The Warsaw Shale is named for Warsaw, Hancock County, and the exposure in Geode Glen at Warsaw has become the type section. The Warsaw is widely present in the bluffs of the Mississippi and Illinois Valleys in western and southwestern Illinois. It consists of as much as 300 feet of siltstone in west-central Illinois, but it thins to less than 100 feet in the outcrop area, where it consists of gray shale containing beds of argillaceous limestone. Quartz geodes are common and locally abundant; some are replacements of fossils. Some contain petroleum. The Warsaw is fossiliferous, with brachiopods, bryozoans, and crinoids especially common.