at the Tennessee River in West Tennessee. Uncertainties in the latter 19th century over the location of the state's border with Virginia culminated in the U.S. Supreme Court settling the matter in 1893, which resulted in the division of Bristol between the two states. An 1818 survey erroneously placed Tennessee's southern border south of the 35th parallel; Georgia legislators continue to dispute this placement, as it prevents Georgia from accessing the Tennessee River. Marked by a diversity of landforms and topographies, Tennessee features six principal physiographic provinces, from east to west, which are part of three larger regions: the Blue Ridge Mountains, Ridge-and-Valley Appalachians, and Cumberland Plateau, part of the Appalachian Mountains; the Highland Rim and Nashville Basin, part of the Interior Low Plateaus of the Interior Plains; and the East Gulf Coastal Plain, part of the Atlantic Plains. Other regions include the southern tip of the Cumberland Mountains, the Western Tennessee Valley, and the Mississippi Alluvial Plain. The state's highest point, which is also the third-highest peak in eastern North America, is Clingmans Dome, at above sea level. Its lowest point, , is on the Mississippi River at the Mississippi state line in Memphis. Tennessee has the most caves in the United States, with more than 10,000 documented. Geological formations in Tennessee largely correspond with the state's topographic features, and, in general, decrease in age from east to west. The state's oldest rocks are igneous strata more than 1 billion years old found in the Blue Ridge Mountains, and the youngest deposits in Tennessee are sands and silts in the Mississippi Alluvial Plain and river valleys that drain into the Mississippi River. Tennessee is considered seismically active and contains two major seismic zones, although destructive earthquakes rarely occur there. The Eastern Tennessee Seismic Zone spans the entirety of East Tennessee from northwestern Alabama to southwestern Virginia, and is considered one of the most active zones in the Southeastern United States, frequently producing low-magnitude earthquakes. The New Madrid Seismic Zone in the northwestern part of the state produced a series of devastating earthquakes between December 1811 and February 1812 that formed Reelfoot Lake near Tiptonville. Topography The southwestern Blue Ridge Mountains lie within Tennessee's eastern edge, and are divided into several subranges, namely the Great Smoky Mountains, Bald Mountains, Unicoi Mountains, Unaka Mountains, and Iron Mountains. These mountains, which average above sea level in Tennessee, contain some of the highest