http://www.missourigeologists.org/Meeting2004/AMGguidebook2004.pdf Association of Missouri Geologists Field Trip Guidebook], 51st Annual Meeting, Rolla, Missouri, October 1–2, 2004, 2004, Missouri Department of Natural Resources, Geological Survey and Resource Assessment Division, p. 15 are a series of circular depressions or deformations stretching 700 kilometers across southern Illinois and Missouri into eastern Kansas at a latitude of roughly 38 degrees north.