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
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:
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
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.
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.
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
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
while the overlying
sedimentary rocks are composed
mostly of limestones, sandstones, and shales. These
sedimentary rocks were deposited from 650 to 290 million
"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