during the thousands of years of native inhabitation on the continent, cultures changed and shifted. archaeologists often name different cultural groups they discover after the site where they are first found. one of the oldest cultures yet found is the clovis culture of modern new mexico. a more recent example is the group of related cultures called the mound builders (e.g. the fort walton culture), found in the mississippi river valley. they flourished from 3000 bc to the 1500s ad.