Layers in the upper martian crust record a complex history of deposition and erosion. We examined a region between 2¿--9¿N, 1¿--8¿W and found that light-toned, layered, sedimentary rocks in northwest Sinus Meridiani also occur in southwest Arabia Terra, where they are mantled by dust. The rocks are divided into four distinct stratigraphic units. Impact craters of diameters 30--60 km are interbedded with the ~200 m-thick units, attesting to their antiquity, revealing the presence of temporal unconformities between units, and indicating that the sequence spans considerable time. A crater at 8¿N, 7¿W, containing hundreds of repeated sedimentary layers, is stratigraphically lower than most of the four-unit section. The layers in the crater formed in a different environment than the layered material outside the crater. Deciphering the geologic history of Mars requires recognition that the planet has more than a cratered surface; its crust is a cratered volume.