DOWN PLUNGE CROSS SECTIONS Main Topics A Cylindrical folds B Downplunge cross-section views C Apparent dip II Cylindrical folds A Surface of a cylindrical fold is parallel to a line called the fold axis. B Cylindrical folds maintain their shape for “long” (infinite) distances in the direction of the fold axis (as opposed to folds bending in the shape of a bowl); they are two-dimensional structures because they do not change in shape along the dimension of the fold axis. C Planes tangent to cylindrically folded beds intersect in lines parallel to the fold axis. D Poles to cylindrically folded beds are contained in the plane perpendicular to the fold axis, so taking the cross-product of the poles gives the orientation of the fold axis. III Down-plunge cross-section views A Down-plunge cross-section views can be obtained directly from a geologic map by looking obliquely at the map down a fold axis. B Beds appear in true thickness C Graphical technique 1 Find orientation of fold axi..