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.