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.

Minnesota Geology Monday - Folded Banded Iron Formation

Minnesota's first iron ore mine opened in 1882 and is called the Soudan Mine, initially there were several open pit mines in the area.  In 1892, the work moved underground and continued until 1962.  During the time that the mine was in operation, over 14 million metric tons of iron ore were removed.  The state bought the mine and approximately 1,200 acres surrounding the mine for $1, on the condition that the area be converted to a park.  Today visitors to Soudan Underground Mine State Park can ride a metal skip to the 27th level (2,341 feet below the surface) for a tour of the mine or visit a physics laboratory that is also operated at this depth.

Near the mine is also a classic pavement outcrop of folded banded iron formation.  Some have called it the most photographed outcrop in Minnesota and it was recently added as one of the 101 American Geo-Sites You've Gotta See.  The day that I visited the site, it was unfortunately late in the day, with a slight drizzle and thunderstorm moving in, but here are some of the pictures taken that day.

The site consists of layers of metallic hematite, red jasper and white chert.  These layers were folded multiple times, though questions arise about whether the folding occurred while the sediments were still soft or after they had lithified.

The site is a short walk away from the mines headframe, the stands over the mine shaft supporting the cables and skips that brought people and ore from the lower levels to the surface.  Because of several small roads in the vicinity, you do not need to enter through the park's main entrance to get to the outcrop, Stunz Bay Road climbs a small hill and leads practically right past the outcrop.

In many places, the layers of red jasper and white chert have small fractures that are filled with milky quartz.  This suggests that the jasper and chert were brittle during the folding event.

Minnesota Geology Monday - Granites of the St. Cloud Area

St. Cloud has the nickname "The Granite City" for good reason.  There were almost one hundred granite quarries in the area over time, but currently there are four in operation in towns like Cold Spring, Rockville and St. Cloud.  Numerous granite plutons are found in the region, together they are known as the East-Central Batholith.  The individual granite plutons have also been informally named, including the Richmond, Rockville and St. Cloud Red, among others.  Radiometric ages of the granites range from 1,800 to 1,750 million years.

The granite is quarried in the area by several companies for use as either dimension stone or aggregate.  Cold Spring Granite maintains several quarries and a processing plant for decorative uses on buildings, countertops, headstones, etc.  The quarry pictured here are from the Rockville Quarry #2, which is sold by the tradename Rockville White. 

Located just above the granite in the area, and seen well in many quarries, are rounded boulders of granite.  These are corestones.  The granites in the area are jointed, weathering of the granite affect the corners of the joints first, eventually smoothing the corners and producing rounded boulders.  The weathering event most likely took place during the Cretaceous, the same period which formed the kaolinite clays found in the upper Minnesota River Valley.

The region is also swarmed by many diabase intrusions.  These intrusions are easily seen within the Martin Marietta aggregate quarry.

Below is a picture of me standing in front of a diabase dike intruding on a body of St. Cloud Red granite.

Because quarries are privately owned and a potential safety hazard, exploring them without guidance or permission is not allowed.  Quarry Park and Nature Reserve is a public park that allows easy access to many of the features found within the granite throughout the region.  Like the name suggests, the area was once home to almost 30 granite quarries.

The granite present in the park is known as the St. Cloud Red granite.  Most of the former quarries are now filled with water, one is used as a swimming pond that is 112 feet deep.  There are also large piles of quarried rock found throughout the park.

Walking the many hiking trails within the park, it's easy to examine the relationship between the granite and diabase intrusions.

Upon closer inspection, you are also able to find places where pieces of granite were broken off by the intruding magma.  These inclusions of granite are now frozen within the diabase intrusion.