To reproduce the conditions within the zone of flow, it will be necessary to load the lateral surfaces of the block instead of leaving them unconstrained as in the above-described experiment. The experiment is best devised as in Fig. 20. Here a series of layers having varying degrees of rigidity is prepared from beeswax as a base, either stiffened by admixture of varying proportions of plaster of Paris, or weakened by the use of Venice turpentine. Such a series of layers may represent rocks of as widely different characters as limestone and shale. The load which is to represent superincumbent rock is supplied in the experiment by a deep layer of shot. [Illustration: FIG. 20.—Apparatus to illustrate the folding of strata within the zone of flow (after Willis).] When compression is applied to the layers from the ends, these normally solid materials, instead of fracturing, are bent into a series of folds. The stiffer, or more competent, layers are found to be less contorted than are the weaker layers, particularly if the latter have been protected under an arch of the more competent layer (pl. 2 A). =The arches and troughs of the folded strata.=—Every series of folds is made up of alternating arches and troughs. The arches of the strata the geologist calls _anticlines_ or _anticlinal folds_, and the troughs he calls _synclines_ or _synclinal folds_ (Fig. 21). When a stratum is merely dropped in a bend to a lower level without producing a complete arch or a complete trough, this half fold is termed a _monocline_. [Illustration: FIG. 21.—Diagrams representing _a_, an anticline; _b_, a syncline; and _c_, a monocline.] Any flexuring of the strata implies a reduction of their surface area, or, considering a single section, a shortening. If the arches and troughs are low and broad, the deformation of the strata is slight, the shortening is comparatively small, and the folds are described as _open_ (Fig. 22 _b_). If they be relatively both high and narrow, the deformation is considerable, a larger amount of crustal shortening has gone on, and the folds are described as _close_ (Fig. 22 _c_). This closing up of the folds may continue until their sides have practically the same slope, in which case they are said to be _isoclinal_ (Fig. 22 _d_). [Illustration: FIG. 22.—A comparison of folds to express increasing degrees of crustal shortening or progressive deformation within the zone of flow: _a_, stratum before folding; _b_, open folds; _c_, close folds; _d_, isoclinal folds.]