It has long been proposed1, and then observed2,3, that faster rates of soil production occur beneath thinner soils. It remains uncertain, however, whether soil thickness is the driving variable regulating production from the top-down2–6, or whether soil thickness is simply responding to changes in bedrock weathering controlled from the bottom-up7,8. Answering this question is difficult because the feedbacks between soil production and soil erosion, the processes that jointly govern soil thickness2,9,10, respond to perturbations on timescales of thousands to millions of years11,12, timescales that are too long for scientists to observe directly. Here we leverage a space-for-time substitution at a transient mountain range along the San Andreas Fault13–15, where the remarkable tectonic setting allows for independent quantification of uplift, soil production and erosion. We show that, following a pulse of tectonic uplift, the conversion of rock to soil accelerates before the overlying soils thin, but at the same time that topographic stresses increase7 and the rock weakens16. This observation challenges the long-standing assumption that soil production rates are controlled predominantly by soil thickness1,2, and instead lends evidence for a bottom-up, rock strength control on soil production.
