Fig. 23-30. Series of diagrams designed to show the hypothetical develop- 

 ment of pitching isoclinal or sigmoidal folds from simple flexures of the 

 Appalachian type. Looking southeast. 



Fig. 23. Stage i. Symmetrical open folding; axes nearly horizontal, axial 

 planes vertical; outcropping limbs either convergent or divergent; axis 

 parallel to strike of axial plane. 



Fig. 24. Stage 2. Asymmetrical open folds; axes as before, nearly hori- 

 zontal ; axial plane sloping 30 degrees northwest, with consequent slight dif- 

 ference in azimuth of pitch and strike of axial plane; limbs still either 

 divergent or convergent, but approximating more nearly to parallelism. 



Fig. 25. Stage 3. Overturned isocline ; axes and axial plane as in stage 2 ; 

 discrepancy between strike of axial plane and direction of pitch due only to 

 inclination of axial plane, but they are still nearly parallel. Limbs of the 

 folds parallel to each other and to the strike of the axial plane, except at and 

 near the flexure points, 0-0. 



Fig. 26. Stage 4. Overturned isocline, as in stage 3; slight tilting of fold 

 en masse in its axial plane has effect of steepening the pitch of the folds and 

 increasing its angular departure from the strike of the axial plane. 



.Fig. 27. Stage 5. Further increase in tilting of stage 4. 



Fig. 28. Stage 6. Further increase in tilting: of stage 5. Sigmoidal char- 

 acter of folding becomes apparent at about this stage of tilting. 



Fig. 29. Stage 7. Further increase in tilting of stage 6. This stage 

 represents approximately the conditions at the Pierrepont sigmoid. 



Fig. 30. Stage 8. Final stage, brouglit about through further tilting of 

 stage 7. Throughout the preceding process of revolution in the axial plane, 

 the limbs of the folds have remained parallel to each other and to the strike 

 of the axial plane, and the pitch of the axes of folding has assumed a greater 

 and greater discrepancy with the strike of the axial plane, or regional folia- 

 tion; at this, the final stage, such discrepancy is at its maximum, the pitch 

 being at right angles to the strike. 



