Paleontology and geology
The Precambrian: Precambrian rocks in Arizona record at least three sequences of deposition, metamorphism, and erosion. Fossils from the Late Precambrian are rare but include stromatolites, indicating that shallow marine conditions prevailed during this time.
The Paleozoic: Paleozoic rocks are well represented in Arizona. Cambrian sedimentary rocks, found in the western part of the state, contain fossils of trilobites, brachiopods, and other organisms that record the shallow marine conditions that prevailed during this time. Most of Arizona stood above sea level during the Ordovician and Silurian. There are a few exposures of Ordovician rocks remaining in the state, but erosion has removed any Silurian rocks and fossils that might have existed. Fossils of corals, brachiopods, and other marine organisms indicate the return of a shallow sea during the Devonian. During the Carboniferous, fluctuating sea levels led to the deposition of marine and coastal plain sediments during times of high water, and to deposition of salts when sea level dropped. In the Permian, tectonic activity to the southeast provided abundant sediment to rivers and streams, and eventually to the sea. In the Late Permian, fossils of marine organisms indicate that a shallow sea covered most of the state once again.
The Mesozoic: During the Mesozoic, much of Arizona was a vast, low-lying landscape, although tectonic activity raised the southwestern part of the state. Remains of forests that grew in the state during the Triassic can now be seen as fossilized logs in Petrified Forest National Park. Dinosaurs roamed the riverbanks and wind-sculpted dunes of the state during the Jurassic, leaving behind their footprints. Mountain building continued into the Cretaceous in the western part of the state, while shallow seas flooded into northeastern and southeastern Arizona. Turtles, plesiosaurs, and other marine reptiles thrived in the warm waters.
The Cenozoic: Cenozoic sedimentary rocks are mostly terrestrial in origin, representing a wide variety of environments inhabited by horses, mastodons, and camels. Arizona’s climate became drier in the Quaternary, and much of the state became desert, with fewer permanent lakes and streams.