The Devonian and Silurian rocks are generally more resistant to weathering than the limestone, and form some of the highest points on the hills, including the highest at Beacon Batch on Black Down, 325 metres (1068 ft) above sea level. Black Down is a moorland area, with its steeper slopes covered in bracken ("Pteridium") and its flatter summit in heather ("Calluna") and grasses rather than the pasture which covers much of the plateau. The main body of the range is an extended plateau, 6–8 km (4–5 miles) wide and generally about 240 metres (800 ft) above sea level.