Firestone Drift, Nenthead, Cumbria.
This level was worked in the Firestone Sill and extracted Firestone, a type of hard sandstone *. The level is small with only around 225m of passage. It is strange that it is named as a drift on LLC plans as there is no indication that it every ran to the surface some 50m above. The only access point is the intersection with Armstong's Rise, and subsequently via Longcleugh Shaft (Frog Shaft).
* Sandstones are made up of grains of quartz which can be cemented together by a number of different minerals, iron calcite and silica are common.
The 'firestone' bed in the North Pennines is a competent sandstone with a silica cement which makes it resistant to heat so it doesn't crumble if used for chimneys, hearths, etc hence the name.