The Geometry and Mechanics of Formation of "Similar" Type Folds

Examples of natural folds are examined to see how closely they resemble ideal parallel and similar fold models. Many natural flexural folds appear to have been modified by flattening, and a method is described whereby quantitative determinations of this flattening may be made. Folds having a perfect similar shape are uncommon. Folds of general similar type appear to be developed by a mechanism of differential flattening. The two- and three-dimensional geometrical aspects of these structures are examined, and the relations of the growth of porphyroblastic minerals to the deforming processes of flattening are described.