Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 175

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often been misleadingly called the “central graben” of Ice-
land (Fig. 1).
The Quaternary rock series overlap in many localities the
TPB, and in addition there are two areas or outliers with
Quaternary rock series, one in Snaefellsnes, westem Iceland,
and another in the northern part of Skagafjördur in north
Iceland. The Quaternary rock series cover about 40% of the
country.
The Quatemary rock series differ in many ways from the
TPB, particularly by their greater variety of rock facies.
They contain in many areas thick sedimentary layers of
marine and fluviatile origin, but also tillite beds indicating
approximately 10 glacials during the Pleistocene. In the
Quaternary rock series elongated tuff ridges and table moun-
tains also occur that were built up in subglacial volcanic
eruptions. The landscape in some areas is characterized by
younger ridges and table mountains but the older ones are
mostly concealed by younger rock series. The Quatemary
series can therefore easily be distinguished from the Tertiary
series with the aid of climatic indicators found in the series
and paleomagnetic studies supplemented by potassium-argon-
dating.
The boundaries between the Tertiary Plateau Basalt forma-
tion and the Quaternary rock series at sea level are shown
in Fig. 1. In the western part of the country the boundary
runs from Reykjavík north-west of Thingvellir, and changes
to an easterly direction north of Langjökull, to Bárdardalur
where it changes direction again to the north and follows
the valley to the sea in Skjálfandi bay. The eastem boundary
has been drawn from Thistilfjördur on the north coast in
a southerly direction to Snaefell where it changes to a south-
westerly direction to Núpsstadaskógur-valley west of Skeid-
arárjökull. On most geological maps of Iceland the boundary
south of Snaefell has been drawn in SSW-direction to Breida-
merkurjökull, but TPB-rocks outcrop in at least 3 or 4 areas
in the Öraefajökull-Skeidarárjökull area, i.e. in the nunatak