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