Weathering of bedrock is a central facet of landscape development, yet surprisingly little is known about processes or their rates. Rock weathering is a precursor to mobile regolith formation, and hence sets the stage for sediment production, and therefore controls the pace of landscape evolution. We explore rock weathering in Boulder Canyon in the Colorado Front Range, where hillslopes comprise both bedrock outcrops and thin mobile regolith cover. The canyon results from upstream propagation of a knickzone from the contact between the Boulder Creek batholith and Mesozoic sedimentary rocks at the range front.

Rock weathering is expected to increase with time that rock has been exposed at the surface, and to be promoted by burial under thin soil cover. We therefore made measurements at sites in Boulder Canyon to test differing surface exposure ages and covered versus uncovered conditions. We chose outcrops in the canyon from the mouth to the top of the knickzone to look for the effect of surface exposure age; we hypothesize that degree of weathering will increase with distance from the knickzone. To study the effect of soil cover, we examined bedrock exposed by debris flows on steep (>25°) slopes during the September 2013 Front Range storm. Based on models of soil production rate, we expect greater weathering in rock that has been covered by mobile regolith, such as rock exposed by debris flows.

At each sample site, we used a Proceq SilverSchmidt hammer to test rock strength, measuring rock rebound values along 5-20 m long transects. We also counted the number of fractures crossing each transect. At selected sites, we collected samples for XRD mineralogical analysis.

In the survey of outcrops in the canyon, we found a slight decrease in rock strength with distance from the knickzone. This accords with rock weathering increasing with time since the passage of the knickzone. In the rock exposed by debris flows, however, we found that rock strength was often greatest in the thalweg of the debris flow channel. We observed decreasing rock strength from debris flow mouth to its initiation point. These observations suggest that debris flows scour their flowpaths, removing weakened rock where the flux of sediment is greatest, thus confounding our expectation of using the debris flow exposures to study rock weathering under regolith cover.