In 1791, Philip Grinder found anthracite coal in what is now Summit Hill, Carbon County, PA. Coal helped the region develop and contributed to America's iron and steel industries. The anthracite coal in the region is known as "stone coal" because of its rock-like hardness. Anthracite is created over millions of years as countless layers of sediment compress plant debris from swamps until it becomes hard. The Delaware and Lehigh National Heritage Corridor follows the route stone coal took from mine to market, winding through northern mountains and along the banks of the Lehigh and Delaware Rivers.