Early model faults and associated folds (H. Caddell and B. Willis, for example) were produced by compressional deformation of a variety of layered materials, and an intuitive association certainly exists between buckle folding and squeezing/shortening a sequence of layers. The left-dipping faults above the anticlinal crests (yellow arrows, below) are an expression of the stretching above the fold crests; this stretching occurred above a shallowly-dipping segment of the deeper fault. Like all sandbox models–and real geologic structures–materials present in the layer packs shown here dictate the geometries of the folds and faults that form.