A tension fracture that develops when a competent rock that overlies incompetent material experiences extension due to gravitational sliding along the bedding, widening joints. Surface material often fills gulls. Small-scale cambering often causes gulling behind cliff tops where blocks have toppled over the edge, producing gulls that are wide at the top and narrow at the bottom.

http://www.ubss.org.uk/resources/proceedings/vol17/UBSS_Proc_17_2_153-174.pdf Describes two gull caves from the Wiltshire and Avon border.