Solution canyons are underground voids 1 to 15 + meters wide, 3 to 45 + meters high, and 30 to 300 + meters long. Floors are stepped, ceilings level. Size increases downstream. Their course is sinuous, with some angularity. They occur parallel to and directly under or slightly offset from the thalwegs of re-entrant valleys tributary to major karst valleys. A section across a re-entrant and underlying solution canyon shows a rough hour-glass shape. Solution canyons are related genetically to solutional vertical shafts, forming where removal of the impermeable sandstone caprock permits the vertical descent of water through jointed limestone. Surface runoff concentrates along re-entrant thalwegs where a large quantity of water goes underground. This water, plus subsurface water flowing over the caprock breached by the valleys, follows the easiest route to baselevel down major vertical joints oriented parallel to the thalwegs. Solution by water seeping down these joint planes forms solution canyons.