Structural geology . folds with referenceto the adjacent competent beds tells the direction of the differen-tial movement. The axial planes are nearly parallel to cleavage(see pp. 119-120). When this movement is of great proportionsthe axial planes of the minor folds may become so rotated as togive the abnormal type of composite fold. Beds of shale mayindicate a differential movement of quartzite beds above and be-low in the direction shown on diagram 56. DRAG FOLDS 115 The position of the major fold is inferred from the differentialmovement indicated by the minor folds. The major fold may int
Structural geology . folds with referenceto the adjacent competent beds tells the direction of the differen-tial movement. The axial planes are nearly parallel to cleavage(see pp. 119-120). When this movement is of great proportionsthe axial planes of the minor folds may become so rotated as togive the abnormal type of composite fold. Beds of shale mayindicate a differential movement of quartzite beds above and be-low in the direction shown on diagram 56. DRAG FOLDS 115 The position of the major fold is inferred from the differentialmovement indicated by the minor folds. The major fold may inturn be found to be one of a series of minor folds related to a stilllarger fold. This is something more than the statement of an academicprinciple. The writer regards it as one of the most fundamentalprinciples in the field study of structures. Adherence to thesimple plan of watching for indications of differential movementleads to surprising results. In the Lake Superior pre-Cambrian / s J-* ^ .d^*> v dr.© £i4. Fig. 56. Figure showing differential movement between competent beds on limbsof a fold with the development of minor drag folds between them. districts it has been possible, by studying the minute crenula-tions of the softer beds, to determine the differential movementof the controlling strata on each side, and thereby to obtain anotion of the position of the next larger unit of structure. Thishas led to a study of still larger units, and so on. In the Mar-quette district of Michigan the slate beds are folded in the mannerto be expected from the control of the harder quartzite layers ofthe Marquette synclinorium. Understanding this relation, thecomposite outlines of the slate folds may be satisfactorily corre-lated with the simple outlines of the quartzite folds. The Mar-quette synclinorium as a whole may be regarded as a minor foldshowing differential movement upon the limb of the major LakeSuperior synclinorium. 116 STRUCTURAL GEOLOGY
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