c. 2.5 billion years ago: Pre-Cambrian igneous and metamorphic bedrock, such as Sioux Quartzite, forms in the area that is now Iowa.
c. 500 million years ago: A warm, shallow sea covers the area that is now Iowa.
c. 500 million years ago: Sedimentary rock begins to form, including limestone, sandstone, dolomite, and shale.
c. 500 million years ago: Cambrian rock forms.
c. 475 million years ago: Ordovician rock forms.
c. 425 million years ago: Silurian rock forms.
c. 375 million years ago: Devonian rock forms.
c. 350 million years ago: Mississippian rock forms.
c. 300 million years ago: Pennsylvanian rock forms.
c. 160 million years ago: Jurassic rock forms.
c. 75 million years ago: Cretaceous rock forms.
c. 3 million years ago: Glaciers form during a cooling of the earth’s surface, and the ice sheets gradually, in several phases, move over the area that is now Iowa.
c. 12,000 years ago: The last glacier melts and the geographical features of Iowa are exposed. First known humans in Iowa.
c. 8,000 years ago: A warming of the climate and reduced rainfall encourages the spread of prairies across the area that is now Iowa.
c. 5,000 years ago: Increased rainfall spurs the growth of hardwood forests in protected areas in the area that is now Iowa.
The First Iowans – The Archaeology
c. 12,000 years ago: Archaeologists believe the first known human beings to live in Iowa, the Paleo period, left fragmentary remains of their hunting practices.
c. 8,500 years ago: For about 7,000 years Iowa is occupied by “Archaic” cultures.
c. 3,000 years ago: Appearance of the Woodland cultures in eastern Iowa, and Great Oasis culture in western Iowa.
c. 2,500 years ago: Middle Woodland cultures construct mounds in Iowa.
c. 1,500 years ago: Late Woodland cultures construct Effigy Mounds in Iowa.
c. 1,000 years ago: Mill Creek culture inhabits northwestern Iowa.
c. 1,000 years ago: Nebraskan Glenwood culture inhabits southwestern Iowa.
c. 900 years ago: Oneota culture inhabits Iowa for several centuries.
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