65 to 60 million years ago: As dinosaurs die out, shallow inland sea drains away. Rockies and Black Hills emerge. Jungle growth transforms dark mud and shale of former sea bed to yellow soil.
37 to 23 million years ago: Periodic floods deposit layers of mud and volcanic ash. Many species of mammals roam the area, their fossils leaving a clear record of the Oligocene Epoch (``golden age of mammals'').
500,000 years ago: Erosion from wind, rain, frost carves the soft rocks into unusual formations, a process that continues.
12,000 years ago: First humans (``the mammoth hunters'') arrive.
1700s A.D.: Lakota establish first long-term settlement.
1890: Army kills 200 Lakota followers of Chief Big Foot at Wounded Knee.
Early 1900s: Homesteaders and ranchers settle the area.
1939: Federal government designates Badlands a national monument as last of the settlers move on.
1976: Government acquires 133,300 more acres in the area, including part of the Pine Ridge Indian Reservation.
1978: Badlands declared a national park.