Abstract

In mapping the closely folded Belt sediments of the Coeur d'Alene district, Idaho, the question frequently arises whether certain beds are overturned or not. The answer can in many cases be found by observing mud cracks, ripple marks, cross-bedding, and vertical gradations of grain size. Even more helpful is cleavage, which is developed throughout the district and therefore can be used in many places where the sedimentation features just enumerated are lacking. The cleavage is believed by the writers to be mainly fracture cleavage. Both flow and fracture cleavage are useful in detecting overturns, because both dip more steeply than the beds in the upright limb and less steeply than the beds in the overturned limb of an overturned fold. Drag in cleavage, as in bedding, may indicate the proximity of faults. All these relations of cleavage to other structures are well illustrated in the Big Creek anticline.

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