The Crazy Mountains basin is a physiographic depression, as well as a structural basin in south-central Montana. The basin is bounded on the north by the Castle Mountains, Shawmut anticlinal trend, and the Huntley-Lake basin fault zone; on the west by the Bridger and Big Belt Mountains, on the south by the Nye-Bowler lineament and the Beartooth Range; and on the east by the Pryor-Big Horn uplift. Separation of the Crazy Mountains basin from the Big Horn basin to the south is arbitrarily placed along the Nye-Bowler lineament.

The Crazy Mountains are a passive intrusive complex occupying the deeper part of the basin. This setting is analogous to the Raton basin of the Colorado and its associated Spanish Peaks intrusives.

The structural pattern of the Crazy Mountains basin is somewhat enigmatic, being most varied and diverse. Structures within the basin resulted from the superposition of Laramide compression on an earlier fragmented Precambrian crystalline basement. The resulting structures are folds related to thrusting; drape folds over faults with dip-slip movement; and en echelon folds with "gash" fractures related both to left-lateral and right-lateral transcurrent fault movement.

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