Forest Service Road 423 is a hiking, biking, and horse trail in Pennington County, South Dakota. It is within Black Hills National Forest. It is 1.7 miles long and begins at 5,749 feet altitude. Traveling the entire trail is 3.4 miles with a total elevation gain of 780 feet. The Thompson Reservoir (elevation 5,728 feet) reservoir is near the trailhead.
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