The Castor River Shut-Ins

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Published on: Jun. 2, 1996

Last revision: Oct. 21, 2010

Over thousands of years, the clear waters of the Castor River have cut into ancient pink granite to create deep gorgelike chutes. Now trapped in these narrow channels, the rushing waters roar.

Old growth pines and cedars dot the high, steep banks, their verdant tones contrasting with the luminous salmon-colored boulders. In May, azaleas bloom in the rocky woodland and add their speckled pink delight.

You can enter this scene, if you visit the 1,630-acre Amidon Conservation Area, which lies on the border of Madison and Bollinger counties in southeast Missouri.

The area is located in the St. Francois Mountains, the principal location in Missouri where granite and rhyolite are exposed at the earth's surface. Granite and rhyolite are two types of igneous or fire-formed rock created over a billion years ago as molten lava flowed over the land surface, and deep magma cooled and solidified.

The deep, narrow channel of the Castor River in the northern part of Amidon Conservation Area is a classic example of a shut-in. Shut-ins are geologic features that are formed as streams erode away relatively soft limestone and dolomite, until they encounter deeper igneous rock, which is much harder to erode.

Streams and rivers running through igneous rock can cut only deep, narrow channels and are given little opportunity to form meanders as most streams do. As a given volume of water passes though these shut-ins, the water's velocity increases, creating the rushing, bubbling effect that makes the scene at Amidon so appealing.

Amidon Conservation Area was named for Evelyn and Ellsworth Amidon, who donated part of the area to the Missouri Department of Conservation in 1984, but many local people still know the area as Hahn's Mill. In the late 1800s, two brothers, Fred and John Hahn, operated two mills on the area.

The Mill Shoals mill, near the north end of the area, was used to grind grain. The river was dammed at the top of the shut-in and a mill race was cut through the adjoining granite. Water was channeled to a water wheel and turbine that were located downstream. This mill was destroyed by a flood in the early 1900s, but holes in the granite are still visible at the mill site, presumably where pins attached the structure to the rock.

The mill at the lower end was located several hundred feet from the river, and a long race carried water across a field to the mill. It was

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