Map, Location of the Interior Plains
Interior Plains Region

The Interior Plains:

The Interior Plains is a vast region that spreads across the stable core (craton) of North America. This area had formed when several small continents collided and welded together well over a billion years ago, during the Precambrian. Precambrian metamorphic and igneous rocks now form the basement of the Interior Plains and make up the stable nucleus of North America. With the exception of the Black Hills of South Dakota, the entire region has low relief, reflecting more than 500 million years of relative tectonic stability.

Paleozoic and Mesozoic:
Throughout the Paleozoic and Mesozoic Eras the mostly low-lying Interior Plains region remained relatively unaffected by the mountain-building tectonic collisions suffered by the western and eastern margins of the continent. During much of the Mesozoic Era, the North American continental interior was mostly well above sea level, with two notable exceptions. During part of the Jurassic (208-144 million years ago), rising seas flooded the low-lying areas of the continent. Much of the Interior Plains eventually lay submerged beneath shallow Sundance sea.

Cretaceous Period:
Once again, during the Cretaceous Period (144-65 million years ago), record high sea levels flooded the continental interior with shallow seas.

The Interior Plains continued to receive deposits from the eroding Rocky Mountains to the west and Appalachian and Ozark/Ouachita Mountains to the east and south throughout the most recent Era, the Cenozoic. The flatness of the Interior Plains is a reflection of the platform of mostly flat-lying marine and stream deposits layed down in the Mesozoic and Cenozoic Eras.

North American Craton:
Continents move about the surface of the earth, bumping into other continents and drifting away. This takes millions of years to happen, but the results can be seen today in the rocks of the continents. The middle of most continents has not been crunched or squished up the way the edges have been. That is because the middle of the continents are made of very strong old rock. This part of the continent is called the stable craton. Indiana is located near the middle of the North American craton.

Interior Platform:
While much of the stable craton is exposed at the surface north of Indiana as the Canadian Shield, the middle of the craton, located in the United States, is covered with sedimentary rocks of the Interior Platform. The sequence of rocks varies from approximately 3,500 in excess of 20,000 feet in thickness. The cratonic rocks are metamorphic and igneous while the overlying sedimentary rocks are composed mostly of limestones, sandstones, and shales. These sedimentary rocks were deposited from 650 to 290 million years ago.

Tectonic Setting
The metamorphic and igneous rocks of the "basement complex" were created 1.5 to 1.0 billion years ago in a tectonically active setting. It was a setting of great pressure and temperature. The younger sedimentary rocks that were deposited on top of this basement complex were formed in a setting of quiet marine and river waters. Much of this time the craton was covered by a large shallow sea, a so-called "epicratonic sea" (meaning literally "on" the craton). Sometimes land masses or mountain chains rose up on the distant edges of the craton and then eroded down, shedding their sand across the landscape.

-- Excerpt from:
USGS/NPS Geology in the Parks Website, 2001, and Indiana Geological Survey Website, 2002

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