The external shape, the internal structure, or the forms preserved on bedding surfaces, generated in sedimentary rocks by sedimentary processes or contemporaneous biogenic activity. Internal sedimentary structures include: those formed by physical depositional processes (cross-stratification, flat bedding (see plane bed), lamination, and heterolithic structures); those due to post-depositional deformation (convolute bedding, slump structures, dish and pillar structures, flame structures, ball and pillow structures, etc.); those caused by organic disturbance (bioturbation, trace fossils); or by post-depositional chemical disturbance (enterolithic structures, collapse and solution structures, concretions, etc.) Structures preserved on the tops of beds include: those formed by depositional processes (ripple marks, primary current lineations); erosional structures (flutes and scour marks, see scour and lag); structures caused by the transportation of an object over the bed (tool marks); and other features such as desiccation and syneresis cracks, sand volcanoes, adhesion ripples and warts, rain prints, and biogenic traces and trails. Structures preserved on the bases of beds (sole marks) include load casts, the casts of flutes, trails and tool marks, and the fill of erosional scours. The external form of sedimentary units (sheet-like, channel-fill, reef or mound (see mud mound), lenticular, etc.) is a function of the depositional environment and sometimes of post-depositional compaction.