Not only was the valley infilled over time, but large impact craters on the surrounding plains were also almost entirely buried (see annotated image). Only small hills in a circular shape, which were once part of the crater rim, indicate the existence of these craters. In the annotated image only one of them is noted – can you spot the others? The plains appear very smooth and are partly dotted with rounded hills and mesas (flat plateaus). These resistant rock formations are remnants of a formerly more continuous surface layer that eroded over time. The infilling material could consist of lava flows. In some areas, the surface appears crumpled, which is a common feature on volcanic plains known as wrinkle ridges (see annotated image). These form due to the contraction of cooling material at the surface, causing it to compress and fold slightly. Although many large impact craters are buried in this scene, some younger craters formed after the infilling occurred. These craters show a prominent ejecta blanket, which may indicate the presence of subsurface ice (see annotated images).