The last stop of our field trip is at the Thomson Dam area near Jay Cooke State Park on the St. Louis River.  A map of the park shows the Thomson Dam just outside the northwest end of the park forming the Thomson Reservoir.  It is a unique area because of the middle Precambrian Thomson Formation rocks as well as late Precambrian Keweenawan basalts.  The Thomson Formation rocks are graywacke sandstone and siltstone along with slate.  Most of the world's iron ore formations were formed during the middle Precambrian period so seeing iron containing rock in this area is not unexpected.  The river flows through an area where a fault has occurred along with small scale folding of metamorphic rocks. 
     On our visit the river is very high with a lot of water flowing over the dam.  This makes the structures more difficult to see compared to this photo of the same area and the potholes we were hoping to see are covered with water.
     This is the geology and landscape description of Jay Cooke State Park on the MN State Parks website:
Geology  The geological makeup of Jay Cooke State Park is one of slate, graywacke and red clay. Streams have exposed the bedrock in many places. Slate beds were formed from original deposits of mud which compacted into shale. Heat, pressure and movement converted the deeply buried shale into slate. Underground movement caused the slate and graywacke beds to fold and fracture. Later, masses of molten rock were forced through fractures in the beds and when they cooled, these intrusions formed dikes which can be seen in the river bed today.

Landscape  The rugged land formations of Jay Cooke State Park enhance the beauty of the hardwood forests. The water-eroded gorge, steep valleys, and massive rock formations are seen throughout the park. In some seasons, the water of the St. Louis River thunders over slabs of ancient, exposed rock. At other times, it slows to a gentle trickle. Visitors enjoy the scenic splendor of Jay Cooke State Park during all seasons.

     The slates and graywackes of the Thomson Formation were formed from marine muds about 2 billion years ago. (1)  The muds became shales from deep burial and compression and later became slates when subjected to heat and pressure.  Still further pressure and heat again folded the slates causing what is called kink-band folding. The photos of the rocks right and below show some of this kink-band folding.  The photos are all of the same rock at the edge of the road on the northeast side of the bridge.

     The photo at the bottom right is of Thomson Dam and the bridge for Highway 210 over the St. Louis River.  You can see from this photo how much higher the Reservoir is than the river below. 

     In the photo at the right it is easy to see the lower slate turned upward with a sandstone cap.  Note the bedding angles on the two kinds of rock with the obvious unconformity between them.  Only about half of the sandstone grains are quartz, the others are feldspars and angular rock fragments. (3)  This indicates that the sandstone deposition must have occurred shortly after erosion with little sorting. 

     After the slates were formed molten rock was forced into cracks in the slate and cooled into igneous dikes.  The photos below show the northwest side of the bridge, including a large white quartz dike that  supposedly contains traces of gold. (2)  The left photo is from a position on the northeast side of the bridge, the right photo is from directly above on the northwest end of the bridge.

     It is also easy to see the folding and tilting of the slates in these photos, evidence of faulting activity.

     This photo is of rock outcrops just below the dam, about 10 yards north of the rock with the sandstone cap above.  Notice how the fractures lie at about a 45o incline.  Since these are slates or graywackes formed from mud layers that must be put down on a nearly horizontal surface and that the fractures occur along these bedding lines, considerable faulting must have occurred to tilt them to this angle.

     The photos below show more tilting and folding with the arrow in the bottom left photo pointing to a fault line.  Notice how the beds tip opposite directions on each side of vertical line.

     The rock at the left is one of  many stepping stones on the trails around the Thomson Dam area.  This one is an example of slate.  Notice the sheet like trends in the layers of the rock, typical of mud layers that have been compressed into shale and then later metamorphosed into slate.
     These photos show what must be a famous anticline feature.  The "A" shaped fold in rock.  This rock shows up as an example of an anticline in many books, (4), (5) as well as in websites.  It is on the southwest side of the bridge and is easily visible from a trail that runs along the river on the southeast side of the river
While this was the last stop on the group field trip, there is much more to Minnesota Geology so over the next few days I extended the field trip on my own.  This trip takes me to St. Cloud, Rockville & Cold Spring; St. Peter, Kasota & Mankato; Minneapolis & St. Paul; Taylors Falls & Interstate Park; Morton & Redwood Falls; Pipestone & Jasper; and long before this trip, to Lake Agassiz in northwestern Minnesota.  Click the next button below to continue this trip.
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1.  Sansdome, Constance J.; Minnesota Underfoot, 1983, p 27
2.  ibid., also Ojakangas, Richard W. & Matsch, Charles L.; Minnesota's Geology, 1982, p 166
3.  Ojakangas, Richard W. & Matsch, Charles L.; Minnesota's Geology, 1982, p 42
4.  Ojakangas, Richard W. & Matsch, Charles L.; Minnesota's Geology, 1982, p 44
5.  Sansdome, Constance J.; Minnesota Underfoot, 1983, p 27