An impact basin is a large complex impact crater. The threshold diameter to distinguish between craters and basins is approximately 150–200 km. In general, basins are characterized by two or more concentric rings, which are ridges or scarps facing toward the basin. One of these rings is the main rim that borders the cavity from which material was excavated and ejected during the impact. Most basins are heavily degraded or have been covered by younger material. All known basins are old impact features created during the first 800 million years of the planet or satellite on which they are found. Examples are Caloris basin on Mercury, the Orientale basin on the Moon, and the Hellas basin on Mars.