 Harford County straddles the border between the rolling hills of the Piedmont Plateau and the flatlands of the Atlantic Coastal Plain along the Chesapeake Bay and its tributaries.The county's development is a mix of rural and suburban, with denser development in the larger towns of Aberdeen and Bel Air and along Route 40 and other major arteries leading out of Baltimore.The highest elevations are in the north and northwest of the county, reaching 805 ft. near the Pennsylvania border in the county's northwestern corner.