Uplift: Why We Rise Above the Valley

Monument Canyon in autumn, showing tilted rock layers along the monocline

NPS Photo / N Scarborough

 

Summary

"Uplift" is when a landscape is pushed up from below, usually over a large area. Several uplifts happened in this area across the eons. The uplift that formed the Uncompahgre Plateau (which includes modern Pinyon Mesa and Unaweep Canyon) activated the Redlands Fault. We rise hundreds of feet above the Grand Valley because of this most recent uplift. Movement of tectonic plates, the pieces that make up Earth's crust, is what causes this uplift.

 

This generalized cross section reveals the bend in the rock layers from the Valley floor to the Monument's cliffs.

NPS Diagram

Monocline: One Slope

A monocline forms the lazy S shape that connects rock layers from the Valley floor to the Monument cliffs. Where rock layers were pushed up, some bent, some broke, and many have eroded. The generalized cross section reveals how we can see the effects of this uplift in the rock layers within and outside the Monument.

 

One Piece of the Grand Valley

The diagram below reveals where the south (left) side of the Grand Valley was uplifted. The area at the edge of the movement where the rock layers broke is called a fault, nowadays called the Redlands Fault. Rock layers that make up the Book Cliffs on the north side of the Grand Valley also once stretched across the entire area. At one time, all these rock layers piled above the present-day Monument. You can imagine extending the lines from the right side of the diagram to the left. All these layers were buried underground until the ancestral Colorado River began carving the Grand Valley.

As the Colorado River quickly cut down through the soft Mancos Shale, streams drained from higher up on the Uncompahgre Plateau (south of the Monument, present-day Pinyon Mesa, Unaweep Canyon, and the highlands between Gateway and Montrose). These streams used to connect with the Colorado River. Nowadays, only intermittent streams make it to the river from within the Monument.

Eventually, the river cut down far enough that the Precambrian rocks were exposed along the Redlands Fault. This metamorphic rock layer is so hard that it doesn't erode as easily as all the sedimentary layers above it. Colorado National Monument is one of the few places in western Colorado that you can see this layer on the surface. It exposes rocks that have been around since before most complex life on Earth.

 

Grand Valley Cross Section revealing the Monument's rock layers and the Book Cliffs across the valley

NPS Diagram