the bottoms of the canyons, most of the time they barely flow, much less move sediment downstream. Even in the spring when snowmelt causes streams to flow, you won't see much evidence of sediment moving downstream. So how can these little streams carve the huge canyons?

Imagine this:
It's summer. Uneven heating of the land surface causes towering thunderheads to build up. A massive storm develops, dumping two inches of water over a few square miles in an hour's time. What happens to all of this water?

As the water level builds up, more and larger sediment is transported. Eventually all of the sediment on the stream bed may start moving, exposing the bedrock below. The result is like a giant wet abrasion machine grinding away at the bed of the stream. Banks cave in, adding more sediment. Erosion and transportation are at work!

Eventually the storm passes or dies away. The influx of water slows and almost stops. Stream levels drop, and, as they do, the erosion process reverses and becomes the depositional process. With lower velocity comes reduced ability to move particles. Sediment starts to drop out and quit moving, large particles first and small ones later. Sediment builds up and again covers the bed of the stream. The storm is over. Preserved throughout the monument is evidence of such storms.

The forces of water and wind, freezing and thawing, acting over vast spans of time, built the spires, domes, and sheer canyon walls of Colorado National Monument. As the Colorado River relentlessly lowered the Grand Valley north of the monument, small tributary streams within the monument labored to keep pace as they cut through the thick sandstone layers. Thwarted by the tough black basement rocks (Precambrian) of the canyon floors, they gradually widened the chasms to reveal the colorful red cliffs that we enjoy today. Remnants of the sandstone walls make up fascinating monoliths, such as Kissing Couple, Coke Ovens, and the 450-foot high Independence Monument.

What does the future hold for Colorado National Monument? As the mighty Colorado River continues to erode down through the soft sandstone, the difference in elevation between the Grand Valley and the monument will become more dramatic — leaving the spectacular hanging canyons of the monument perched even higher above the valley floor.

 

Erosion at work, run-off scours a streambed after a big rainstorm.

NPS