Fig. 2.

Controls on the fresh-bedrock profile and thickness of the weathered zone under hillslopes. (A) Conceptual illustration of the dependence of the weathered zone thickness on the mean hillslope gradient, S_h, and mean groundwater table slope, S_w, and thus soil transport and hydraulic properties (all terms defined in the text). (B) The dependence of the ratio of fresh-bedrock relief to hillslope relief at the ridgetop, Z_b0/Z_s0, on S_h and S_w illustrates the parameter space for which the weathered zone is expected to be limited in vertical extent by drainage of fresh bedrock. Observations from three sites (solid symbols) and data associated with the example profiles shown in Figs. 3 C–F (open symbols) are plotted. Open circles represent results of the linear soil flux model (Eq. 5) and open squares represent the nonlinear model (Eq. S12). The dark gray area indicates where S_w exceeds S_h and thus bedrock is expected at the surface. The light gray area indicates where S_w is so low relative to S_h that Z_b0 is essentially at the elevation of the channel. (C–F) Four example profiles for a fixed hillslope length (L = 100 m) and lowering rate (C_o = 0.1 mm/y) demonstrate the influence of soil diffusivity, D, and the ratio of hydraulic conductivity to porosity, , on the thickness and residence time of the weathered bedrock zone. Fresh bedrock is denoted by dark gray. The surface topography was calculated using the nonlinear model (29) assuming a critical slope, S_c, of 1.2 (Eq. S10). The corresponding linear profiles are shown in Fig. S2.