A karst feature, grikes are cracks formed when an exposed area of limestone rock is eroded in a particular fashion. Small fissures occur in a limestone bed, which are worn away and widened by rainwater, which dissolves limestone. These fissures reach widths of tens of centimetres, leaving raised beds (clints) of uneroded limestone between extensive networks of cracks. This characteristic pattern is known as clint-and-grike topography.
Technically a grike can reach gigantic size, but the word is most often used to refer to smaller fissures, with larger ones being called bogaz or zanjones. See also Caving.
Reference: http://www.geocities.com/cavevader/caving/cave_term_a.htm