Hot Springs National Park is in the Zigzag Mountains, a small range of the Ouachita (Wash'-i-tah)Mountain system in the Interior Highlands, which form the only major topographic relief for a vast area of the midwestern and south-central United States. The topography was formed in late Paleozoic times (400 million years ago) by tremendous geological forces that acted to uplift, fold, fault, fracture, and harden inland seabed sediments. Subsequent erosion has led to the formation of the present ridge and valley landscape. The narrow steep ridges of the Zigzag Mountains, are capped with novaculite rock outcrops. These outcrops are unique to the Ouachita Mountains, and the finely grained structure of the novaculite is known for its superior quality as a natural whetstone.