In geology, a facies (/ˈfeɪʃɪ.iːz/ FAY-shih-eez, US also /ˈfeɪʃiːz/ FAY-sheez; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formation) and the changes that may occur in those attributes over a geographic area. A facies encompasses all the characteristics of a rock including its chemical, physical, and biological features that distinguish it from adjacent rock.
The term "facies" was introduced by the Swiss geologist Amanz Gressly in 1838 and was part of his significant contribution to the foundations of modern stratigraphy, which replaced the earlier notions of Neptunism.