A subdivision or the American Upper Silurian system,
   embracing the Medina, Clinton, and Niagara epoch. The rocks of the
   Niagara epoch, mostly limestones, are extensively distributed, and at
   Niagara Falls consist of about eighty feet of shale supporting a
   greater thickness of limestone, which is gradually undermined by the
   removal of the shale. See Chart of Geology.