246 247 (13)

F0RMAT10N AND CHARACTER OF ICE

247

erosion. A berg calves whcn a piccc brcaks off; this disturbs its equilibrium, so that it may float at a different anglc or it may capsize. In cold water melting takes place mainly on thc watcr-linc. In warm water a berg melts mainly from below and calves frequently. Erosion is causcd by wind and rain.

Table 17.2. Ratio of hcights of iccbcrgs above water to their depths undrr water

Typc	Ratio Exposed hcight/Submerged depth
Blocky, precipitous sides	1:5
Rounded	1:4
‘Picturcsąuc’ Greenland berg	^1:3
Pinnacled and ridged	1:2
Last stages, horned and winged	1 :i

(Ali of thcse typcs, except blocky bergs, arc classified as ‘glacier bergs’.)

Arctic Bergs

In the Arctic the irregular glacier berg of varying shapc constitutcs thc largest class. The height of this berg varies grcatly and frcquently reachcs 70 metres; occasionally this is exceeded and one of about 170 metres abovc sca lcvel has been measured. These figures refer to the height soon after calving, but the height ąuickly decreases. The highest berg so far measured south of Newfoundland was about 80 metres high, and thc longcst about 520 metres. Glacier bergs excecding 1000 metres in length have been seen further north.

An entircly different form of iceberg is thc blocky, flat-topped and prccipitous-sidcd berg which is thc nearest countcrpart in the Arctic to the great tabular bergs of the Antarctic (see below). These bergs may originate either from a large glacier tonguc or from an ice shclf. If of thc latter origin they are truć tabular bergs, but in either case they arc tabular in form.

A third class of Arctic bergs is the icf. island, a namc popularly used to describe a rare form of tabular berg found in thc Arctic. Ice islands originate by brcaking ofT from icc shclvcs which arc found principally in north Ellesmere Island and north Greenland. They stand about 5 metres out of the water and havc a total thickness of about 30 to 50 metres; in contrast, thc tabular bergs of the Antarctic commonly stand about 30 metres out of the water and have a total thickness of about 200 metres.

The larger ice islands have hitherto been found only within the Arctic Ocean whcrc they drift with thc sca icc at an avcragc ratc of from 1 to 3 nautical miles per day. The best known, named T3 or Fletchcr’s Icc Island, was sighted in 1947 and was occupied intermittcntly by United States scientific partics for a number ofycars between 1952 and 1971. In 1962 its horizontal dimensions were estimated to be 3 nautical miles by 6 nautical miles. For most of its known life, T3 was drifting in a eloekwise direction in the Beaufort Sca current gyre.

Smali ice islands have been sighted in thc waters of thc islands of thc Canadian Arctic and off Greenland where they havc been carricd out of thc Arctic Ocean by wind and current. In addition, tabular bergs, somc of which may wcll be vcry smali ice islands, have been reported in the vicinity of Svalbard and in waters north of thc USSR.