The term facies is widely used in geology, particularly in the study of sedimentology in which sedimentary facies refers to the sum of the characteristics of a sedimentary unit. These characteristics include the dimensions (sorting, rounding), sedimentary structures, grain sizes and types, colour (colour can be very useful in the interpretation of depositional environments, Black Colour indicates deposition in the absence of oxygen in either the ocean, lakes, or swamps whereas Red Colour indicates deposition in the presence of abundant oxygen in a warm, humid terrestrial environment), and biogenic content of the sedimentary rock.
Walthers Law of Facies, states that the vertical succession of facies reflects lateral changes in environment. Conversely, it states that when a depositional environment migrates laterally, sediments of one depositional environment come to lie on top of another.