longmynd rocks

from The Collaborative International Dictionary of English v.0.48

Longmynd rocks \Long"mynd rocks"\n. pl. (Geol.)
   The sparingly fossiliferous conglomerates, grits, schists,
   and slates of Great Britain, which lie at the base of the
   Cambrian system; -- so called, because typically developed in
   the Longmynd Hills, Shropshire.
   [1913 Webster]