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Geologic Time

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We usually think of time in years, generations and civilizations. Yet geologic time extends over billions of years. It identifies the age o rock groups and which unit of rock is older than another. Older rock are normally found below younger ones.

The geologic time scale places rocks and fossils in sequence. The order is established by which rock unit is older than another and confirmed by index fossils and radioactive dating.  Iowa rocks do not show a continuous rock record, vast periods of time and geologic deposits are missing. Their absence indicates periods of erosion or an environment that did not produce rock deposits.



  • Quaternary (Recent, Pleistocene)

  • Tertiary (Pliocene, Miocene, Oligocene, Eocene, Paleocene)

The last 3 million years (the Pleistocene period) is represented across Iowa by glacial deposits. Erosion characterized Iowa during the remainder of the Cenozoic period.


  • Cretaceous

  • Jurassic

  • Triassic

The time from 65 to 230 million years ago. Only portions of the Cretaceous and Jurassic periods are represented in Iowa. These deposits are stream and coastal sediments mixed with marine sediments of limestone, shale, and gypsum.


  • Permian

  • Carboniferous

  • Pennsylvanian

  • Devonian

  • Slurian

  • Ordovician

  • Cambrian

The sediments were formed 230 to 600 million years ago. Repeated episodes of marine, stream, and coastal environments produced limestone, sandstone, coal, and shale as characteristic deposits.
The oldest rock on earth from 600 million to 4.5 billion years old. A long and complex time of deposition, mountain building, and erosion. Rocks of this age are visible in Northwest Iowa.


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