Johnson's Shut-Ins State Park
The last stop for the day is the very popular Johnson's Shut-Ins State Park in the southwestern region of the St. Francois Mountains. The term "shut-in" refers to an area of a river which is compressed into a narrow valley surrounded by steep sides due to lithologic changes. In this case, the shut-ins are constructed as the East Fork of the Black River erodes through the Cambrian sandstone and dolostone reaching the Precambrian rhyolite. These harder igneous rocks are more difficult to erode, however the uncovering of these rocks removes a tremendous pressure from them. They respond
to the pressure release by expanding and fracturing. The water flowing through these rocks exploits these fractures forming chutes through the rhyolite. As the water is forced into the small chutes, the water picks up speed (as described by law of continuity in hydrodynamics). The load carried by fast moving water polishes the surfaces that it flows over. As the load is swirled about in the hydrualics of the water flow, potholes are carved out of the resistant ash flows. The rhyolitic rocks are leached of their characteristic pinkish colors and adopt a blue-grey color. Immediately below the shut-in region is a deeper pool of quite water as the small conduits are replaced by a wider river channel. The change in velocity of the river water also drastically changes the sediment load and a large cobble to pebble beach occurs below the shut-ins.
Following the river downstream several hundred meters from this region we reach the lower shut-ins. In this region the river channel is not quite as "maze-like" as the upper shut-ins, forming more of an in-line chute. Following this the river widens again and flows out of the shut-ins area.
One reason for the popularity of this park is the ability to while away a warm day in the cool pools formed in the upper shut-ins region. In the picture on the right we see (from right to left) Amanda Reibel, Duane Ford, Sarah Coddington and Stuart Perez considering the pros and cons on whether to enter the cool spring fed waters. Also apparent in these and subsequent images are the intersecting fractures in the rhyolite.
In the next image on the left we see that several of the students (Aimee Forgey, in the
foreground with backpack on, Stuart Perez, in the background with hat, Amanda Riebel, on right foreground in white tee shirt, and Duane Ford, on right background in white tee shirt) have overcome their initial misgivings and are enjoying wading through the cool water.
Alas, the slippery rocks take their toll as in the next image on the right we see the aftermath of a harmless but somewhat embarrassing dunking in the water for Stuart Perez
(still wearing the hat). Aimee Forgey (on left), Duane Ford (right-center) and Amanda Riebel (far right), offer their unique form of sympathy and support.
To learn more about Johnson's Shut-Ins State Park, check out the park pamphlet by clicking on the icon to the left. Inside this site is a link to the Missouri Department of Natural Resources
web page for the park.
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The last stop for the day is the very popular Johnson's Shut-Ins State Park in the southwestern region of the St. Francois Mountains. The term "shut-in" refers to an area of a river which is compressed into a narrow valley surrounded by steep sides due to lithologic changes. In this case, the shut-ins are constructed as the East Fork of the Black River erodes through the Cambrian sandstone and dolostone reaching the Precambrian rhyolite. These harder igneous rocks are more difficult to erode, however the uncovering of these rocks removes a tremendous pressure from them. They respond
to the pressure release by expanding and fracturing. The water flowing through these rocks exploits these fractures forming chutes through the rhyolite. As the water is forced into the small chutes, the water picks up speed (as described by law of continuity in hydrodynamics). The load carried by fast moving water polishes the surfaces that it flows over. As the load is swirled about in the hydrualics of the water flow, potholes are carved out of the resistant ash flows. The rhyolitic rocks are leached of their characteristic pinkish colors and adopt a blue-grey color. Immediately below the shut-in region is a deeper pool of quite water as the small conduits are replaced by a wider river channel. The change in velocity of the river water also drastically changes the sediment load and a large cobble to pebble beach occurs below the shut-ins.
Following the river downstream several hundred meters from this region we reach the lower shut-ins. In this region the river channel is not quite as "maze-like" as the upper shut-ins, forming more of an in-line chute. Following this the river widens again and flows out of the shut-ins area.
