Náttúrufræðingurinn - 1970, Blaðsíða 26

120
NÁTTÚRUFRÆÐINGURINN
Sveinn Jakobsson: The Geology and Petrology of the Westman Islands. A Pre-
liminary Report. í Surtsey Research Progress Report IV, June 1968, s.
113-130.
Jóhann G. Ólafsson: Vestmannaeyjar. Árb. Ferðafél. íslands 1948.
Þorkell Jóhannesson: Ornefni í Vestmannaeyjum. Þjóðvinafélagið, 1938.
Trausti Einarsson: The ring-mountains Hverfjall, Lúdent, and Hrossaborg in
Northern Iceland. Vís. ísl. Greinar IV, 1. bls. 1—28. 1965.
— Suðurströnd Islands og myndunarsaga hennar. Tímarit Verkfræðingafél.
íslands, 51. árg. 1966, bls. 1—18.
Sigfús M. Johnsen: Saga Vestmannaeyja, I—II. 1946.
Ólafur Lárusson: Byggð og saga. 1944.
SUMMARY
Review of the geological history of the Westman Isles
by
Trausti Einarsson,
Department of Engineering and Science Institute,
University of Iceland.
A 1565 m deep drill hole on the main island revealed that the volcanic
Westman Islands submarine rocks reach a depth of 200 m below sea-level,
where they rest on thick fine-grained marine sediments. Glacial layers are con-
tained in the volcanic series, showing that the volcanic activity reaches hack
into the Pleistocene. A shore line is found in the drill hole at 37—40 m below
present sea-level. It is concluded that the steep walls of the islands and the
deep sea caves were mostly formed at this low sea-level. Before this time an
elongated block, Fig. 4, on whicli tlie main island group lies, had been up-
lifted by at least some 240 m, bringing the mostly submarine tuffs of the is-
lands, wliich had hardened by that time, to a present height of about 200
m (layer B in Sæfjall, Fig. 2). This block lifting is compared with similar
activity on tlie Reykjanes ridge and on the Reykjanes peninsula, Fig. 5. At
the low, -t-40 m, sea-level the subaerial Stórhöfði volcano was formed. The
superposed Garðsendi flora, whose C14 age is 5000—6000 years, was probably
also formed during the low sea-level. The rise to the present level, which was
due to subsidence of the active block, had already taken place rather soon after
this when the Helgafell lavas were formed. All the main soil on the islands
started to form after the Helgafell lavas, i.e. in the mild Atlantic time.
The sancl bars that formed the early liarbour were probably formed mainly
during the stormy sub-Ailantic, 2000—2500 B.P. By the time of Settlement,
late in the lOth century, the eastern bar had a shallow opening. Some con-
siderations are presented concerning the first Settlement which is assumed to
have been centred on the harbour.