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State Fair Booth Rock Specimens list

1.  AGATE

Carlton County

 

Minnesota’s state rock is the Lake Superior Agate, so named because it is found along the coast of Lake Superior.  But a shrewd agate hunter would do best to focus on gravel pits.  The glaciers brought agate-bearing gravel into the central and southern parts of the state.  Carlton County has been and continues to be an excellent source of agates.

 

Agates are formed in gas cavities (vesicles) that were left behind as lava cooled.  The lava flows, after many thousands of years of cooling, were buried, some to depths of several miles.  Mineral bearing groundwater slowly percolated through the fractures in the hardened lava rock and deposited minerals, layer by layer, into the vesicles and cracks.  (Once filled, the vesicles are called amygdules).  Agates are made of chalcedony, an extremely fine-grained quartz, that coated the interior of the vesicles, starting from the outside surface of the cavity.

 

 

2.  BRACHIOPOD FOSSILS

Spring Valley, Minnesota

 

Brachiopod is the name given to marine animals that cover their soft bodies with shells.  The shells left an impression of their shape and sometimes a pearly luster after having been buried under sediment.  During the Ordovician period, approximately 500 million years ago, southeastern Minnesota was covered by an ocean.  Many brachiopod shells were present in these waters.  As they died and were swept up by waves, many of them were broken into fragments.  Sediments buried these fragments along with whole shells, and this specimen is an example of the fossilized casts, or impressions, left by the brachiopod shells.  (also see Specimen#8)

 

 

 

 

3.  DULUTH GABBRO

Duluth Area

1.1  Billion Years Old

 

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 extensive, but relatively low-grade deposits of copper, nickel, and platinum group elements.

 

 

 

4.  RHYOLITE

North Shore, Minnesota

 

This volcanic rock is granitic, with a high silica content.  Rhyolite rocks have less iron than basalt which leaves them lighter colored, but there is enough iron to color many of them red or pink.  Rhyolitic lava flows erupted while basaltic volcanism continued, so today we see them interlayered with basalt flows.  This piece of rhyolite is from Minnesota’s north shore, and was formed approximately 1.1 billion years ago.

 

 

 

 

5.  IRON ORE (HEMATITE IN CHERT)

Tower - Sudan Area

2.0 Billion Years Old

 

This rock occurs within the approximately 2.7 billion year old greenstone lava of northern Minnesota (also see Greenstone specimen, #25).  It formed as iron-rich particles precipitated and settled to the sea floor during quiet periods in volcanic activity.  The red layers are jasper; the white – chert (mostly quartz); and the black are iron bearing minerals – mostly magnetite (magnetic) and hematite (non-magnetic).

 

 

 

6.  BASALT WITH THOMSONITE

1.1 Billion Years Old

North Shore

 

About 1.1 billion years ago, this continent began splitting apart along a rupture called the Midcontinent Rift, which extended from the Lake Superior region southwest to Kansas.  During a period of about 20 million years, thousands of lava eruptions flowed out over a flat landscape.  Layer upon layer of flows accumulated until the growing stack reached a thickness of up to 20 kilometers in the Lake Superior area.  Most of these flows solidified into the dark volcanic rock called basalt.  Gas bubbles in the hot lava caused the basalt to harden with cavities in it.  These cavities gradually filled with other minerals, such as quartz (agates) and Thomsonite.

 

 

7.  MICA SCHIST WITH STAUROLITE

Banks of the Mississippi, Royalton

 

Staurolite occurs in metamorphosed rocks such as gneiss and mica schist.  It forms as a right-angled (90 degrees) cross or oblique angled (60 degrees) cross.  Over time these staurolite crystals will weather out of the rock and may be found in sands and along rivers and streams.  Mica Schist (see specimen #26) doesn’t always contain staurolite crystals.  This specimen (#7) shows staurolite crystals just below the surface of the schist.  Each lump in this rock contains an emerging staurolite crystal.

 

 

 

 

 

8.  BRACHIOPOD FOSSILS

Olmstead County

 

Brachiopod is the name given to marine animals that cover their soft bodies with shells.  The shells left an impression of their shape and sometimes a pearly luster after having been buried under sediment.  During the Ordovician period, approximately 500 million years ago, southeastern Minnesota was covered by an ocean.  Many brachiopod shells were present in these waters.  As they died and were swept up by waves, many of them were broken into fragments.  Sediments buried these fragments along with whole shells, and this specimen is an example of the fossilized casts, or impressions, left by the brachiopod shells.  (Also see specimen #2)

 

 

9.  CALCITE

Pigeon River

 

Large chunks of calcite litter the banks of the Pigeon River, along the Minnesota/Canada border.  Calcite is a sedimentary mineral formed by chemical precipitation through the evaporations of solutions rich in calcium bicarbonate, or by extraction through the action of marine and freshwater organisms.  Calcite forms in many rock types.

 

 

 

10. LIMESTONE, PLATTEVILLE FORMATION (Fossiliferous)

Twin Cities

475 Million Years Old

 

The Platteville formation was deposited in a shallow marine bank.  It is a 9 meter thick limestone with an abundance of fossils.  It dates from the Ordovician Period 430 – 500 million years ago.  One of the places where the Platteville Formation can be seen is at the top of Minnehaha Falls

 

 

 

 

11. ANTHRAXOLITE

Cliff Erie Mine, Biwabik

 

This specimen of anthraxolite was obtained on one of the GSM field trips to the Mesabi Iron Range.  It was found in the Cliff Erie iron mine, located near Biwabik.  Anthraxolite is an organic bitumen – coal.  It is 95% pure carbon.  Present throughout the iron formation, it resembles obsidian, and has a black vitreous luster and conchoidal fracture.

 

 

 

12. VARIOLITIC PILLOW BASALT

Ely

 

Pillow basalt forms when a lava flow is extruded onto the sea floor as glassy-skinned lava blobs, sort of like the way toothpaste is squeezed from a tube.  As each blob, or pillow cools, continued extrusion breaks through and creates another pillow, and another, and another.  This specimen is one small piece of a pillow and has a variolitic structure.  The varioles (spots) are commonly composed of fanlike sprays of feldspar fibers.

 

 

13. LEAF FOSSIL in Cretaceous Clay

Cortland

 

This leaf fossil was found on a GSM field trip to a quarry near Cortland.  We are not certain what kind of leaf it is, nor its age, but since it was found in Cretaceous age clay, it could be as old as 66 – 100 million years.

 

 

 

14. SACRED HEART GRANITE

Granite Falls

 

Granite is found throughout northern and central Minnesota.  It varies in age from 2.6 billion years to about 1.7 billion years old.  Minnesota granites are composed predominately of the minerals feldspar, quartz, mica, and hornblende.  Granite forms deep below the surface in the roots of mountain ranges.  These once deeply buried rocks are now exposed at or near the surface due to uplift and erosion.

 

 

15. CEPHALOPOD FOSSIL

Lillydale Park (a.k.a. The Brickyards)

475 Million Years Old

 

The Ordovician Period ended approximately 438 million years ago.  During the preceding 65 million years, there was a lot of activity in the oceans, which covered much of Southeastern Minnesota.  One creature that swam in that ocean was the Cephalopod.  This fossil represents part of a cephalopod, some of which grew to over 13 feet long.  To see an artist’s depiction of what a living cephalopod may have looked like, look on page 80 of the book “Minnesota’s Geology”, which is on the table.

 

 

 

 

16. OOLITIC LIMESTONE

Burnsville

 

Oolitic limestone forms in warm, shallow, and strongly agitated marine conditions.  The constant action of tides, currents and waves encourages the precipitation of calcium carbonate around quartz sand grains.  The quartz sand grains, coated with concentric layers of calcium, are called ooliths.  This rock is composed of closely packed ooliths.  Examine it with a magnifying glass and you will see the closely packed spheroidal ooliths.

 

 

 

17. PIPESTONE (CATLINITE)

Pipestone

 

Pipestone is a relatively soft, red mudstone, and easily carved when first quarried and wet.  Native Americans have been using pipestone to carve ceremonial pipes and other objects since approx. 900 AD.  The pipestone quarries at the Pipestone National Monument are still used by Native Americans.  Pipestone lies just beneath the Sioux Quartzite (see specimen #21) and gets its red-pink color from hematite.  George Catlin, an American artist, visited the pipestone quarry in 1836, and painted views of the quarry and of the plains Indians.  His paintings and writings were important as ethnographic records.  Pipestone is also called Catlinite in his honor.

 

 

 

18. MARY ELLEN JASPER

Mary Ellen Mine, Biwabik

 

Jasper is a form of microcrystalline quartz.  It is formed from silica that has been dissolved out of silica rich minerals like quartz.  The silica traveled as fine particles in water until settling out as the water evaporated.  Jasper is stained red with deep burgundy sands of hematite.  Mary Ellen Jasper is a variety of jasper that contains fossilized algae mounds, called stromatolites (see specimen #28)

 

 

19. PYROXENITE

Granite Falls, Minnesota

 

Pyroxenite forms in small independent igneous intrusions that are usually associated with ultrabasic rock.  It is composed almost entirely of one or more pyroxenes, and may have small amounts of olivine, hornblende, chromite, biotite and iron oxide.

 

 

 

 

20. DOLOMITIC MARBLE

2 Billion Years Old

Denham Area, Pine County

 

The Denham Formation is the name assigned to a unique package of metamorphosed sedimentary and volcanic rocks that outcrop in Northwestern Pine County, southeast of the town of Denham.  The assemblage of interbedded rock types, formed approximately 2 billion years ago, originally consisted of shale, siltstone, sandstone, conglomerate, dolomite and basaltic volcanic rocks.  After millions of years of metomorphosis, the dolomite completely recrystallized into marble.  The marble layer is at least 500 feet thick.

 

 

 

21. SIOUX QUARTZITE

Renville County

2.55 – 2.7 Billion Years Old

 

Quartzite is formed by quartz sand grains being consolidated and slightly altered into a very hard rock.  The Sioux quartzite is exposed in Rock, Pipestone, Cottonwood, Brown, Watonwan Counties, and along the Minnesota River east of New Ulm, in Nicollet County.  Blue Mound State Park and the Jeffers Petroglyphs are well-known Sioux quartzite sites.  The red color of the Sioux quartzite results from a fine hematite coating on the sand grains.  Many of the buildings in downtown Pipestone, including the historic Calumet Hotel, are built with Sioux quartzite obtained from nearby quarries.

 

 

22. ELY GREENSTONE – SOUDAN IRON FORMATION

With Pyrite vein

2.7 Billion Years Old

 

This specimen of Iron Formation is included to illustrate the pyrite vein, which some people mistake for gold – hence the term “fool’s gold”.  This was found on a GSM field trip, in a road cut along Hwy 1 between Tower and Soudan.

 

 

 

23. MAGNETITE

Cliff Erie Mine, Biwabik

 

 

Magnetite is a common iron oxide mineral, named for an ancient region of Greece where metal production was prominent. It is the only mineral that exhibits strong magnetism, although hematite may have a weakly magnetic behavior. Most magnetite occurs in very small grains. Large chunks of magnetite like this one are called lodestones. The earliest form of the sailor's compass was a lodestone rod mounted on cork and floating in a bowl of water. The rod aligns with the Earth's magnetic field to point roughly north-south.

 

 

 

24. MUSCOVITE (MICA)

Banks of the Mississippi, Royalton

 

Mica occurs as reflective flakes or “books” of flakes.  It occurs in a wide range of colors as well as colorless and black.  White mica is called “Muscovite”.  Mica is a prominent constituent of both igneous and metamorphic rocks.  This specimen is from a metamorphic rock layer.

 

 

 

 

25. ELY GREENSTONE

Ely

2.7 Billion Years Old

 

Greenstone is a weakly metamorphosed basalt that is greenish to gray in color.  This 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.  Island arcs are produced by the convergence of two oceanic plates, or where oceanic crust is subducted beneath continental crust.

 

 

 

26. MICA SCHIST

Pine County

 

The erosion of ancient rocks produced sediment.  These sediments, fine-grained sand and mud, were later deformed by the same forces that caused the uplift of mountains in northern Minnesota.  The resulting high temperatures and pressures formed metamorphic rock called schist.  Schist is composed predominately of mica minerals, which impart a platy or layered texture to the rock.  Schist is common in central and northern Minnesota.

 

 

 

27. ANORTHOSITE

Hwy 61 - Silver Bay

 

Anorthosite is a coarse-grained intrusive igneous rock composed almost entirely of a single mineral – plagioclase feldspar.  This specimen is from a large outcrop along Hwy 61 in Silver Bay.  Studies by NASA geologists indicate that anorthosite was formed near the base of the earth’s crust (20-25 miles below the surface) and was broken off and carried up by the diabase magma during the great rifting event 1.1 billion years ago.  (Also see specimen #31)

 

 

 

 

 

28. STROMATOLITE IN DOLOSTONE (Fossil Algae)

Burnsville

450 Million Years Old

 

A stromatolite is a mounded sedimentary fossil formed from layers of blue-green algae, calcium carbonate, and trapped sediment.  Look closely at this specimen, and you will see the curved mound outlines. This rock is evidence that blue-green algae once lived in a shallow and warm saltwater sea about 480 million years ago, over what is now Burnsville. Algae grow in clusters or filaments, and secrete a gelatinous sheath to protect the living cells from dehydration.  After sunset, algal growth stops, and the gelatinous sheath forms a sediment trap where a layer of fine particles accumulates overnight.  During the daylight hours, the photosynthetic algae grow through and over this sedimentary layer.  As the day and night laminations accumulate, a stromatolite forms.

 

 

29. ST. CLOUD GRANITE

St. Cloud

1.2  Billion Years Old

 

Granite is found throughout northern and central Minnesota.  It varies in age from 2.6 billion years to about 1.7 billion years old.  Minnesota granites are composed predominately of the minerals feldspar, quartz, mica, and hornblende.  Granite forms deep below the surface in the roots of mountain ranges.  These once deeply buried rocks are now exposed at or near the surface due to uplift and erosion.

 

 

 

 

30. MORTON GNEISS

Morton

3.5 Billion Years Old

 

Gneiss (pronounced “nice”) is a coarsely crystalline, foliated metamorphic rock.  This example comes from Morton, Minnesota and is 3.5 Billion years old.  Morton Gneiss, also called “Rainbow Gneiss”, is quarried and cut into pieces used as decorative stone in buildings and monuments.  Gneiss originated as a granitic igneous magma that cooled slowly beneath the earth’s surface over millions of years.  Many more millions of years of great heat and pressure turned the rock into gneiss. 

 

 

 

31. ANORTHOSITE

Carlton Peak

 

In 1902 “Corundum” was found on the North Shore.  A new company, called Minnesota Mining and Manufacturing (3M), was formed to mine this rock for use as an abrasive.  They built crushing and loading facilities and shipped material out for several years.  Eventually is was recognized that the rock being mined was not corundum, but anorthosite, a rock composed mostly of plagioclase feldspar, a much softer rock.  This sample of anorthosite comes from a quarry near Carlton Peak.  (also see specimen #27)

 

 

32. CONGLOMERATE

Glacial Outwash, S.W. Minnesota

10,000 Years Old

 

Conglomerate is a Latin term which means “rolled together”.  Conglomerate is formed by rounded rock debris which is cemented together with a clayey, calcareous, siliceous bonding agent.  The proportion of coarse material must be more than 50% if the name “conglomerate” is to be applied.  In general, several rock types are present in conglomerate.

 

 

 

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