Some areas of the White Peak exhibit contemporaneous basalt flows (e.g. Ravens Tor at Millers Dale), as well as subsequent dolerite sill intrusion at a much later stage (e.g. near Tideswell Dale), whilst mineralisation of the carboniferous limestone in a subsequent period created extensive lead and fluorite deposits which have formed a significant part of Derbyshire's economy, as did coal mining. Lead mining has been important here since Roman Times. The more recent river gravels of the Trent valley remain a significant extractive industry today in south Derbyshire, as does the mining of limestone rock in central and northern parts of the county. Coarse sandstones were once extensively quarried both for local building materials and for the production of gritstone grinding wheels for use in mills, and both former industries have left their mark on the Derbyshire landscape.