The Bass Formation is the lowermost formation of the Unkar Group in the eastern Grand Canyon, resting on much older Vishnu Basement Rocks and preserving a major ancient erosional surface at its base. First deposited as river sands and gravels in eroded valleys, it later accumulated mostly in shallow marine settings as the sea advanced from the west, with changing conditions recorded by stromatolites, ripple marks, mudcracks, and other sedimentary features. Its mixed sedimentary rocks vary across the canyon and were later intruded by basaltic sills and tilted by faulting. Dated to about 1,254.8 million years ago, it is a key record of Precambrian sedimentation, marine flooding, tectonic deformation, and erosion in the Grand Canyon.