In the very southeast corner of Kansas, the 55 square mile Ozark Plateau preserves geological remnants of Mississippian times in a limestone cave region. Its cherty limestones developed in marine environments during the late Mississippian Period 345 million years ago. Today the area is characterized by sinkholes, caves, swift streams, and steep cliffs, and its well drained cherty soils are poorly suited for agricultural use. Rich veins of lead and zinc beneath the surface made this tri-state district a lead and zinc mining center of worldwide importance in the 20th Century. Much of the original Ozark Oak-Hickory forestlands still remains in the region, where white and Shumard's oaks and bitternut and shagbark hickorys dominate steep limestone bluffs and clear streams.