Fig. 401.
Fig. 402.
Fig. 401.—Map record of dip and strike showing anticlinal structure.
Fig. 402.—The structure of the area shown in Fig. 401, in cross-section.
Fig. 403.—Map record of dip and strike showing plunging (dipping down at ends) anticline.
As found in the field, folds are usually much eroded, and often completely truncated ([Fig. 398]). The determination of anticlinal or synclinal structure is then not based on topography, or even on such sections as shown in Figs. [394] to [397], for such sections are relatively rare. The structure is determined by a careful record of dips and strikes. On the field map, the record may be made as shown in Figs. [399] to [401], where the free ends of the lines with but one free end point in the direction of dip, while the other lines represent the directions of strike. Applying this method, the structure shown in [Fig. 400] represents a syncline, and that in [Fig. 401] an anticline. In cross-section, the structure presented by [Fig. 401] would appear as in [Fig. 402]. [Fig. 403] shows a doubly plunging anticline; that is, an anticline the axis of which dips down at either end. [Fig. 404] shows a combination of synclines and anticlines, and [Fig. 405] a cross-section along the line ab of [Fig. 404]. The outcrops of rock where the dip and strike may be determined may be few and far between, but when they are sufficiently near one another, the structure of the rock, as shown in [Fig. 405], may be worked out, even though the surface be flat.
Fig. 404.—Map record of dip and strike showing complex structure.
