Glacier velocities from aerial photographs in North and North-East Greenland

General descriptions of the glaciers of North and North-East Greenland have been given by Koch (1928), Davies & Krinsley (1962) and Weidick (1975). These descriptions, however, provide little in the way of quimtitative data on glacier velocities, although Davies & Krinsley cancluded that a large number af glaciers and small ice caps in North Greenland exhibited stable conditions, with a significant number showing evidence of recent retreat. Comparisons of vertical aerial photographs taken in 1959-63, 1971 and 1978 permit measurements ef glacier velocity to be made on floating ice tongues which have preserved a distinctive surface pattern of meandering streams and crevasses. These show the largest glaciers draining the Inland Ice in North and North-East Greenland to have average velocities ranging from 300 to 900 m/year. This study of vertical aerial photographs has also demonstrated that for floating glacier tongues the position of the glacier terminus is not areliable indicator of advance or retreat. In the fjords of North Greenland semi-permanent sea ice often maintains the integrity of advancing floating glacier fronts for periods of ten to twenty years (Koch, 1928; Weidick, 1975); the break-up of the floating tongue in rare summers when the sea ice melts completely may give the impression of a sudden retreat, but this 'retreat' is unrelated to changes in the mass balance.