Structural geology . the center (see Fig. 62).The shallowest deformation found is miles. Making calcula-tions for five sub-sections, he finds a gradual increase in the depthaffected toward the center of his section, which suggests that thedeformed zone is bounded by planes dipping approximately 45°from the surface at either end of the section and intersecting about32 miles below the surface near the center. The intersection ofthese hypothetical planes at 45° with each other and with theearths surface suggests to Chamberlin that they are really shearing 1 Chamberlin, T. C, and Salisbury, R.
Structural geology . the center (see Fig. 62).The shallowest deformation found is miles. Making calcula-tions for five sub-sections, he finds a gradual increase in the depthaffected toward the center of his section, which suggests that thedeformed zone is bounded by planes dipping approximately 45°from the surface at either end of the section and intersecting about32 miles below the surface near the center. The intersection ofthese hypothetical planes at 45° with each other and with theearths surface suggests to Chamberlin that they are really shearing 1 Chamberlin, T. C, and Salisbury, R. D., Geology, Vol. II, 1906, pp. 125-126. 2 Chamberlin, R. T., Appalachian folds of central Pennsylvania; Jour. Geol.,Vol. 18, 1910, pp. 228-251. 3 Willis, Bailey, Physiography and deformation of the Wenatchee-Chelan dis-trict, Cascade Range: Prof. Paper No. 19, U. S. Geol. Survey, 1903, pp. 95-97. 4 Chamberlin, T. C, The fault problem: Econ. Geol., Vol. II, 1907, p. 596. 5 Op. cit., p. 245. 126 STRUCTURAL GEOLOGY. DEPTH OF FOLDS . 127 planes developed by tangential shortening in the manner of frac-ture planes formed in a block under pressure. The above inference implies that the pressure has been appliedwith equal intensity on all unit areas on the sides of the deformedblock; it implies a non-rotational strain; it implies, further, thatshearing planes find expression in the zone of rock flowage. Whileshearing stresses are undoubtedly present in this zone duringdeformation, it is not so clear that they would find expression indefinite planes bounding the deformed region or that such planeswould have the position assumed for them. They would not if thestrain were rotational, developed by tangential stresses. Thestructure consonant with such deformation in the zone of flowageis a vertical cleavage, as is implied by Willis conclusion concerningthe Cascade The depth reached by the deforming move-ments of the Cascade uplift has been calculated by Willis to befrom 375 to
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