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Abstract To investigate how bedrock transforms to soil, we mapped the topography of the interface demarcating onset of weathering under an east‐west trending shale watershed in the Valley and Ridge province in the USA Using wave equation travel‐time tomography from a seismic array of >4,000 geophones, we obtained a 3D P‐wave velocity (Vp) model that resolves structures ∼20 m below land surface (mbls). The depth of mobile soil and the onset of dissolution of chlorite roughly match Vp = 600 m/s and Vp = 2,700 m/s, respectively. Chlorite dissolution initiates porosity growth in the shale matrix. Depth to the 2,700 m/s contour is greater under the N‐ as compared to S‐facing hillslopes and under sub‐planar as compared to concave‐up land surfaces. Broadly, the geometries of the ‘soil’ and ‘chlorite’ Vp contours are consistent with the calculated potential for shear fracture opening under weak regional compression. However, this calculated fracture potential does not consistently explain observations related to N‐ versus S‐facing aspect nor fracture density observed by borehole televiewer. Apparently, regional compression is only a secondary influence on Vp: the primary driver of P‐wave slowing in the upper layers of this catchment is topographic control of reactive water flowpaths and their integrated effects on weathering. The Vp result is best explained as the long‐term integrated effect of groundwater flow‐induced geochemical weathering of shale in response to climate‐driven patterns of micro‐ and macro‐topography.