The Geological Society of Iowa has produced an series of guidebooks covering virtually all aspects of Iowa's field geology. They comprise the most comprehensive set of guidebooks available for Iowa.
All guidebooks are in print and available at a nominal cost.
I got them from a creek bed along the Muddy Creek in Henry County near Lowell, Iowa on April 30th, 1995. That's about all there is to tell. Iowa named the geode their state rock in 1967. The prime hunting location for geodes is in a 35-mile radius of Keokuk, Iowa, and they range in size from 2 to 6 inches in diameter, although some as large as 30 inches have been found. About twenty different minerals have been found in the Keokuk geodes with quartz being the most common.