In regions which have been closely folded the larger flexures of the strata may be found with folds of a smaller order of magnitude superimposed upon them, and these in turn may show crumplings of still lower orders. It has been found that the folds of the smaller orders of magnitude possess the shapes of the larger flexures, and much is therefore to be learned from their careful study (Fig. 25). It is also quite generally discovered that parallel planes of ready parting, which are described as _rock cleavage_, take their course parallel to the axial plane within each minor fold. As was long ago shown by the pioneer British geologists, these planes of cleavage are essentially parallel and follow the fold axes throughout large areas. ┌────────────────────────────────────────────────────────────────────────┐ │ PLATE 2. │ │ │ │ [Illustration: _A._ Layers compressed in experiments and showing the │ │ effect of a competent layer in the process of folding (after Willis).] │ │ │ │ [Illustration: _B._ Experimental production of a series of parallel │ │ thrusts within closely folded strata (after Willis).] │ │ │ │ [Illustration: _C._ Apparatus to illustrate shearing action within the │ │ overturned limb of a fold.] │ └────────────────────────────────────────────────────────────────────────┘