The Fanny Peak quadrangle is located on the west flank of the Black Hills. The exposed strata total over 4000 feet in thickness and range in age from Mississippian to late Cretaceous. Local surficial Cenozoic deposits are common.
The upper 200 to 300 feet of the Minnelusa formation is a breccia whose origin is attributed to removal of anhydrite by ground water and consequent collapse of the overlying strata. Crude bedding indicates uniform collapse; breccia pipes result from more intensive brecciation. The anhydrite probably occurred as lenses at different levels. 3recciation started after the Black Hills were uplifted and the Minnelusa was breached. Calcium sulphate in spring waters attests to present removal of anhydrite.
The Minnekahta limestone exhibits secondary structures attributed to gravity sliding. Cascade folds and slip sheets with displacements of not more than 30 feet developed in exposed beds whose dip is more than 30 degrees. Numerous intraformational folds and faults formed when beds within the limestone slid over each other. Shortening of the Ninnekahta limestone of about 200 feet per mile is estimated.
Four monoclines and associated structural terraces are interpreted as reflecting six fault blocks at depth with minimum displacement of 6000 feet. The intersecting Fanny Peak and Black Hills monoclines represent lines of major deformation. The Fanny Peak monocline passes into a near-vertical normal fault with a throw of approximately 800 feet two miles north of the quadrangle.
