THE INFLUENCE OF BEDROCK TORS ON MOBILE-REGOLITH FLOW IN A CREEP-DOMINATED LANDSCAPE, BOULDER CREEK CRITICAL ZONE OBSERVATORY

The downslope movement of mobile regolith is a crucial process in the evolution of soil-mantled landscapes. Hillslope creep by a variety of biological and physical processes is likely the dominant form of mobile-regolith movement on unglaciated, montane hillslopes in the Colorado Front Range. However, many montane hillslopes are interrupted by a high density of bedrock tors, which likely impede creep-related processes. We hypothesize that bedrock tors affect creep movement of mobile regolith by: (1) creating a sediment pond on the upslope side of tors; (2) causing mobile regolith motion to bifurcate around tors once this sediment pond generates sufficient lateral slopes; (3) causing a deficit of sediment downslope from tors.

We investigated mobile regolith depth and soil properties around mid-slope tors on north- and south-facing slopes in Gordon Gulch, within the Boulder Creek Critical Zone Observatory (BcCZO). We excavated four trenches from the tor/mobile-regolith contact, extending 2 m directly upslope, 1 m downslope and 1 m adjacent to each tor along contour. Soil samples from each trench site and detailed topographic profiles of the tor and surrounding hillslope were also collected.

Preliminary data from 4 tor sites indicate that there is indeed a ponding effect, at least topographically. These ponds are often more shallow than expected. We hypothesize that this is due to the soil pond encroaching upon the upslope side of the tor. Although the tor is likely growing in height, due to the contrast between lowering rates on tors relative to soil-mantled bedrock, the upslope contact is being slowly buried by mobile regolith. Contrary to our hypothesis, trenches downslope of the bedrock tors often revealed thicker mobile regolith than expected; however, these soils are more oxidized and appear to be derived from weathered rock below rather than mobile-regolith discharge from above. Trenching revealed no consistent trend in the mobile-regolith depth on the side of the tors. Overall, mobile-regolith depths in trench sites on the north-facing slope are greater than those on the south-facing slope. This may reflect the role of increased vegetation impeding mobile-regolith creep. Ultimately, these data will be used to calibrate a 2-D numerical model of mobile regolith flow that accounts for tor-hillslope dynamics.