Significance of Thick Deposits of Colluvium on Hillslopes: A Case Study Involving the Use of Pollen Analysis in the Coastal Mountains of Northern California

Distinct bedrock hollows with a colluvial fill are common in soil-mantled hillslopes. It has been proposed that these hollows are sites of recurrent emptying by landsliding and subsequent slower refilling with colluvium. On a hillslope in Northern California we have quantified the time since emptying and the three-dimensional configurations of the underlying bedrock surface. Correlation of the pollen record in the colluvium with a nearby pollen core from a lake with an accurate age control indicates that the oldest colluvium was deposited 11,000 to 13,500 years in the bedrock hollow. Since then deposition of colluvium in the hollow has been equivalent to 1.5 m of landscape lowering in the surrounding source area and to a local lowering rate of about 0.1 mm/yr. The longitudinal profile of the hollow is concave upward, a form that causes the groundwater flow to rise toward the surface downslope, leading to saturation and possibly to landsliding.