Understanding the geology of Minnesota going back to the ancient birth of our planet, is a good way to understand why the landscape looks as it does and what influences our environment has had upon the Native American populations here over time. The contexts of space and time cannot exist apart from each other, they are co-existing and the setting for the human story in our state. Trade routes, settlement patterns, food procurement, tool making and cultural diffusion have all been affected by Minnesota's geology and geography.
The Glacial History of Minnesota |
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Nebraskan Stage
Bibliography |
The first four billion years in Minnesota's geological history is
called
the Precambrian Period. It is divided into the Early, Middle and
Late
Precambrian Periods. This is a time of multiple volcanic and
marine
intrusions that have formed the underlying bedrock of the state.
This period lasted from 4.5 to 2.6 billion years ago. Deep seated volcanic magma came up to the planet's surface and formed the granites which are common in Southern Minnesota around 3.9 billion years ago. At 3.6 billion years ago to 3 billion years ago the Earth's crust heated up partially due to building internal pressures. This meant that the granites previously deposited sometimes became mixed with sedimentary rocks to form gneisses, the rocks with lighter and darker bands within them. About 2.7 billon years ago in the Algoman Orogeny, huge amounts of magma shot up from the earth mixing with the gneisses. This timeframe provided much of the bedrock for Minnesota. It was a time of tremendous folding of the Earth's surface. Many batholithic domes of granite cooled within the gneisses. When the magma came to the surface in northeastern Minnesota, it met an underwater cooling in an ancient underwater sea, much akin to the way that Hawaii's underwater magma cools today. The cooling magma pillowed into Ely Greenstone. When the seawater hit the lava and leaked into the later cooled rock, iron leached out in large deposits forming the Tower-Soudan Iron Range.
From 2.6 through 1.7 billion years ago the previous volcanic
activity
subsided. Quiet geologic erosion formed sediments in the waters of
two
seas that retuned to Minnesota, thereby forming thick lsyers of
sedimentary rock over time. This period formed the slates, shales and
mudstones that are exposed along Lake Superior's North Shore today. The iron
ore deposits continued to form in the northeast until 1.8 billion
years ago
when igneous activity began once again.
From 1.8 billion through 570 million years ago, this period exhibited
wide transformations for central North America. A huge crack in the Earth's
surface opened up from Duluth, Minnesota to the Canadian border at Grand
Portage, depositing 5-mile-wide lava flows in some places along this rift about
1.1 billion years ago. In the southerly direction, a long rift opened up from
Lake Superior down the Saint Croix River in eastern Minnesota and ending up in
southern Kansas. Geologic forces at this time formed a huge depression which
formed the Superior Syncline
where Lake Superior is today, and a large body
of water there eroded rocks into aandstone found as far south as Hinckley,
Minnesota, and along the Saint Louis River in the same area.
From 570 to 500 million years ago, invertebrate life like trilobites
formed in Minnesota's seas. Only extreme northern Minnesota and the Arrowhead
Region were not covered by this ancient sea. The Epicontinental Sea connected
to both the Pacific and Atlantic Oceans. A deep layer of sediment formed under
the sea, and one can still see conglomerate rock beaches in northern Minnesota
from this sea. Jordan Sandstone was formed along east central Minnesota at this
time. Under the Twin Cities, an underground water reservoir of sandstone was
formed at this time. Then the sea retreated due to volcanism in some parts of
Minnesota.
Ordovician Period
From 500 to 435 million years ago, the Epicontinental Sea returned, and covered south and east central Minnesota, leaving behind depositional rocks like limestone, dolomite, sandstone and shale. Cephalopod animals left their remains over a large area.
From 435 to 395 million years ago erosion was the true story. Minnesota was becoming a lowland.
From 395 to 345 million years ago the seas came back to Minnesota, but then retreated again leaving 300 feet of dolomite and limestone rock under Southern Minnesota in many places.
From 225-70 million years before today, this era contained the emergence and spread of biological lifeforms. The pine forests of Northern Minnesota are thought to have come onto the scene at this time.
From 225 to 195 million years ago the whole state was in a huge erosion and leveling phase.
From 195 to 136 million years ago, the Sundance Sea came into North-Western Minnesota from Eastern Alaska, leaving shales and gypsum and a Red Amaranth, which is a red bedrock type. The beginning of this period is marked by the breakup of the supercontinent Pangaea. At this time, Minnesota began its move on the planet from an area now 1000 miles West of Portugal due to plate tectonic shift.
136-70 million years ago yet another sea invaded the western part of Minnesota. It left behind shales, sand and clay. The later glaciers pushed away superior materials exposing this layer today. Sharks teeth and large reptile remains are found in western Minnesota from this period.
From 70 million to 10 million years ago there were no seas here or igneous activity. Much of the previous sediments were eroded away to expose bedrock.
Jumping ahead in time, this epoch which begins about two million years ago, marks the beginning of modern geological and climatological history of Minnesota. The planet began to cool, glaciers formed out of Northern North America and the well drained very flat plains of Minnesota would become hilly, poorly drained and eratic due to glacial till.
Bray, Edmund C. "Billions of Years In Minnesota" North Central
Publishing Company; St. Paul, MN, 1977.
Willard, Daniel E. "The Story of the North Star State" West
Publishing Company; St. Paul, MN, 1922.
by - S.L. Burgstahler