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America's Volcanic Past
Minnesota

"Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994

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Map, Location of Minnesota

Volcanic Highlights and Features:
[NOTE: This list is just a sample of various Minnesota features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Minnesota.]

  • Minnesota
  • Minnesota Regions
  • Minnesota - Brief Geologic History
  • Minnesota's Volcanic Rocks
  • Minnesota's Agates
  • Bear Head Lake State Park
  • Cascade River State Park
  • Central Minnesota
  • George Crosby Manitou State Park
  • Gooseberry Falls State Park
  • Grand Portage
  • Interstate State Park
  • Jay Cooke State Park
  • Moose Lake State Park
  • Palisade Head
  • Red Lake Indian Reservation
  • Sawtooth Mountains
  • Shores of Lake Superior
  • Shovel Point
  • Split Rock Lighthouse
  • Soudan Underground Mine State Park
  • Southeastern Minnesota
  • St. Cloud
  • St. Croix State Park
  • Temperance River State Park
  • Tettegouche State Park

Minnesota

Rocks can tell us a lot about what happened in Minnesota long ago. They show a history of volcanoes, seas, mountain ranges, earthquakes, and glaciers. Volcanic action creates lava and ash. The rocks along Minnesota's North Shore of Lake Superior, for example, were created from cooling lava. Bentonite clay in southeastern Minnesota was formed from volcanic ash.


Excerpts from: Minnesota Department of Natural Resources Website, 2002

   
Minnesota Regions

Superior Upland Province:6
Every continent has a core of very ancient metamorphic rocks. The Superior Upland Province is the southern extension of the Laurentian Upland Province, part of the nucleus of North America called Canadian Shield. The basement rocks of the Laurentian Upland Province were metamorphosed about 2,500 million years ago in a mountain-building collision of tectonic plates called the Kenoran orogeny. The rocks of the Superior Upland are mostly Precambrian metamorphic rocks and overlying Paleozoic rocks (Cambrian) covered by a thin veneer of glacial deposits left behind when glaciers melted at the end of the Pleistocene Ice Age. If we could strip away all of the younger rocks deposited on top of buried Precambrian basement, you would see a landscape of low relief. The topography of the Precambrian rocks is very subdued, with barely 500 feet difference between the highest point and the lowest. Clearly, this region was exposed to a very long period of erosion in the very distant past which beveled the original mountainous surface to a gently undulating surface. The present surface is not much different. Hills rise just a few hundred feet above the surrounding countryside. The highest of these, such as Rib Hill, Wisconsin, are made up mostly of resistant quartzite or granite.




The Interior Plains:6
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.


   
Minnesota - Brief Geologic History

Precambrian:1
Minnesota's geologic framework is moderately well known from a combination of outcrop mapping where exposure permits, and interpretation of high resolution geophysical data and drilling where it does not. The state is underlain by rocks of Precambrian age that are covered in part by veneers of Paleozoic and Mesozoic (Phanerozoic) marine strata and rather extensively by Quaternary glacial deposits. Minnesota is situated at the southern edge of the Canadian Shield -- the nucleus of the continent of North America that formed during Precambrian time. This period of time encompasses about 85% of earth's history. Geologists consider Precambrian time to have begun with the formation of planet earth about 4,550 million years ago and to have ended about 570 million years ago, when organisms with hard parts, such as shells, first appeared. The rocks formed in Minnesota during this enormous span of time record a complicated geologic history that involved volcanoes, ocean islands, mountain chains, and unstable geologic conditions that were very different from the Minnesota of today. Precambrian Minnesota resembled modern-day Indonesia for a while; later, it resembled modern-day California, and still later it resembled parts of the Middle East and eastern Africa.

Midcontinent Rift:7
The 1,100 million-year-old Midcontinent rift system extends north-northeast from Kansas for about 2,000 kilometers to Lake Superior, and then southeast through Michigan. The rift is almost completely buried beneath a cover of Paleozoic and younger sedimentary strata; its position has been deduced from gravity and magnetic anomalies. However, significant exposures of rift-related rocks do occur around the margin of Lake Superior. There, surface rocks are a bimodal suite of mostly tholeiitic basalt and some alkaline basalt along with minor silicic rocks. The alkaline rocks are more abundant in the lower part of the stratigraphic sequence whereas tholeiitic rocks become more abundant up-section. U-Pb zircon dating of basal volcanic rocks and related sills around the shores of Lake Superior imply that rift-related magmatism began about 1,108.8 + 4/-2 million years ago during a period of reversed magnetic polarity. The magnetic polarity shifted back to normal between 1,097.6 8.7 million years ago and 1,086.5 + 1.3/- 3.0 million years ago. Although volcanism was active in the Lake Superior region for about 22 million years, most of the activity was restricted to the 3 to 5 million years interval shortly after 1,097 million years. Very few drill holes penetrate the rift-related rocks south and west of Lake Superior; however, several sampled volcanic sequences in southern Kansas have chemical characteristics similar to those associated with the rocks exposed around Lake Superior. The basaltic sequence in Kansas is subalkalic to alkalic and follows tholeiitic trends: a number of individual flows of high-alumina basalt are scattered throughout the sequence.

   

Minnesota's Volcanic Rocks

Minnesota's Anorthosite:2
Similar in composition to rocks on the moon, anorthosite is a coarse-grained intrusive igneous rock composed almost entirely of a single mineral -- plagioclase feldspar. In Minnesota, anorthosite was intruded into the lowermost lava flows that formed during the opening of the Midcontinent rift. Anorthosite is part of the Duluth Complex and the closely related Beaver Bay Complex exposed along the North Shore of Lake Superior. Split Rock Lighthouse sits atop an outcrop of anorthosite.

Minnesota's Basalt:2
About 1.1 billion years ago, the continent that had been building for billions of years began to split apart across what is now Minnesota. The "Midcontinent rift," as it is called, is where the crust began to separate to form a new ocean basin. The same process is currently underway between Africa and Saudi Arabia. The rifting process stopped short of producing a new ocean basin in central North America, but the abundant dark red-brown basaltic rocks. now exposed along Lake Superior's north shore are a testament to the massive outpouring of lava through fractures or cracks along the rift. Gooseberry Falls State Park is an ideal place to explore these ancient lava flows.

Minnesota's Gabbro:2
Gabbro is an intrusive rock formed when molten rock is trapped beneath the land surface and cools into a hard, coarsely crystalline mass. It is the intrusive equivalent to basalt. Minnesota's best examples of gabbro are in the part of the 1.1 billion year old Midcontinent rift exposed in the large hills at Duluth, known as the Duluth Complex. The rock is dense, dark-colored and contains varied percentages of the minerals plagioclase, pyroxene, and olivine. The Duluth Complex contains extensive, but relatively low-grade deposits of copper, nickel, and platinum group elements. None are currently being mined.

Minnesota's Gneiss:2
Some of the oldest rocks in the world include the gneiss found in the Minnesota River Valley. The Morton Gneiss, which is 3.6 billion years old, is a coarsely crystalline, foliated metamorphic rock. The texture and mineral assemblage of the Morton Gneiss give clues as to how the rock formed. The fact that it is a crystalline rock with large visible grains indicates that it originated as a granitic igneous rock that cooled slowly beneath the Earth's surface. The foliation, or alignment of the mineral grains, indicates that the original rock was subjected to great heat and pressure deep below the Earth's surface. Gneiss is quarried for use as building stone and monuments. You can find outcrops of gneiss near Morton (the famous "Rainbow Gneiss"), Redwood Falls, Sacred Heart and Ortonville.

Minnesota's Granite:2
Granite is found throughout northern and central Minnesota. It varies in age from 2.6 billion years in the Minnesota River Valley and northern Minnesota to about 1.7 billion years near St. Cloud. Minnesota granites are composed predominantly of the minerals feldspar, quartz, mica, and hornblende. These rocks formed deep below the surface in the roots of major mountain ranges. These once deeply buried rocks are now exposed at or near the surface due to uplift and erosion. Granite is quarried for use as building stone and monuments. You can find outcrops of granite in Stearns, Pine, and Mille Lacs counties, and also in places in northeastern Minnesota, including the Boundary Waters Canoe Area.

Minnesota Granite in Des Moines, Iowa:4
The Iowa state capitol building in Des Moines, constructed from 1872-1884 from a variety of building stones, is a spectacular example of late 19th-century stone construction. The granite base was secured from Buchanan County boulders and quarries in Minnesota. Limestone blocks comprising the foundation and lower levels were quarried in Iowa at locations in Johnson and Madison counties. The bulk of the exterior was constructed from sandstone blocks derived from quarries in Missouri. Additional stone, both local and imported, was used in the interior construction, including a number of decorative marbles.

Minnesota's Greenstone:2
Greenstone in northern Minnesota is somewhat younger than gneiss. Greenstone is a weakly metamorphosed (altered) basalt that is, as its name suggests, greenish to gray. This type of rock formed about 2.7 billion years ago when the area that is now northern Minnesota was part of a volcanic island arc, much like the islands of Japan are today. Greenstone and other associated volcanic and related rocks have in the past been prospected for deposits of economic metals such as gold, copper, zinc, lead, and iron. Iron mines at Ely and Soudan are now closed. As yet, no other significant metal deposits have been found.




Minnesota's Agates

Minnesota's Agates:2
Minnesota's state rock is the Lake Superior agate -- so named because it is found predominantly along the coast of Lake Superior. Agates form in cavities in basalt. As mineral-rich water circulates through the cavities, silica (SiO2), or quartz, is deposited in layers along cavity walls. Eventually, the cavities completely fill with this banded variety of quartz. The color variations are due to slight mineral impurities in the water. Iron, for example, causes much of the red and orange color seen in Lake Superior agates. Although agates originated in the basaltic rocks along the North Shore, some of the best places to hunt for agates are in gravel pits scattered across the state. Specifically, agates are likely to be found where operators are mining glacial sand and gravel deposits associated with glaciers that advanced into Minnesota from the northeast, bringing agate-bearing gravel into the central and southern parts of the state. [See Shores of Lake Superior below]




Bear Head Lake State Park

Bear Head Lake - Volcanoes and Ancient Seas:4
Volcanoes and ancient seas produced the bedrock of the Bear Head Lake area. The bedrock of Ely greenstone and Giants-Range granite was scoured and gouged by glaciers. Eagles Nest and Bear Head Lakes, along with the park's rolling hills, were formed from the glacial action. Along the lakes and road cuts, visitors can see many different shapes and sizes of rock left by the glaciers.




Cascade River State Park

Cascade River State Park:4
Aptly named, the Cascade River flows down one ledge after another for a total drop of 900 feet in the last three miles of its journey to Lake Superior. The park setting is a boreal hardwood-conifer forest of aspen, birch, fir, spruce and cedar. Visitors can stand on the footbridge that spans the river, or at any of the viewing spots above the river, and feel the vibration of the rushing torrent of water as it cascades down a volcanic canyon. The geology of the North Shore and the Cascade River is complex. It started billions of years ago when the ancient bedrock was covered with molten lava from volcanoes. Later, shallow seas covered the area and deposited sediment over the lava beds. The sedimentary rock formed from the seas was bent by great mountain building forces. Other complex geological processes took place before the glaciers scoured the area several times leaving the present Lake Superior basin.


Central Minnesota

Central Minnesota Geology:2
There are four basic subdivisions to the geology of the central Minnesota area. The youngest material, which makes up nearly 100% of the present land surface, consists of unlithified glacial till and glacial outwash. These Pleistocene glacial deposits cover older bedrock units, and are typically in the range of 100 to 200-feet thick but may be as much as 400 feet thick. Cretaceous sedimentary rocks, the next oldest unit, consist primarily of poorly lithified shale, sandstone, and limestone. The Cretaceous rocks in central Minnesota occur mostly as scattered outliers or erosional remnants on top of older Precambrian rocks, but become substantially thicker to the south and west. Paleozoic sedimentary rocks, mainly restricted to southeastern Minnesota, consist of interlayered sequences of sandstone, shale, and limestone that decrease in thickness to the northwest of the Twin Cities. Paleozoic rocks are not present in most of central Minnesota, but are very important aquifers in the Twin Cities and southeastern Minnesota. The counties fringing the northwestern metropolitan area are sites of recharge for these aquifers of the Twin Cities.

Precambrian rocks are the first bedrock encountered in most of central Minnesota. The Precambrian is subdivided by age into three major groups - the middle Proterozoic (Keweenawan) (youngest), the Early Proterozoic, and the Archean (oldest). Rocks of the Keweenawan system run along the axis of Lake Superior and continue south along the Minnesota/Wisconsin border. These rocks occur along what is known as the midcontinent rift, and in central Minnesota consist primarily of basalt flows overlain by clean sandstones, which are very similar to overlying Paleozoic sandstones. The next oldest subdivision of the Precambrian, the Early Proterozoic, consists of a wide variety of rocks ranging from slate, schist, and gneiss to granite and gabbro. The northern extent of these rocks is roughly outlined by the Mesabi and Cuyuna iron ranges to the north, and they continue south and southeast beneath Cretaceous, Paleozoic, and Keweenawan rocks. The oldest Precambrian rocks, Archean, are made up primarily of bands of various types of granites situated between linear greenstone belts, and in west-central and southern Minnesota, gneiss.

Northeast-trending, steeply-dipping diabase dikes are common in central Minnesota. The dikes follow preexisting brittle fractures in granitic rocks.

Granite is found throughout northern and central Minnesota. It varies in age from 2.6 billion years in the Minnesota River Valley and northern Minnesota to about 1.7 billion years near St. Cloud. Minnesota granites are composed predominantly of the minerals feldspar, quartz, mica, and hornblende. These rocks formed deep below the surface in the roots of major mountain ranges. These once deeply buried rocks are now exposed at or near the surface due to uplift and erosion. Granite is quarried for use as building stone and monuments. You can find outcrops of granite in Stearns, Pine, and Mille Lacs counties, and also in places in northeastern Minnesota, including the Boundary Waters Canoe Area.

Saprolite is composed of clay minerals that result from in-situ weathering of bedrock by acidic groundwater. Weathering is concentrated along vertical and horizontal fracture planes that may intersect to leave rounded, relatively unweathered corestones (a relict mass of unaltered bedrock, surrounded by saprolite, Corestones are very numerous in the thick saprolites typically found in Minnesota, and may account for the substantial number of "pre-rounded" glacial erratic boulders found in the state). The texture of the original granite is visible in the saprolite as patches of green and white clay. Saprolite in Minnesota is locally overlain by thin deposits of upper Cretaceous sedimentary rocks and may have formed either before, or during Cretaceous time. Kaolin-rich saprolite is mined in the southwestern part of the state as a filler in the manufacture of cement.

Large, smooth, granitic rocks are found in the valley of the Sauk River. This type of surface is thought to have formed when softer and more easily eroded saprolite was removed by fast-moving water in what is now an abandoned glacial outwash channel. In effect these rounded surface boulders were at one time corestones, and reflect the irregular distribution of weathering patterns in the saprolite.




George Crosby Manitou State Park

Country Wilderness Park:4
George Crosby Manitou State Park is a country wilderness park where waterfalls cascade through a volcanic canyon surrounded by majestic forest. Volcanoes spewed fiery lava which cooled and built up in thick layers. Later, when glaciers moved down from the north, they scraped and dislodged the rock. As the melting glacier retreated, they left piles of rock and soil again changing the shape of the park. The last glacier left a large meltwater lake behind the shoreline ridge which drained into Lake Superior. This drainage route is now the Manitou River.


Gooseberry Falls State Park

Gooseberry Lava Flows:4
Geologists have determined that about one billion years ago, the earth began to split apart along the area now known as the North Shore. Lava flowed out onto the earth and cooled to form volcanic bedrock. Several lava flows can be seen at the Upper, Middle, and Lower Falls and south of the Gooseberry River along Lake Superior. These basalt lava flows are also the birthplaces of Lake Superior agates. About two million years ago, glaciers (up to a mile high) advanced into the region. As they ground across the area, they changed the landscape dramatically. About 10,000 years ago the last glacier melted back, filling the infant Lake Superior and beginning the erosional process that creates waterfalls. Today, water, wind, and weather continue to shape the North Shore.




Grand Portage

Grand Portage:4
The geology of high falls is a story of fire and ice. According to geologists, it begins in the middle of the Precambrian era about 1.9 billion years ago. An ancient sea covered what is now Minnesota and deposited mud that hardened over time into a layer of shales and slates known as the Rove Formation. These are the layered rocks you can see along the inside of the high falls gorge. About 1.2 billion year ago, the continent began to spread along a rift that extended from what is now eastern Lake Superior to Kansas. Basaltic lava flowed over this zone, while here in Grand Portage, diabase intrusions penetrated the older Rove Shales turning the contact area shales into slate. The cliff the falls run over is a diabase dike and is very resistant to erosion. Huge glaciers bulldozed away the softer rock (shale and slate) leaving the harder and more resistant rock (basalt) standing.




Interstate State Park

Interstate State Park:3
In this park you can see the effects of glaciers 11,000 years ago. Potholes were created by rivers draining the glacial lake to the north. The basalt rock formation from 1.1 billion years ago confined the river to a gorge, and the river cut through overlying siltstones and sandstones and some of the basalt to create potholes.

Geologic Background:4
About 1.1 billion years ago, volcanoes erupted from Taylors Falls to Lake Superior. At least 10 different lava flows were the result. The hardened basalt rock from these lava flows was partly responsible for the formation of the Dalles of the St. Croix and the bottom of the river. From 530 million years ago up until 70 million years ago, the state was washed by advancing and retreating seas. Evidence of these ancient seas is revealed in the sedimentary rocks and formations found in the park. These rocks contain fossil remains of ancient animals, evidence of various creatures, and ripple marks left in stone by the now vanished seas. Actually, there have been many different St. Croix Valleys down through the ages. The original one was formed about 70 million years ago. The last one, which exists today, was formed about 10,000 years ago. This last glacier formed both Glacial Lake Duluth, now Lake Superior, and Glacial Lake Grantsburg, now extinct. Glacial Lake Duluth was much larger than Lake Superior is now. As this giant body of ice thawed, its meltwaters roared south to carve out the broad valley of the St. Croix. Only very resistant basalts were able to partially withstand the torrent, resulted in the dalles, potholes, and cliffs.




Jay Cooke State Park

Volcanic Dikes:4
The geological makeup of Jay Cooke State Park is one of slate, graywacke and red clay. Streams have exposed the bedrock in many places. The Precambrian Thomson Formation is located within Jay Cooke State Park. The formation is a layer of mud that was compressed and hardened into shale, then compressed further into slate, and finally folded. This formation is about two billion years old. Later, masses of molten rock were forced through fractures in the beds and when they cooled, these intrusions formed dikes which can be seen in the river bed today.


McCarthy Beach State Park

McCarthy Beach:4
The glaciers that moved through the area gouged and tore at the bedrock, the cooled lava of ancient volcanoes. During that period, glaciers flowed south, retreated north, and flowed south again. The first glacier stopped where McCarthy Beach is now located. It left low rolling hills with steep sides called moraines. Between the hills, the glaciers gouged valleys and in the larger valleys, lakes were formed. Trails from the beach lead visitors along the ridge tops of the park's moraines. From these vistas, visitors can imagine the titanic forces that moved and shaped the region.


Moose Lake State Park

Moose Lake State Park:2,3
Learn more about Minnesota's state gemstone, the Lake Superior agate, while visiting this park. The Lake Superior agate is the oldest of the world's agates, 1.1 billion years old. The agate is composed of quartz with distinctive red and white banding. Agates form in cavities in basalt. As mineral-rich water circulates through the cavities, silica (SiO2), or quartz, is deposited in layers along cavity walls. Eventually, the cavities completely fill with this banded variety of quartz. The color variations are due to slight mineral impurities in the water. Iron, for example, causes much of the red and orange color seen in Lake Superior agates. [See Shores of Lake Superior below]


Palisade Head

Palisade Head:4
The north shore of Lake Superior is a combination of rock cliffs, pebble beaches, and bold headlands. The landscape owes its character to the erosion of bedrock by running water and glaciers. Glacial action resulted in thin soils, scoured lake basins, and numerous rock outcroppings. The North Shore drainage pattern is one of short, steep rivers with many waterfalls and deeply eroded gorges. Some present day features, however, are the result of much older geologic processes. About 1.1 billion years ago, North America began to spread apart along a rift that extended from what is now Lake Superior all the way to Kansas. The deep basaltic lava that poured out of this rift formed virtually all of the bedrock of the North Shore. Removal of lava from beneath what is now Lake Superior caused the flows to tilt to the southeast. Spectacular examples of these lava flows are Palisade Head and Shovel Point.


Red Lake Indian Reservation

Red Lake Indian Reservation:5
The Red Lake Indian Reservation contains 564,447 acres in parts of eight northern Minnesota counties. Bedrock underlying reservation land is Archean granite-greenstone terrane that comprises the southern margin of the Canadian Shield. Although glacial cover obscures most bedrock, the main rock types present are mafic volcanic rocks, felsic volcanic rocks, graywacke-type sedimentary rocks, and granitic intrusive rocks. These define southern exposures of the Wawa and Wabigoon greenstone belts, which extend northward into Canada. Major structural features include suture zones, shear zones, and several generations of folding in which early recumbent nappes were refolded into structures with steeply dipping limbs.




Sawtooth Mountains

Brule River Rhyolite Flow and the Sawtooth Mountains:4
The bedrock exposed along Lake Superior's North Shore has a geologic history that goes back some 1.2 billion years. During the mountain-building, volcanic activity of that time, molten lava poured through great fissures that developed in the earth's crust. One particular flow complex, the Brule River rhyolite flow is thought to be as much as 3,500 feet thick. As these flows accumulated, the land along the rift zone sank to form a great basin, presently occupied by Lake Superior. Long periods of erosion followed. The local Sawtooth Mountains of the Grand Marais area in Minnesota are the remnants of this ancient mountain range. More recently, glaciers also took their toll on the area as massive ice sheets gouged out basins and scoured the surface of the bedrock.


Shores of Lake Superior

Shores of Lake Superior3
Rocks can tell us a lot about what happened in Minnesota long ago. They show a history of volcanoes, seas, mountain ranges, earthquakes, and glaciers. Volcanic action creates lava and ash. The rocks along Minnesota's North Shore of Lake Superior, for example, were created from cooling lava. More than a billion years ago, the North American continent began to split apart into two separate continents. This catastrophic event, spurred by molten rock moving deep within the earth, poured out massive, iron-rich lava flows. These flows now are exposed along the north and south shores of Lake Superior. The tectonic forces that attempted to pull the continent apart, and which left behind the lava flows, also created the Superior trough. The trough eventually became the basin of Lake Superior and the lava flows became the birthplace of Lake Superior agates.

Minnesota Agates:3
More than a billion years ago, the North American continent began to split apart into two separate continents. This catastrophic event, spurred by molten rock moving deep within the earth, poured out massive, iron-rich lava flows. These flows now are exposed along the north and south shores of Lake Superior. The tectonic forces that attempted to pull the continent apart, and which left behind the lava flows, also created the Superior trough. The trough eventually became the basin of Lake Superior and the lava flows became the birthplace of Lake Superior agates.

Water vapor and carbon dioxide became trapped within the solidified flows in the form of millions of bubbles, called gas pockets or vesicles. Later, groundwater carrying ferric iron, quartz, and other dissolved minerals passed through the trapped gas vesicles. These quartz-rich groundwater solutions crystallized into concentric bands of fine-grained quartz called chalcedony. Over the next billion years, some of these quartz-filled, banded vesicles -- agates -- were freed by running water and chemical disintegration of the lavas, since these vesicles were now harder than the lava rocks that contained them. The vast majority, however, remained lodged in the lava flows until the next major geologic event that changed them and Minnesota.

About 2 million years ago, the world's climate grew colder signaling the beginning of the Great Ice Age. A lobe of glacial ice, the Superior lobe, moved into Minnesota 10,000 to 15,000 years ago. It followed the agate-filled Superior trough. The glacier picked up surface agates and carried them south. Its crushing action and cycle of freezing and thawing at its base also freed many agates from within the lava flows and transported them, too. The advancing glacier acted like an enormous rock tumbler, abrading, fracturing, and rough-polishing the agates.


Shovel Point

Shovel Point:4
The north shore of Lake Superior is a combination of rock cliffs, pebble beaches, and bold headlands. The landscape owes its character to the erosion of bedrock by running water and glaciers. Glacial action resulted in thin soils, scoured lake basins, and numerous rock outcroppings. The North Shore drainage pattern is one of short, steep rivers with many waterfalls and deeply eroded gorges. Some present day features, however, are the result of much older geologic processes. About 1.1 billion years ago, North America began to spread apart along a rift that extended from what is now Lake Superior all the way to Kansas. The deep basaltic lava that poured out of this rift formed virtually all of the bedrock of the North Shore. Removal of lava from beneath what is now Lake Superior caused the flows to tilt to the southeast. Spectacular examples of these lava flows are Palisade Head and Shovel Point.


Split Rock Lighthouse

Anorthosite:2
Similar in composition to rocks on the moon, anorthosite is a coarse-grained intrusive igneous rock composed almost entirely of a single mineral -- plagioclase feldspar. In Minnesota, anorthosite was intruded into the lowermost lava flows that formed during the opening of the Midcontinent rift. Anorthosite is part of the Duluth Complex and the closely related Beaver Bay Complex exposed along the North Shore of Lake Superior. Split Rock Lighthouse sits atop an outcrop of anorthosite.

Magnificent Bluffs:4
The magnificent bluffs upon which the lighthouse is built are formations of anorthosite. The bedrock is a result of ancient lava flows. Then over an immense time span, many complex geological processes occurred. Water covered the area depositing sediment over the bedrock. Later this rock was bent and compressed by mountain-building forces. Eventually a series of glaciers scraped the landscape, scouring out the Lake Superior Basin and shearing off the shoreline cliffs. Then the meltwaters of the glaciers flowed over the upland rock cliffs, carved away the less resistant rock and filled the basin we know as Lake Superior.


Soudan Underground Mine State Park

Soudan Underground Mine State Park:3
Go almost 1/2 mile below the surface to view the world of underground mining. Opened in 1883, the Soudan Mine is Minnesota's oldest and deepest iron ore mine. Ely Greenstone, volcanic rocks and sediments formed in oceans over 2.7 billion years old, may be seen here.




Southeastern Minnesota

Volcanic Ash:3
Bentonite clay in southeastern Minnesota was formed from volcanic ash.


St. Cloud

St. Cloud:3
The first granite quarry was opened in the St. Cloud area in 1868.




St. Croix State Park

Ancient Lava Bedrock:4
During the last glacial period, about 10,000 years ago, the St. Croix River valley served as a major drainage channel for glacial meltwater from Lake Superior. As these waters carved the way for the river seen today, the waters left behind a variety of soils and sediment that cover the ancient lava bedrock far below, including a glacial outwash plain of sand left by Glacial Lake Grantsburg. Many springs along the river banks occur where the river valley has eroded through the glacial gravel to release water trapped between the sediments. The final wave of glacial meltwater cleaned debris from the Kettle River valley, leaving much of the basalt and sandstone bedrock exposed. A stop at the Kettle River Highbanks is a step back over millions of years.


Temperance River State Park

Potholes:4
One of the most interesting geologic features in the park is the narrow Temperance River gorge with its many waterfalls. The rapidly falling river cut through the soft lava of the river bed. Swirling water carried sand and gravel which wore away the soft lava and created potholes. Over thousands of years, these potholes were dug deeper and wider, eventually connecting and creating the deep, narrow gorge. Nearby, more potholes were left high and dry as the river found its new and present channel.


Tettegouche State Park

Lava Flows:4
The north shore of Lake Superior is a combination of rock cliffs, pebble beaches, and bold headlands. The landscape owes its character to the erosion of bedrock by running water and glaciers. Glacial action resulted in thin soils, scoured lake basins, and numerous rock outcroppings. The North Shore drainage pattern is one of short, steep rivers with many waterfalls and deeply eroded gorges. Some present day features, however, are the result of much older geologic processes. About 1.1 billion years ago, North America began to spread apart along a rift that extended from what is now Lake Superior all the way to Kansas. The deep basaltic lava that poured out of this rift formed virtually all of the bedrock of the North Shore. Removal of lava from beneath what is now Lake Superior caused the flows to tilt to the southeast. Spectacular examples of these lava flows are Palisade Head and Shovel Point.


Excerpts from:
1) University of Minnesota "Minnesota Geology"; University of Minnesota "Minnesota at a Glance", 2001
2) Minnesota Geological Survey Website, 2001
3) Minnesota's Department of Natural Resources Website, 2002
4) Iowa Department of Natural Resources Website, 2001
5) Bureau of Indian Affairs, Division of Energy and Minerals Website, 2001
6) USGS/NPS Geology in the Parks Website, 2001
7) Morey, G.B., 2001, Compositions of Rift-Related Volcanic Rocks of the Keweenawan Supergroup Atop the ST. Croix Horst, Southeastern Minnesota: Minnesota Geological Survey Information Circular 47, 27p.

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01/26/03, Lyn Topinka