A facies is a sedimentary unit formed under specific depositional conditions with consistent characteristics. Facies models depict idealized vertical successions of facies that would be expected from migrating depositional environments within a particular setting, such as a sandy braided river or wave-dominated delta. To construct a facies model, one identifies important surfaces like erosional unconformities, interprets facies using knowledge of sedimentary processes and modern environments, and determines the characteristic vertical succession of facies.