Jökull - 01.12.1980, Síða 6

erosion, during which an extensive shelf was
carved into the Icelandic plateau. The “Red
Crag” deposits of Tjörnes were thought to
have been deposited during the highest level
of this period of abrasion, resting unconform-
ably on the tilted and subsided basalts. Thor-
oddsen described the eastern part of Tjörnes as
a basaltic horst, where the old basalts dipped
4—5° towards northwest, compared to a dip
of 4° towards north within the “Red Crag”
deposits. In Thoroddsen’s scheme the Pala-
gonite Formation and the Dolerite Formation
were considered to have been formed simult-
aneously during the time from early Pliocene
up into the Glacial Period, which Thoroddsen
considered to have been uninterrupted by in-
terglacials, and to have begun with the for-
mation of mountain glaciers in late Pliocene
time. The youngest rocks of Iceland belonged
to a formation consisting of moraines and
other sediments and lava flows from the
Glacial Period and Postglacial time.
Thoroddsen’s “classic” division was chal-
lenged by Pjetursson in a number of papers
during the first decade of the twentieth cen-
tury. Tjörnes played an important part in
Pjetursson’s new contribution (Pjetursson 1901,
1905a, 1905b, 1906a, 1906b, Pjeturss 1908,
1910, 1924). Fle divided the Icelandic
sequence into three major formations (Table
1). A Regional Basalt Formation was built up
during middle Eocene to early Miocene time
as a part of the North Atlantic basalt region.
In the uppermost part of the Regional Basalt
Formation Pjetursson (1905b) found evidence
of repeated Tertiary glaciations, and
originally defined a “Grey Stage” at that level.
This conclusion was partly based on the
assumption that intercalated tillites west of
Tjörnes belonged to a stratigraphical level
below the Pliocene deposits on Tjörnes. Later,
Pjeturss (1939) abandoned the idea of Miocene
glaciations in Iceland. According to Pjetursson
(1905b) the piling up of the Regional Basalt
Formation was followed by a reduction in
volcanic activity, and the thick Pliocene Crag
deposits on Tjörnes were deposited in a littoral
environment during subsidence, disturbed
towards the end of the Pliocene by local
uplifts. During the Pliocene volcanic lull Ice-
land had become separated from the rest of
the North Atlantic basalt region, after which
extensive eruptions set in again and the Insu-
lar Basalt Formation was now built up during
the Pleistocene. On Tjörnes Peninsula Pjeturs-
son found dolerites and glacial sediments, in-
cluding an interglacial marine sequence bet-
ween two lithified “moraines” in Breiðavík
above the Crag Formation, but near the base
of the Insular Basalt Formation. He did not
exclude the possibility that the uppermost
part of the Crag deposits reached up into the
Pleistocene (Pjeturss 1908). Pjetursson found
evidence of at least four or five glaciations
separated by interglacials in various sections
through the Insular Basalt Formation, and
suggested that marine fossils found at several
localities might yield information about
climatic conditions during the Pleistocene.
Clarification of the stratigraphy
and palaeontology
Schlesch visited Tjörnes in 1921 and studied
the palaeontology of the Tjörnes sequence
(Schlesch 1924, 1925a, 1925b, 1931). He
presented an outline .of the geological
relationships, largely based on the work of
Pjetursson. Schlesch (1924) identified 146 species
of molluscs out of which 39 were extinct or
unknown at that time. He concluded that the
fauna showed similarities with the English
and Belgian Crags although the Tjörnes Crag
did have a more northern character. Schlesch
suggested that the very mixed faunistic cha-
racter indicated that the climate had under-
gone some changes during the time of
deposition.
The first detailed investigation of the
stratigraphy of the coastal sections between
Héðinshöfði and Voladalstorfa (Fig. 2) was
carried out by Bárðarson (1925), who mapped
individual beds and analyzed the fossil
material. Several workers have referred to
Bárðarson’s work as a classic one for the
sequence on Tjörnes. Earlier, Bárðarson (1918,
1922) had published general descriptions of
4 JÖKULL 30. ÁR