Minnesota Geology Pictures - Mill City Conglomerate & 600 Million Years Missing

Within the Minnesota portion of Interstate State Park lies the informally named 504 million year old Mill City Conglomerate.  The picture above shows one of the outcrops of the conglomerate found in notches exposed through erosion.

The conglomerate consists of basalt boulders cemented together by a reddish matrix of sand and silt.  The size of the basalt boulders and rounded edges indicates a high energy environment during deposition, though some fossils of trilobites and brachiopods have been found in the conglomerate.  Quite likely, the conglomerate was deposited on the shoreline of the Cambrian seas near basaltic islands.  The basalt is part of the 1,100 million year old Mid-Continent Rift System. 

Not far from the campground, the contact between the 504 million year conglomerate and the 1,100 million year old basalt.  600 million years of time is missing between these two rock types.

Minnesota Geology Pictures - Stromatolites in the Biwabik Iron Formation

In northern Minnesota, the Biwabik Iron Formation was deposited 1,900 to 1,885 years old along the shores of the Animikie Sea.  At this time, surface water was busy eroding the continent located to the north bringing fine materials like silt and clay into this shallow sea that would later be incorporated into the iron formation.  Before the time of iron deposition though, reduced iron was easily dissolved and concentrated into the sea water because there was not free oxygen molecules in the atmosphere or seas at the time.

This was a unique time period in Earth's history, as photosynthetic organisms begin to appear in the rock record worldwide.  These organisms, called stromatolites, formed algal reefs along the shoreline of these ancient seas and as a waste product of photosynthesis, produced oxygen.  It is this oxygen that caused the iron to precipitate out of the seawater and be deposited as iron.  This time period is called the Great Oxidation Event (GOE) and is responsible for most of the iron deposition throughout the world.

Each domed structure above would have been an algal mound located at or near the surface of the water 1,900 years ago, busy precipitating iron to the seafloor as a consequence of photosynthesis.  This particular field of stromatolite domes is found within an inactive iron mine and is being preserved, there are no future plans to mine the iron in this location.

Minnesota Geology Pictures - Pillowed Greenstone

In Gilbert, Minnesota, is a site of 2,700 million year old pillowed greenstone.  Basaltic lava would have originally have erupted into an ocean or sea forming rounded structures called pillows.  Through weak metamorphosism, involving higher temperatures and pressures, original minerals found in the basalt like plagioclase and pyroxene changed to chlorite and epidote.  The edges of the pillows have crystals much smaller than the interior.  As the lava erupted into the colder water, the edges cooled faster resulting in small crystal growth, while the interior cooled relatively slower resulting in larger crystals.

Much later in time, during the late Wisconsin glaciation, glaciers from the Rainy Provenance scoured the area, largely cutting the pillows in half and leaving numerous striations across the pillows.

Minnesota Geology Pictures - Folded Banded Iron Formation

Very near the Soudan Underground State Park administered by the Minnesota Department of Natural Resources is what some people call 'the most photographed outcrop in the state.'  This is a pavement outcrop of folded banded iron formation.  The outcrop consists of metallic hematite, red jasper and white chert.  These originally horizontal layers have been folded multiple times.  In some areas, the jasper and chert have fractures filled with milky quartz.

The picture above gives a larger sense of scale for the site.

Frozen Minnehaha Falls

A week ago, a colleague of mine visited Minnehaha Falls and took the picture above to show me.  The bedrock of the Minnehaha Falls area consists of the resistant Platteville Limestone, the Glenwood Shale and the St. Peter Sandstone.  As water falls over the more resistant limestone, it erodes the weakly cemented St. Peter Sandstone at the base of the 53-foot waterfall.

What's interesting to me in the picture isn't how the water has slowly frozen in the waterfall through the course of a Minnesota winter, it's all of the other frozen water to the right and left of Minnehaha Falls.  In between the limestone and sandstone is the Glenwood Shale.  As water soaks downward through the upper layers of soil it comes to this impermeable layer of shale and flows laterally until eventually reaching the eroded valley carved by Minnehaha Creek.  At this point the water cools to the point of freezing, forming a ring of large icicles.

On September 29, 2012, I took a gigapan of Minnehaha Falls (with a very low amount of water).  For a full size version of the gigapan, please click here.

For a more detailed post on the Minnehaha Falls, please click here.

Minnesota Geology Pictures - Gunflint Iron Formation & Magnetic Rock

In areas of Northeastern Minnesota, the bedrock is comprised of the 1,878 million year old Gunflint Iron Formation.  The iron formation consists of black or gray chert alternating with black iron-rich layers.  During the late Wisconsin glaciation, the southwest trending Rainy lobe left a large slab of iron formation laying vertical.  The bedding planes of the surrounding iron formation is nearly horizontal, contrasting with the vertical bedding planes of this slab.  The name given to this 25 foot slab is Magnetic Rock (though many areas of the iron formation are magnetic).

For a more picture heavy look at the hike into Magnetic Rock, click here.  The trail is over a mile in length to the rock.

Minnesota Geology Pictures - Sudbury Impact Breccia

At the beginning of 2012, I started what I thought would be a weekly series called Minnesota Monday Geology, with posts regarding different geological topics around the state.  For nine months the series went by well, until the beginning of the school year where my time was taken up with work associated with school, like planning lessons, grading jail, and taking care of my three children at home.  Although I have numerous posts to finish the series somewhere in my mind, as well as several including geology from neighboring states like South Dakota and Wisconsin (including a recent trip to Van Hise rock) and several with gigapans that I've begun taking, I'm starting another series today.

Nearly 1,850 million years ago, the region that is now Sudbury, Ontario was struck by an asteroid at least six miles in diameter.  A record of this catastrophic moment in time is preserved in the rock record along the Gunflint Trail, near the Canadian border in Minnesota.  The impact breccia at this location has been interpreted as a submarine debris flow 450 miles from the impact site.  At the time of the impact, this area of Minnesota was located near the shoreline of an ancient sea.  Found directly beneath the breccia layer is the 1,878 million year old Gunflint Iron Formation, directly above the breccia layer is the 1,850 million year old mudstones and shales of the iron-free Rove Formation.  The breccia layer is almost 7 meters thick, with the lower levels containing angular pieces of Iron Formation several meters across.  The iron formation found in the upper levels of breccia are much smaller.

A more complete post on the Sudbury Impact Breccia can by found here.

Both of my sons went on a hike to the breccia outcrops last June (2012).  It can be a challenging hike through some dense underbrush for 6 year old legs.

My 4 year old was more interested in the leaf that he 'captured' than what was around him, oh well.

Minnesota Geology Monday - Mississippi River Headwaters

The Mississippi River is the fourth longest river (2,552 miles) in the world and drains parts of 31 states and 2 Canadian provinces.  The river also has the 4th largest drainage basin in the world (over 1,245,000 square miles), representing almost 40% of the continental United States landmass.  Historically important for early Native Americans and used during the fur trade, today the river is important for tranport of materials via commercial barges.

Expeditions in the early 1800s sought to discover the headwaters of the Mississippi River.  In 1832, led by Native American guides, Henry Schoolcraft identified Lake Itasca as the source of the river.  Schoolcraft named the lake Itasca as a combination of two Latin words, veritas (meaning truth) and caput (meaning head).  The lake is now the namesake of Lake Itasca State Park, which was created in 1891 to preserve the area's remaining virgin pine forests and is Minnesota's oldest state park.

Lake Itasca is at an elevation of 1,475 feet above sea level and is a typical kettle lake found in the area, a result of late-Wisconsin glaciation.  The lake is 1.8 square miles and has an average depth of 20-35 feet deep.  Retention time, the amount of time needed for water to flow from Lake Itasca to the Gulf of Mexico, is approximately 90 days.

The headwaters area was originally swampland.  To give tourists better access, the Civilian Conservation Corps in the 1930s drained the surrounding swamp, dug a new channel and installed a rock rapids.  Many visitors to the area walk across these rock rapids and some subsequently fall into the brand new, Mississippi River.

Minnesota Geology Monday - Dry Waterfalls

At the beginning of August, 1,584 counties in 32 states had been declared disaster areas due to the ongoing drought affecting the United States.  Southern Minnesota has recently received enough rain to move to from the status of 'Drought - Severe' to 'Drought - Moderate' or even 'Abnormally Dry'.  The dry summer months has caused numerous problems, least of which is the drying of the regions waterfalls.

Minnemishinona Falls and the nearby Minneopa Falls are dry, or nearly so.  Both falls occur as water falls over the late Cambrian-aged Jordan Sandstone.  Differential cementation in the sandstone forms the falls, better cemented areas form the top of the falls while while weaker cementation areas are easily eroded at the bottom of the falls.

The picture below shows Minnemishona Falls on June 2, 2012.  Not a high flow rate, but still flowing.

The picture taken on August 4, 2012 shows the falls dry after two months of very little rain.  It is apparently not unusual for this particular waterfall to go dry during late summer months due to the creek's small watershed.

Minneopa Falls is a regional tourist attraction for the beauty of the falls.  The picture below was also taken on June 2, 2012.

Two months later and there is hardly a trickle of water flowing over the falls.

Minnesota Geology Monday - Sioux Quartzite

The most prominent bedrock in southwestern Minnesota is comprised of the 1,700 million year old Sioux Quartzite that extends into South Dakota.  The quartzite ranges in color from pink to purple depending on the amount of iron oxides present.  The quartzite is derived from quartz sand, the source of which was the erosion of the Penokean Mountains, which were located across central Minnesota and into Wisconsin during Proterozoic time.  As the mountain ranges were weathered and eroded, most minerals were broken down into clay minerals and transported away.  The resistant quartz grains were transported south and deposited in a braided stream environment.

Cross beds and ripples found in the quartzite indicate the direction of movement of the flowing river systems as the sandgrains were being deposited.  Most of the cross beds and ripples indicate a southward direction.  Across the border in South Dakota, there are a few locations that have southward dipping cross beds that are found directly beneath northward dipping cross beds.  This potentially suggests that the depositional environment found in this location would have been near an ocean shore under the influence of tides.

Being highly weathering and erosion resistant, the Sioux Quartzite tends to be cliff forming in several locations around the region.  At Blue Mounds State Park, near Luverne, pioneers moving westward used the cliffs as a navigational marker.  The cliffs appearred blue at a distance and were given the name 'blue mounds'.

At Pipestone National Monument, the quartzite also forms cliffs.  The quartzite slowly weathers, primarily via ice wedging along vertical fractures.

During late-Wisconsin glaciation, glaciers also left their mark on the Sioux Quartzite.  Rocks and pebbles embedded in the ice left many glacial striations on exposed quartzite throughout the region.

Early Native Americans also found uses for such a resistant bedrock.  At the Minnesota Historical Site called the Jeffers Petroglyphs, carvings were made into the quartzite beginning 7,000 years ago and continued until recent times.  Carvings at the site include birds, turtles, bison, thunderbirds, weapons and a great number of other objects.  The site most likely represents religious illustrations of impartant aspects of the lives for these earlier Native Americans.

The Sioux Quartzite has been (and still is) used widely as both dimension stone and aggregate.  Modern quarries of Sioux Quartzite can be found as far east as New Ulm and extend west into South Dakota.  Early quarries used for these purposes include this late 1880s quarry found near Luverne.

Many towns in the region have buildings made out of the resistant Sioux Quartzite, including the Rock County Courthouse.

Minnesota Geology Monday - Pipestone National Monument

Pipestone National Monument is located on US Highway 75, approximately 25 miles north of Interstate 90.  The monument focuses on how Native Americans have quarried catlinite out of the region to be carved into pipes or other structures.  Catlinite is a metamorphosed mudstone with a fine-grained texture and it can be easily carved.  Due to the presence of iron oxides, the catlinite is deep red color.

At the entrance to the park office is the first stop, a series of granite boulders called the Three Maidens where Native Americans would leave offerings before proceeding to the quarries.  Native Americans realized that these boulders did not match the local bedrock and thus were significant spiritually.  These glacial erratics were transported from the Ortonville, MN area and broken into many pieces by the process of ice wedging.

Prior to the late 1880s, there were 35 pieces of rock containing petroglyphs that had been placed at the Three Maidens location.  These were removed in 1888 or 1889 because some of the petroglyphs had been defaced, but were returned to the monument in the mid-1900s.  These petroglyphs are now on display at the visitor center.

Bedrock in southwestern Minnesota is the 1,700 million year old Sioux Quartzite.  The quartzite ranges in color from white to pink to purple and derived from quartz sand.  The source location of the sand grains is the erosion of the Penokean Mountains which were located across central Minnesota and into Wisconsin during Proterozoic times.  Cross-beds and ripples found in the quartzite indicate the direction of movement of the flowing water as the sand grains were being deposited.  The majority of the cross-beds and ripples suggest a southward direction of flow.  The Sioux Quartzite correlates well with other quartzites in the region, including Wisconsin's Baraboo Quartzite.

Because the quartzite fractures along vertical joints, it tends to form cliffs.  Though weathering of this nature has also formed feature known locally as the Old Stone Face.

Or this structure known as the Oracle.

Because the quartzite is erosion resistant and cliff-forming, a creek has developed a waterfall in the park over the escarpment called Winniwissa Falls.  It must be one of the very few waterfalls present in southwestern Minnesota.

Further south of the waterfall, the escarpment continues, at the base of the escarpment are evidence of plunge pools and scour holes.  Looking west of the escarpment (the direction the creek flows) there appear to be dry channels.  As the glaciers where melting in the late Pleistocene, meltwater would have been pouring through the area creating a much larger waterfall.

Native Americans continue to quarry the catlinite in several quarries like shown below.  The park service needs to pump water out of the quarries before work proceeds.

Quarrying would have (and still does) as demonstrated by one of the park's informational signs.  The letter A represents the catlinite, B is the overlying Sioux Quartzite, C is the soil layer, while D is the rubble pile created by removing the Quartzite and soil.  Since the quartzite and catlinite layers dip be 5-10 degrees to the east, more and more quartzite must be removed in order to reach the catlinite layer.

The park service has cleaned up one of the older quarries for tourists to enter.  At the bottom of the quarry face is the catlinite layer, the layer is approximately 10-15 inches thick and much softer than the overlying Sioux Quartzite.

Because of the cultural and geological significance, Pipestone National Monument was recently included as one of the 101 American Geo-Sites You've Gotta See in the book by Albert Dickas.