U.S. Route 16A The route passes through Keystone, South Dakota; Mount Rushmore National Memorial; The Norbeck Wildlife Management Area and Black Elk Wilderness within the Black Hills National Forest; and Custer State Park (including State Game Lodge and Legion Lake); before rejoining the parent highway. US 16A is famous for its scenic, one-lane tunnels aligned to frame the faces on Mount Rushmore, its "pigtail bridges", and its sections of divided highway but with single (and narrow) lanes on each roadway. It is the only route which can be used to drive through Custer State Park without having to pay an entrance fee for
Mount Rushmore selected Mount Rushmore as the site for several reasons. The rock of the mountain is composed of smooth, fine-grained granite. The durable granite erodes only every 10,000 years, thus was more than sturdy enough to support the sculpture and its long-term exposure. The mountain's height of above sea level made it suitable, and because it faces the southeast, the workers also had the advantage of sunlight for most of the day. The Mount Rushmore area is underlain by well drained alfisol soils of very gravelly loam (Mocmount) to silt loam (Buska) texture, brown to dark grayish brown. Mount Rushmore has