 These rocks are prevalent in the Muncy Hills and the lowlands in the southern portion of the county.The Devonian Period rocks in Montour County include the Catskill Formation, the Marcellus Shale, the Helderburg Formation, the Mahantango Formation, the Oriskany Formation, the Marine Beds, and the Onondaga Formation.The other one third of the rocks in Montour County are from the Silurian Period.