What’s 215 feet high, 90 feet long, and has been admired in great art and literature for centuries—from Thomas Jefferson’s “Notes on the State of Virginia” to Herman Melville’s “Moby-Dick” to Frederic Edwin Church’s landscape paintings? Here’s another hint: Jefferson bought it, along with 157 acres of land, from King George III of England for 20 shillings in 1774. And he owned it until he died. The answer is Virginia’s Natural Bridge, a naturally occurring arch over 400 million years old that geologists believe was once the roof of an underground river cave. The Natural Bridge is one of the most frequently depicted sites in 19th-century American landscape painting, according to Christopher C. Oliver, the assistant curator of American art at the Virginia Museum of Fine Arts. Oliver mentioned the fact as part of an impassioned online opening talk for the museum’s new exhibition, “Virginia Arcadia: The Natural Bridge in …