Big Basin is a sinkhole about a mile in diameter and more than a hundred feet deep that, though it has the appearance of a valley, is entirely surrounded by higher ground. Like several other smaller sinks in this section of Kansas, it was formed thousands of years ago by the dissolving and collapse of massive gypsum and salt formations lying several hundred feet below the surface. Just beyond its east rim is the smaller Little Basin, which contains St. Jacob's Well, a pool of water never known to have gone dry. Archeological finds indicate that St. Jacob's Well attracted visitors for many centuries, beginning with prehistoric peoples and continuing into the early days of European settlement. Although Big Basin and Little Basin lie on the eastern edge of the High Plains, they exhibit the physical characteristics of the nearby Red Hills region, where white gypsum and gray dolomite alternate with brick-red shales, siltstones, and sandstones to create a striking butte-and-mesa topography unlike any other in Kansas.