 This Allegheny Plateau is so dissected by valleys formed by small streams and springs that it also seems mountainous, but with elevation differences between valley floors and hilltop peaks most often less than a few hundred feet.The plateau is underlain by sedimentary rocks of Mississippian and Pennsylvanian age, which bear abundant fossils, mineral deposits such as iron, as well as natural gas, coal and petroleum.These regions fostered 17th-19th century industries locally even into the 1930s days of mass production.