The population of rocks larger than 15 cm in diameter was measured at 36 locations imaged by the Spirit rover over ~ 4 km of the traverse across the Gusev plains and Husband Hill in Gusev crater on Mars. The rock population observed on plains surfaces is consistent with impact fragmentation of rubbly/fractured volcanics and reveals little evidence for modification by secondary processes or weathering. Interpretation of counts from Husband Hill suggest an influence by bedrock for rocks larger than 0.5 m across, but the distribution of smaller rocks is consistent with ejecta possessing contributions from pre-plains impact events. Results indicate plains surfaces experienced only tens of centimeters of eolian erosion/deposition since the Hesperian, whereas meters rather than tens of meters of erosion modified Husband Hill since the early Hesperian.