Lehigh County occupies an area of 347 square miles in southeastern Pennsylvania. The northern part of Lehigh County is underlain by the Martinsburg Formation, which consists chiefly of shale and slate. The central part of the county, where most of the population centers are located and much of the urbanization is occurring, is underlain by alternating beds of limestone and dolomite. From oldest to youngest, these carbonate rocks are the Leithsville Formation, the Allentown Formation, the Beekmantown Group, and the Jacksonburg Formation. The southern part of the county is underlain chiefly by the shales, sandstones, and conglomerates of the Brunswick Formation and by metamorphic and igneous rocks.