A fault system is described that is located in Armstrong County in central South Dakota. Superficially it is similar to the system described by Hollinger in the Ironstone district in England. The fault spacing is fairly close, the displacement usually small and the individual faults are commonly upthrown toward the Missouri River. The Ironstone faults were ascribed to the tilting of vertically jointed blocks toward the valleys under the influence of gravity. As some of the faults in the South Dakota area are fifteen miles from the major stream valley the force developed by gravity would not seem adequate. If these faults are surficial the writer suggests that a tangential stress may have been stored in the rocks since the folding of the Lemmon syncline and that the cuttin of the rivers relieved these stresses permitting the adjacent rocks to move toward the valleys. Because relief would be easier near the surface there would be a tilting of joint blocks toward the river with a small fault displacement at each joint. A weak point in this mechanism is that there is some evidence suggesting that the faults extend below the bed of the Missouri River.

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