Paleontology and geology
The oldest Cretaceous rocks form the bedrock of northwestern Iowa and originated from sediments deposited in ancient river systems that drained westward to an interior seaway. Floodplains and coastal lowlands were covered with lush subtropical vegetation. Later in the Cretaceous, a vast shallow sea flooded eastward across the state, depositing layers of mud and limy sediments in western Iowa. Bones of large marine reptiles known as plesiosaurs have been found in these rocks. In southwestern Iowa, a fragment of fossil bone recovered from Cretaceous river deposits revealed a microscopic structure very similar to that seen in many dinosaur bones and may represent the first fossil dinosaur material found in the state.