Detrital thermochronology has been proposed as a method for measuring average basin-wide erosion rates. This paper illustrates how to use detrital apatite fission track (AFT) thermochronology to map out where in a basin erosion takes place. Five samples of detrital AFT ages were collected on an alluvial fan that is fed by the Marble Creek drainage basin in the northern White Mountains (California). Using a digital elevation model to characterize the basin hypsometry and a few published basement samples to constrain the age-elevation curve of the catchment, the detrital AFT distribution was predicted. Comparing the observed with the predicted age distributions reveals that localized rock fall events provide the sediment source of the currently active Marble Creek, but a composite sample from the entire alluvial fan indicates that over longer timescales, the entire catchment contributes material to the debris-flow dominated fan, with more material being derived from lower elevations than from higher up the Marble Creek Canyon. The observed AFT age distribution is compared with the hypsometric predictions using the ``Cumulative Age Distribution'' (CAD) which is the cumulative distribution of the measured ages. In contrast with probability density estimators and their cumulative equivalents, the CAD requires no numerical smoothing while properly accounting for (unequal) measurement uncertainties.