Soil-Bedrock Interface
The presence of a soil-bedrock interface enhances interflow. The soil-bedrock interface typically occurs in steep terrain where the soil layer is considerably more permeable than the underlying bedrock. Rainwater or snowmelt infiltrates rapidly to the bedrock interface and then moves rapidly downslope along the interface.
Sometimes a feature called a fragipan exists. A fragipan layer has low permeability, like rock or clay, and may also serve to focus the lateral subsurface flow. Fragipan features can exist at relatively shallow depths and play an important role in enhancing both interflow and even surface runoff after the soil layers above the fragipan are saturated.