While Earth’s most dramatic geologic fold structures result from compressional deformation, extensional deformation can also produce closed, upright folds–in both the real world and in physical models. The image below shows a suite of sandbox models I made with the development of extensional folding in mind (or lack of folding, in the top model) . All of the folding developed in the models is some sort of expression of forced, fault-bend folding, which develops when the hanging wall makes geometric adjustments to match changing footwall shape.