The primary mountains within the county (From west to east: Wills, Evitts, Dunning, and Tussey mountains) extend from the southern border with Maryland to the northeast into Blair County, and are held up by the Silurian Tuscarora Formation, made of quartz sandstone and conglomerate. Chestnut Ridge is a broad anticline held up by the Devonian Ridgeley Member of the Old Port Formation, also made of sandstone and conglomerate. Broad Top, located north of Breezewood, is a plateau of relatively flat-lying rocks that are stratigraphically higher, and thus younger (Mississippian and Pennsylvanian), than most of the other rocks within the county (Cambrian through Devonian). Broad Top extends into Huntingdon County to the north and Fulton County to the east.