Gilwern Hill is formed from a series of rock strata of Carboniferous age, each of which dip moderately southwards. The summit plateau is formed from sandstones - the Farewell Rock and the Twrch Sandstone which overlie the Carboniferous Limestone. The latter has been extensively quarried around the eastern, northern and western margins of the hill where it is exposed. Derelict tramways are in evidence in connection with the working of these quarries for limestone in connection with iron and steel-smelting.