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Many people carried out theoretical work but the one that is noteworthy is the work by Dietrich (1969,1970) 

reproduced in the figure. The top part shows that under a viscosity contrast of 42.1, the folds acquire limb dips of 40 at 33 per cent shortening and layer thickness is unchanged. These folds become nearly isoclinal at 63.2 per cent shortening. The short lines are the trajectories of planes normal to applied stress which of course deflects at layer boundaries. If the viscosity contrast is reduced to 17.5, the limb dips of about 30 are accomplished only at 63.2 per cent shortens; the layer thickness at this stage has more than doubled at this stage than the original. Even at 77.7 per cent shortening, the limb dips do not exceed 60 and at this stage the layer thickness has increased more than four times the original. In other words, the layer shortening was predominant over buckling even up to the stage of 77.7 per cent shortening of the layer-matrix complex. At extremely small values of viscosity contrast of 5.2, the layer seems to buckle so little that the limb dips of about 10 to 15 are achieved only at about 63 per cent shortening. The initial thickness of the layer at this stage though has increased nearly four fold. At 77.7 per cent shortening, the layer thickness is increased about eight times but the limb dips are only about 20-25. Thus when the viscosity contrast is small, i.e. less than 20 then, folds of cuspate style begin to form and continue to do so, no matter what the amount of shortening is.

If the viscosity contrast is high, the layer parallel strain is as good as nothing and initial thickness is unchanged and buckling is rapid and large limb dips are achieved at small amount of shortening. If the viscosity contrast is very high, as predicted by theoretical analysis, the ptygmatic folds may develop amplifying very rapidly.