Big Basin Prairie Preserve is a 1,818-acre nature preserve managed by the Kansas Department of Wildlife, Parks & Tourism and named for the large collapse basin or sink just north of the site. This 1-mile-diameter, 100-foot-deep basin formed by solution-subsidence, as surface water reached and dissolved underground deposits of salt, gypsum, or limestone, causing the overlying rock and mineral layers to sink into the emptied space. Just east of Big Basin are other collapse features, including Little Basin, 280 yards in diameter and 35 feet deep, and a recently formed deep sinkhole on its rim known as St. Jacob's Well, which contains permanent water. The preserve lies just west of the 100th Meridian, where tall and mixed grass prairie give way to more drought-tolerant short grass prairie, and mid-grasses such as little bluestem, buffalo grass, and blue grama predominate, while big bluestem grows in moister places such as old sink scars. At the western edge of the Red Hills, a Kansas region with unusually rugged topography for the Great Plains, the preserve includes exposed rocks from the latter Permian Period, about 260 million years ago, when several thousand feet of brick-red shale, siltstone, and sandstone with interbedded gypsum and dolomite were deposited and later revealed by erosion; these iron oxide deposits rust when exposed to oxygen, creating bright red soils visible through the grasses and along basin sidewalls. The rocky prairie also resembles the Kansas of 100 years ago, and bison, native grassland grazers, range within the preserve and help maintain the natural character of an ecosystem that has declined dramatically in the United States over the past century. In 1978, the Secretary of the Interior designated the site the Big Basin Preserve National Natural Landmark in recognition of its outstanding collapse features and thriving bluestem-grama mixed-grass prairie.