Secondary crater fields are important stratigraphic markers that can shed light on resurfacing processes that have occurred since their formation. We examine the morphologies of secondary craters formed from the ejection of material from two large impacts on the Martian South Polar Layered Deposits (SPLD): McMurdo crater at 84.5¿S, 0¿W, and an unnamed impact at 80.8¿S, 284¿W. The morphologies of these secondary craters allow us to impose constraints on the modification history of the SPLD. We have quantified crater morphologies using data sets from the Mars Global Surveyor and Mars Odyssey missions. We find a complete lack of secondary craters smaller than 300 m in diameter in both crater fields, which implies that at least the upper 30 m of the deposits have been resurfaced since the time of these impacts. Secondary crater depth-to-diameter ratios are low (average of 0.016), indicating that significant degradation has occurred since their emplacement. We find that vertical resurfacing alone is not enough to explain the observed depth-to-diameter distribution and suggest that viscous relaxation of craters coupled with a small amount of vertical resurfacing best fits the data. In the McMurdo field, high depth-to-diameter craters are found preferentially on steeper terrain associated with scarps cutting through the secondary field. This observation suggests that crater modification exhibits a dependence on slope. We comment on possible mechanisms that may explain this observation. The morphologies of secondary craters on the SPLD point to modification processes without lunar parallel and not yet fully modeled for Mars.