An Unusual Occurrence of Pre-Cambrian Crystalline Rocks beneath Glacial Drift near Manson, Iowa

Highly brecciated pre-Cambrian gneiss, granite, and diabase are described from a core in crystalline rocks directly beneath glacial drift near Manson, Iowa. Surrounding the crystalline mass, whose top is about 1 1/2 square miles in area, is a depressed zone about 280 square miles in area, consisting of a thick section of strongly disturbed Lower Cretaceous sedimentary rocks. A comparison of the characteristics of the structure with those of cryptovolcanic structures suggests a similar origin. The basement complex appears to have been violently intruded into the sedimentary rocks subsequent to Lower Cretaceous time and before the Pleistocene. The fact that the crystalline rocks were exposed and deeply weathered before being covered by glaciers suggests that this stiucture was quite deeply eroded.