several hatchings such as cross lines and dots are used to represent the depth within this portrait. as seen in this image the darkness of an area is shown by doing a much more of a closer/tight hatchings.
Artist John Franzen creates textured drawings reminiscent of wrinkled fabric, or waves of water, by drawing tediously placed rows of lines with black ink. 'Each Line One Breath': Morphogenetic Freehand Drawings By John Franzen. Click through for videos.