Contemporary Minnesota is much quieter geologically than in the past. Outcroppings of lava flows and magma intrusions are the only remaining traces of the volcanism that ended over 1,100 mya. Landlocked within the continent, the state is far from the seas that once covered it, and the continental glacier has receded entirely from North America. Minnesota's landscape is a relatively flat
peneplain; its highest and lowest points are separated by only of elevation. While the state no longer has true mountain ranges or oceans, there is a fair amount of regional diversity in
landforms and geological history, which in turn has affected Minnesota's settlement patterns, human history, and economic development. These diverse geological regions can be classified several ways. The classification used here principally derives from Sansome's ''Minnesota Underfoot: A Field Guide to Minnesota's Geology
, but is also influenced by Minnesota's Geology'' by Ojakangas and Matsch. These authorities generally agree on areal borders, but the regions as defined by Ojakangas and Matsch are more geographical in their approximations of areas of similar geology, while Sansome's divisions are more irregular in shape in order to include within a region all areas of similar geology, with particular emphasis on the effects of recent glaciation. As glaciation and its residue has largely dictated regional surface geology and topography, Sansome's divisions are often coextensive with ecological provinces, sections, and subsections.
Northeastern Minnesota: ancient bedrock Northeastern Minnesota is an irregularly-shaped region composed of the northeasternmost part of the state north of Lake Superior, the area around
Jay Cooke State Park and the
Nemadji River basin southwest of Duluth, and much of the area east of
U.S. Highway 53 that runs between Duluth and
International Falls. Excluded are parts of the beds of
glacial lakes Agassiz and
Upham, the latter now occupied by the upper valley of the
Saint Louis River and its tributary the
Cloquet. This area is coextensive with the
Northern Superior Uplands Section of the
Laurentian Mixed Forest. and
Gunflint Ranges, once part of a single mountain range, were split when the center was engulfed by magma blistering up during the Midcontinent Rift, creating the
Duluth Complex. Lava from the rift formed dense volcanic rocks which sank, creating the Lake Superior basin, edged by the
North Shore Volcanic Group along the lakeshore. Known as the
Arrowhead for its shape, this region shows the most visible evidence of the state's violent past. There are surface exposures of rocks first formed in volcanic activity some 2,700 mya during construction of the Archaen-Superior province, including Ely
greenstone, metamorphosed and highly folded volcanics once thought to be the oldest exposed rock on earth;
Proterozoic formations created about 1,900 mya that gave the area most of its mineral riches; and more recent intrusive
gabbro and
extrusive basalts and
rhyolites of the
Duluth Complex and North Shore Volcanic Group, created by magma and lava which upwelled and hardened about 1,100 mya during the
Midcontinent Rift. The
Precambrian bedrock formed by this activity has been eroded but remains at or close to the surface over much of the area. The entire area is the raw southern edge of the
Canadian Shield. Topsoils are thin and poor and their parent soils derived from the rock beneath or nearby rather than from glacial till, which is sparse. Many of this region's lakes are located in depressions formed by the differential erosion of tilted layers of bedded rock of the Canadian Shield; the crevasses thereby formed have filled with water to create many of the thousands of lakes and swamps of the
Superior National Forest. In post-glacial times Northeastern Minnesota was covered by forest broken only by these interconnected lakes and wetlands. Much of the area has been little changed by human activity, as there are substantial forest and wilderness preserves, most notably the
Boundary Waters Canoe Area Wilderness and
Voyageurs National Park. In the remainder of the region, lakes provide recreation, forests are managed for pulpwood, and the underlying bedrock is mined for valuable ores deposited in Precambrian times. While copper and nickel ores have been mined, the principal metallic mineral is iron. Three of Minnesota's four iron ranges are in the region, including the
Mesabi Range, which has supplied over 90% of the state's historic output, including most of the natural ores pure enough to be fed directly into furnaces. The state's iron mines have produced over three and a half billion metric tons of ore. While high-grade ores have now been exhausted, lower-grade
taconite continues to supply a large proportion of the nation's needs.
Northwestern Minnesota: glacial lakebed Northwestern Minnesota is a vast plain in the bed of Glacial Lake Agassiz. This plain extends north and northwest from the
Big Stone Moraine, beyond Minnesota's borders into Canada and
North Dakota. In the northeast, the Glacial Lake Agassiz plain transitions into the forests of the Arrowhead. The region includes the lowland portions of the Red River watershed and the western half of the
Rainy River watershed within the state, at approximately the level of Lake Agassiz’
Herman Beach. In ecological terms, it includes the Northern Minnesota Peatlands of the Laurentian Mixed Forest, the Tallgrass Aspen Parklands, and the Red River Valley Section of the Prairie Parklands. Bedrock in this region is mainly
Archean, with small areas of Lower
Paleozoic and Upper
Mesozoic sedimentary rocks along the western border. By late
Wisconsinan times this bedrock had been covered by clayey glacial drift scoured and transported south from sedimentary rocks in Manitoba. The bottomland is undissected and essentially flat, but imperceptibly declines from about 400 meters at the southern beaches of Lake Agassiz to 335 meters along the Rainy River. There is almost no relief, except for benches or beaches where Glacial Lake Agassiz stabilized for a time before it receded to a lower level. In contrast to the lakebed, these beaches
rise from the south to the north and east at a gradient of approximately 1:5000; this rise resulted from the
isostatic rebound of the land after recession of the last ice sheet. In the western part of the region in the Red River Valley, fine-grained glacial lake deposits and decayed organic materials up to 50 meters in depth form rich, well-textured, and moisture-retentive, yet well-drained soils (
mollisols), which are ideal for agriculture. To the north and east, much of the land is poorly drained
peat, often organized in rare and distinctive patterns known as
patterned peatland. At marginally higher elevations within these wetlands are areas of black spruce, tamarack, and other water-tolerant species.
Southwestern Minnesota: glacial river and glacial till in the valley of the
glacial River Warren. To the northwest, the faint U-shaped
Big Stone Moraine; the triangular wedge in the southwest is the
Coteau des Prairies. The rough-appearing areas to the north and east are part of Central Minnesota.
Southwestern Minnesota is in the watersheds of the
Minnesota River, the
Missouri River, and the
Des Moines River. The Minnesota River lies in the bed of the
glacial River Warren, a much larger torrent that drained Lake Agassiz while outlets to the north were blocked by glaciers. The
Coteau des Prairies divides the Minnesota and Missouri River valleys, and is a striking landform created by the bifurcation of different lobes of glacial advance. On the Minnesota side of the coteau is a feature known as
Buffalo Ridge, where wind speeds average 16 mph (26 km/h). This windy plateau has been developed for commercial
wind power. Between the river and the plateau are flat
prairies atop varying depths of glacial till. In the extreme southwest portion of the state, bedrock outcroppings of
Sioux Quartzite are common, with less common interbedded outcrops of an associated metamorphosed mudstone named
catlinite.
Pipestone, Minnesota is the site of historic Native American quarries of catlinite, which is more commonly known as "pipestone". Another notable outcrop in the region is the
Jeffers Petroglyphs, a Sioux Quartzite outcropping with numerous petroglyphs which may be up to 7000–9000 years old. Drier than most of the rest of the state, the region is a transition zone between the prairies and the
Great Plains. Once rich in wetlands known as
prairie potholes, 90%, or some three million acres (12,000 km²), have been drained for agriculture in the Minnesota River basin. Most of the prairies are now farm fields. Ojakangas and Matsch extend the region west past the moraine to a line running north from the Iowa border between
Mankato and
New Ulm to the latitude of the
Twin Cities, then encompassing the latter metropolis with a broad arc east to the
St. Croix River. This moraine runs south from the Twin Cities in the general area of
Minnesota State Highway 13 and
Interstate 35. Sansome attaches this moraine to her description of West-Central Minnesota, given its similarity in glacial features to that region. Under Sansome's classification (followed here), Southeastern Minnesota is generally coterminous with the Paleozoic Plateau Section of the Eastern Broadleaf Forest Province. The bedrock here is lower
Paleozoic sedimentary rocks, with
limestone and
dolomite especially prevalent near the surface. It is highly dissected, and local tributaries of the Mississippi have cut deep valleys into the bedrock. It is an area of
karst topography, with thin topsoils lying atop porous limestones, leading to formation of caverns and
sinkholes. The last glaciation did not cover this region (halting at the Des Moines terminal lobe mentioned above), so there is no glacial drift to form
subsoils, giving the region the name of the
Driftless area. As the topsoils are shallower and poorer than those to the west,
dairy farming rather than cash crops is the principal agricultural activity.
Central Minnesota: knob and kettle country Central Minnesota is composed of (1) the
drainage basin of the St. Croix River (2) the basin of the Mississippi River above its confluence with the Minnesota, (3) those parts of the Minnesota and Red River basins on the glacial uplands forming the divides of those two basins with that of the Mississippi, (4) the Owatonna Moraine atop a strip of land running from western
Hennepin County south to the Iowa border, and (5) the upper valley of the
Saint Louis River and the valley of its principal tributary the
Cloquet River which once drained to the Mississippi before they were captured by
stream piracy and their waters were redirected through the lower Saint Louis River to Lake Superior.
Glacial landforms are the common characteristics of this
gerrymander-like region. The bedrock ranges in age from Archean granites to Upper Mesozoic
Cretaceous sediments, and underlying the eastern part of the region (and the southerly extension to Iowa) are the Late Precambrian Keweenawan volcanics of the Midcontinent Rift, overlaid by thousands of meters of sedimentary rocks. At the surface, the entire region is "Moraine terrain", with the
glacial landforms of
moraines,
drumlins,
eskers,
kames,
outwash plains and
till plains, all relics from recent glaciation. In the multitude of glacier-formed depressions are wetlands and many of the state's "10,000 lakes", which make the area prime vacation territory. The glacial deposits are a source of aggregate, and underneath the glacial till are high-quality granites which are quarried for buildings and monuments.
East Central Minnesota: bedrock valleys and outwash plain enters from northwest and crosses diagonally to the southeast;
Minnesota River in the valley of
River Warren enters from the south;
St. Croix enters from northeast. East Central Minnesota is composed of the eastern half and all territory on the image north of the Mississippi. of
Minnehaha Creek The subregion of
East Central Minnesota is that part of Central Minnesota near the junction of three of the state's great rivers. Included are
Dakota County, eastern
Hennepin County, and the region north of the Mississippi but south of an east-west line from
Saint Cloud to the
St. Croix River on the
Wisconsin border. It includes much of the
Twin Cities metropolitan area. The region has the same types of glacial landforms as the remainder of Central Minnesota, but is distinguished by its bedrock valleys, both active and buried. The valleys now hold three of Minnesota's largest rivers, which join here. The St. Croix joins the Mississippi at
Prescott, Wisconsin. Upstream, the Mississippi is joined by the Minnesota River at historic
Fort Snelling. When
River Warren Falls receded past the confluence of the much smaller
Upper Mississippi River, a new waterfall was created where that river entered the much-lower River Warren. The new falls receded upstream on the Mississippi, migrating eight miles (13 km) over 9600 years to where
Louis Hennepin first saw it and named
St. Anthony Falls in 1680. Due to its value as a power source, this waterfall determined the location of
Minneapolis. One tributary of the river coming from the west,
Minnehaha Creek, receded only a few hundred yards from one of the channels of the Mississippi.
Minnehaha Falls remains as a picturesque and informative relic of River Warren Falls, and the limestone-over-sandstone construction is readily apparent in its small gorge. At St. Anthony Falls, the Mississippi dropped over a limestone ledge; these waterfalls were used to drive the
flour mills that were the foundation for the city's 19th century growth. Other bedrock
tunnel valleys lie deep beneath till deposited by the glaciers which created them, but can be traced in many places by the
Chain of Lakes in Minneapolis and lakes and dry valleys in St. Paul. North of the metropolitan area is the
Anoka Sandplain, a flat area of sandy
outwash from the last ice age. Along the eastern edge of the region are the
Dalles of the St. Croix River, a deep gorge cut by runoff from
Glacial Lake Duluth into ancient bedrock.
Interstate Park here contains the southernmost surface exposure of the Precambrian lava flows of the
Midcontinent Rift, providing a glimpse of Minnesota's volcanic past. ==References==