geological formation

In the study and mapping of the earth's surface, geologists classify a geological formation as any basic unit of rock that stands out from its surroundings. A geological formation, therefore, can be small or very large, but must be distinct in appearance as well as substantial enough to chart on a map. Geological formations are often given names based on their location and their dominant rock type. For example, the El Malpais sandstone bluffs are a unique formation of sandstone, located in the El Malpais National Monument near Grants, New Mexico.