By John L. Roberts (Auth.)
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Additional info for Introduction to Geological Maps and Structures
Sample text
8. Definition of crest and trough in a fold profile. There is a standard classification used to describe the attitude of a fold in terms of the plunge shown by its hinge, as follows: Horizontal Gently Plunging Moderately Plunging Steeply Plunging Vertical 0° 10° 30° 60° 80° to to to to to 10° 30° 60° 80° 90° Accordingly, horizontal folds have hinges plunging at less than 10°, plunging folds have hinges plunging between 10° and 80°, and vertical folds have hinges plunging at more than 80°. Inclination of a Fold.
Structure contours should be constructed, at least in imagination, whenever the outcrop pattern is studied to determine the dip of the bedding. A useful rule to remember is that the V-shaped outcrop of a geological boundary across a valley always points towards the area where the contact is present below the topographic surface. Although this direction does not necessarily correspond to the dip of the contact, it is the direction in which the sedimentary rocks overlying the contact are encountered.
This means that simply describing the trend of a fold without reference to the plunge of the fold hinge or the strike of the axial plane can only lead to confusion. It is therefore emphasised that considerable care must be taken in order to describe the orientation of a fold unambiguously. This is best done by providing a full description of the orientation shown by the various elements which define the fold, so that a complete picture of the fold can be constructed. Tightness and Angularity. The region near the hinge where the folded surface has an appreciable curvature is termed the hinge zone.