Raindrop imprints may form when the sediment surface is exposed to the atmosphere. Graded bedding develops when sedimentation occurs rapidly from suspension. In graded bedding, grain size changes from fine at the base of a bed to coarse to its top. Dune cross-bedding may form in air, rivers, and on the sea floor. A formation is a distinctive layer or series of rock layers that are geographically extensive.
Formations are parts of larger stratigraphic units called members.
Lithologic units classify rocks only on the basis of their fossil content. Formations generally thicken and thin, laterally, rather than retaining a uniform thickness. Geologic time units are based primarily on a rock's fossil content.