WAS CLARENCE DUTTON RIGHT? DOES LATERAL ESCARPMENT RETREAT, RATHER THAN VERTICAL INCISION, ACCOUNT FOR MOST OF THE EROSION OF THE COLORADO PLATEAU? --NEW FINDINGS FOR THE BOOK CLIFFS OF UTAH

Since the earliest explorations, geologists have invoked escarpment retreat as substantial part of the Colorado Plateau’s landscape evolution. Clarence Dutton envisioned “the beds thus dissolving edge wise until after the lapse of millions of centuries their terminal cliffs stand a hundred miles or more back from their original position” when he wrote about this landscape in 1882. Assuming that dryland escarpments, as controlled by sub-horizontal layered strata, maintain about the same relief and profile over geologic time, geometry dictates that the lateral component of erosion is faster than vertical, an ~7:1 ratio. However, despite the advent of multiple geochronometers and the many studies establishing rates of vertical incision for the rivers that drain the Colorado Plateau, only a couple studies have quantified escarpment retreat, and indeed there are only a few well constrained rates of lateral retreat measured the world over.

We apply empirical approaches to quantify escarpment retreat through the study of remnant colluvial slopes, aka. talus flatirons. The study area along the Book Cliffs of central Utah is marked by three generations of talus flatirons constrained by 25 OSL dates to depositional episodes spanning the past 120 ky. Our terrain analysis involves projection of 46 paleo-landforms and discrimination of lateral and vertical distances of erosion through a novel profile-area-integration approach.

Results confirm the geometric expectation that lateral cliff retreat proceeds at 1-3 m/ky, several times faster than vertical incision around the Book Cliffs. In fact, our measurements are in line with early, unconstrained estimates of retreat rates by Clarence Dutton and later Karl-Heinz Schmidt. Although cliffs move laterally faster than canyons are cut, that is not the same thing as hillslope processes accounting for more mass removal than fluvial processes, which will require further analysis.