The state can be divided into three natural geographic areas which then can be subdivided into five distinct cultural regions. The natural environment is divided from east to west by the Atlantic coastal plain, the Piedmont, and the Blue Ridge Mountains. Culturally, the coastal plain is split into the Lowcountry and the Pee Dee region. While, the upper Piedmont region is referred to as the Piedmont and the lower Piedmont region is referred to as the Midlands. The area surrounding the Blue Ridge Mountains is known as the Upstate. The Atlantic Coastal Plain makes up two-thirds of the state. Its eastern border is the Sea Islands, a chain of tidal and barrier islands. The border between the low country and the upcountry is defined by the Atlantic Seaboard fall line, which marks the limit of navigable rivers.