Eye of the Beholder ESP_028957_2085 Science Theme: Sedimentary/Layering Processes Nilosyrtis Mensae is an ancient terrain with a wonderful variety of landforms and rock types. What is this circular landform? It probably got its shape from an impact crater long ago, but was subsequently eroded and filled in and eroded again, so that now it is a low mesa surrounded by a boulder-rich geologic unit.