Island in the bay and Chincoteague, one of 23 barrier islands on the Atlantic coast. The Tidewater is a coastal plain between the Atlantic coast and the fall line. It includes the Eastern Shore and major estuaries of Chesapeake Bay. The Piedmont is a series of sedimentary and igneous rock-based foothills east of the mountains which were formed in the Mesozoic era. The region, known for its heavy clay soil, includes the Southwest Mountains around Charlottesville. The Blue Ridge Mountains are a physiographic province of the Appalachian Mountains with the highest points in the Commonwealth, the tallest being Mount Rogers at . The Ridge-and-Valley region is west of the mountains, carbonate rock based, and includes the Massanutten Mountain ridge and the Great Appalachian Valley, which is called the Shenandoah Valley in Virginia. The Cumberland Plateau and Cumberland Mountains are in the southwest corner of Virginia, south of the Allegheny Plateau. In this region, rivers flow northwest, with a dendritic drainage system, into the Ohio River basin. The Virginia Seismic Zone has not had a history of regular earthquake activity. Earthquakes are rarely above 4.5 in magnitude, because Virginia is located away from the edges of the North American Plate. The Commonwealth's largest earthquake in at least a century, at a magnitude of 5.8, struck central Virginia on August 23, 2011, near Mineral. Due to the area's geologic properties, this earthquake was felt from Northern Florida to Southern Ontario. 35million years ago, a bolide impacted what is now eastern Virginia. The resulting Chesapeake Bay impact crater may explain what earthquakes and subsidence the region does experience. A meteor impact is also theorized as the source of Lake Drummond, the largest of the two natural lakes in the state. The Commonwealth's carbonate rock is filled with more than 4,000 limestone caves, ten of which are open for tourism, including the popular Luray Caverns and Skyline Caverns. Virginia's iconic Natural Bridge is also the remaining roof of a collapsed limestone cave. Coal mining takes place in the three mountainous regions at 45 distinct coal beds near Mesozoic basins. More than 72million tons of other non-fuel resources, such as slate, kyanite, sand, or gravel, were also mined in Virginia . The largest-known deposits of uranium in the U.S. are under Coles Hill, Virginia. Despite a challenge that reached the U.S. Supreme Court twice, the state has banned its mining since 1982 due to environmental