Monday, July 23, 2012

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.

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