The shape of a crater can depend on factors including the angle of impact and the pre-impact slope and topography.
This image shows a roughly 3-kilometer impact crater, formed on the sloping walls of Tithonium Chasma, part of the large Valles Marineris canyon system. We can see that this crater is non-circular, measuring about 3 by 4 kilometers. The ejecta—the debris that is generated and thrown out by an impact—will typically distribute itself evenly around the outside of the crater rim where the pre-impact surface is flat and the angle of impact is not too low. However, due to the highly inclined nature of the surface here, the ejecta deposited preferentially downslope, forming a tongue-like deposit. … (HiRISE)