Black Mountain Sandstone is deposited on top of State Circle Shale conformably. It is made up from thick beds of grey quartz sandstone mostly, but has some beds included of siltstone and grey shale. The grain size is fine to medium. It was originally named by Öpik after the mountain—Black Mountain where it is found. Originally it was believed to be Ordovician, but is actually from the Silurian period, late Llandoverian epoch. Some of the slopes of Black mountain are covered in fanglomerate. The deposition was in a marine proximal turbidite fan, with the turbidity current flowing to the east. There are no fossils, but there is some sedimentary structure including plane, cross or convolute laminations, load casts, slump units and flute moulds.