If the hinge line is curved then the folds are said to be noncylindrical (more precise treatment can be seen elsewhere in this program). If the hinge line is curved, the inflexion line is also curved although the degree of curvature of one may be different from the other. In cylindrical folds, both enveloping surfaces and median plane are strictly planar and parallel to each other, in noncylindrical folds, they are not.
surface can be reconstructed by rigid translation of a line through space. That line is the fold axis, an important symmetry element. serial orthogonal cross sections are identical. concentric is a special case, where fold axis follows part of circular arc. many folds approximately cylindrical. great circle girdle of poles on stereonet. planar strain a good approximation. can model surface in Excel.conical geometry:
fold surface reconstructed by rotating one line about another.
cone axis and apical angle are important descriptors.
change in serial cross sections (smaller amplitude).
as apical angle approaches 0 the fold form approaches cylindrical.
these folds die out along their length. - nonplanar strain.
often a good approximation of the end of doubly plunging fold
small circle girdle of poles on a stereonet.
composite geometry: e.g. a cylindrical middle portion and conical ends.
More complex patterns occur due to refolding and the variety of interference patterns that can develop.