I was able to spend only a short time at Interstate Park in Taylors Falls, MN.  With cooperation of Wisconsin, Interstate Park is on the shores of the St. Croix River crossing state boundaries.  To reach some of the most interesting geology of the park, the potholes in basaltic rock, follow US highway 8 into Taylors Falls and turn right at the first stop light.  The parking area and Visitor Center are at the end of a 1/4 mile self guided tour.
    The basic geology of the area started with the shifting of the midcontinental rift that goes from Lake Superior through Minneapolis, central Iowa and into Kansas.  This area is called the Midcontinent Gravity High because the higher density rock causes an increased gravitational attraction in this area.  Lava erupted along this line to produce the basalt bedrock of Lake Superior and Taylors Falls.  At least seven different lava flows are visible in the park as well as deposits from two glaciers.  During the Wisconsinan Glacial Period the Grantsburg Sublobe directed meltwater from Glacial Lake Grantsburg and Glacial Lake Duluth (it was blocked on the eastern end by the Superior Lobe) to flow through Taylors Falls. (1)  The erosion formed the gorge and the Dalles (rapids flowing over a flat bottom in a narrow part of a river - Webster, 1992) of the St. Croix.  The "falls" are no longer visible because of a dam built to provide electrical power further downstream.
     Here at the entrance to the pothole section of the park (map) the cliffs of basalt rock stand against the erosion of the river.  The photo above is just one of the many examples of beautiful cliffs in the area.

     In the photo above right there is a gabbro inclusion in the basalt rock, very similar to the inclusion in basalt near the Lester River in Duluth.

     The photo at the right shows some pillow lava formation in the basalt.  It is evidence that the lava flowed into a marine environment as it was formed. 

     These photos were taken from Angle Rock (see the detail on the map) as the river turns sharply right.  Earthquakes caused a fault that the river followed as it reached this rock.  In photo at the right you might be able to see some of the fault lines as well the traces of swirling current that at least in part caused the huge potholes all around this area.

     The two photos below show two of over one hundred potholes in the park.  Some of the potholes are only a few inches across and deep while the largest measured (bottom right) is 12 to 15 feet wide and 63 feet deep.  The widest pothole , "The Caldron", is over 20 feet wide.  Potholes form when the swirling water carrying sand cuts circular holes in the rock.  How it starts in not known but as they grow more sand and rocks get trapped in the eddies and continue to drill away more rock.  Even larger potholes are suspected in the marshy area next to the road landing. (2)

    The potholes are an excellent place to study the formation of basalt from the lava flows.  In the series of photos above taken from inside one pothole you should be able to see that at the bottom of the pothole (top left photo) there are no air pockets or holes in the basalt but as you look higher (moving left to right in the photos) you should be able to see that there are holes that become larger the closer the rock is to the surface.  This happens because as the lava starts to cool to form rock the air bubbles are also rising.  The largest bubbles would rise the fastest so they would be closer to the top while at the bottom no air bubbles remain.
This photo shows one of the many fault lines visible in the park.  I was not able to spend much time at the park on this visit but my family is already making plans for a return visit.
     The St. Croix River occupies the gorge in the old basalt.  To get out of the river valley a road cut on US Highway 8 through the younger sedimentary rock shows some of the effects of time.  The photo above shows the limestone and shale cap and the photo above right shows the sandstone in a similar configuration to the Mississippi River Valley.

1.  MN Department of Natural Resources, The Geology of Interstate Park, 1994, pp 1-5
2.  MN Department of Natural Resources, The Geology of Interstate Park, 1994, pp 8-9

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