Article: SD Petrified Forest
At South Dakota’s Petrified Forest of the Black Hills, a museum open since 1929 near Rapid City, the earth has preserved a cypress swamp 120 million years old. This was the time of dinosaurs—when the Black Hills didn’t exist and the entire area was a flat, tropical swamp (similar to the cypress swamps found now in the southeast U.S.).
To view a petrified cypress forest up close, you can find this exhibit west of Rapid City just about one mile from Interstate-90 at Exit 46. Use your tow vehicle or dinghy for this “off-the-beaten-highway” side trip.
from this Good Sam blog entry