Structural geology . Fig. 53. Folding of brittle and soft layers contrasted in jaspercracks in the brittle layers. Note the tension movements are not ordinarily described as faults, on account oftaking place parallel to the bedding. In the zone of fracture rocks are relatively competent; theydo not crumple by interior adjustment; the folds therefore tend tobe simple and open. In the zone of flowage rocks are folded by interior adjustment ofall parts of the mass with development of cleavage. Beds arethickened and thinned. No part of the rock mass is competent towithstand the load without interi
Structural geology . Fig. 53. Folding of brittle and soft layers contrasted in jaspercracks in the brittle layers. Note the tension movements are not ordinarily described as faults, on account oftaking place parallel to the bedding. In the zone of fracture rocks are relatively competent; theydo not crumple by interior adjustment; the folds therefore tend tobe simple and open. In the zone of flowage rocks are folded by interior adjustment ofall parts of the mass with development of cleavage. Beds arethickened and thinned. No part of the rock mass is competent towithstand the load without interior adjustment and result is a much more composite or complex folding. Thebed thereby becomes thickened and strengthened, enabling it to. 11(1 FOLDS OF FRACTURE AND FLOW CONTRASTED 111 support the load. The folding of rocks in schistose areas, that is,areas which indicate that they have been in the zone of flowage, isintricate and close, and contrasts strongly with the more open andsimple folding of rocks in the zone of fracture. For instance, thefolding in the Piedmont area of Virginia, the rocks of which weredeformed in the zone of flowage, is much more minute and com-plex than that of the Knox dolomite in the Appalachians to thewest, which occurred partly in the zone of rock fracture. In thefolds of the zone of flowage the readjustment takes place notonly between the beds but in every part of the bed. The curva-ture in each bed tends to remain the same as in the strata aboveand below. This is called the similar type of folding. (SeeFigs. 51b and 54.) The distortion in the layers in ideal similarfolds is greater in proportion as the bends are gentle on the anti-clines and synclines. Hence, to avoid this distortion, th
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