At the margins of the north polar deposits of dusty water ice on Mars, steep scarps reveal layered deposits that preserve a record of the planet's recent climate changes. Some of these scarps are currently the most active places on Mars with annual avalanches and block falls. We estimated the erosion rate of a steep north polar scarp by automatically identifying newly apparent blocks throughout a time series of HiRISE images. The total depositional volume over three Mars years corresponds to a minimum erosion rate of ~0.3 m3 per Mars year per meter along the scarp, or a minimum average scarp retreat rate of ~0.2 m/kyr. This rate cannot balance published 0.01-1 m/yr viscous flow rates, implying that either lower rates of the latter occur or that other processes contribute more than block falls to the slopes' steepness.