266 Willis and Hayes — Conditions of 



resistance to bending offered by the strata, both of which 

 forces tend to hold down this portion of the competent stratum. 

 But the radial components directed upward in the convex 

 curve at and near A / must carry up the crown of the arch, if 

 it develops as a competent structure, and in so doing they 

 cause the beginning of a concave curve in the section PA'. 

 As this curve becomes sharper, the force P is resolved into 

 components tangential and radial to the curve, the latter being 

 directed downward. Thus the limb S' A', iig..8, occupies a 

 position between forces acting downward at one end and up- 

 ward at the other and a neutral point n becomes a center of 

 rotation for that limb. Moreover the force P is absorbed in 

 the development of these radial components and with increas- 

 ing curvature there is decreasing pressure in the other limb 

 A' S and consequently less and less tendency to further bend- 

 ing. The flow of material from the point of support S' of the 

 competent arch toward the zone of relief K! beneath the arch 

 also tends to promote the rotation of K' S'. Thus a conse- 

 quent stepfold is developed parallel to and overturned in the 

 same direction as the original fold which depended upon 

 initial dips. The same process may continue with the forma- 

 tion of an indefinite number of parallel folds all turned in the 

 same direction and each consequent upon the one next preced- 

 ing it in time and position. 



Another factor in determining the direction of overturn of 

 the consequent fold, as pointed out by Dutton,* may be deep 

 seated isostatic flow. If such flow of material from the area 

 of sedimentation toward the area of erosion actually takes 

 place its effect may be to displace S and S 7 , the supports of 

 the anticlinal arches, toward the shore line and so to add to 

 the rotation of the seaward limbs of the anticlines. 



Next to the parallelism and continuity of their axes the 

 most remarkable feature of the Appalachian folds is the uni- 

 formity in the direction of their steeper sides. This indicates 

 a corresponding uniformity in the initial dips of the synclines 

 of deposition which determined the form of the original step- 

 folds. It also indicates the prevalence of conditions favorable 

 for the westward overturn of the consequent folds. As 

 shown in the above analysis of the process by which the con- 

 sequent folds are formed, an important condition is the long 

 western limb of the original syncline convex upward and with 

 gentle dip eastward. 



Two exceptions to this westward inclination of the folds are 

 known in the southern part of the province. These are the 

 northern portions of the Wills Valley and Murphree Valley 



* On some of the greater problems of Physical Geology, C. E. Dutton, Bull. 

 Phil. Soc. Washington, vol. xi, 1889, p. 60. 



