Ársrit Skógræktarfélags Íslands - 15.12.1975, Side 37

striations, erratics and moraine deposits. Near
to the lake Elliðavatn remains of eskers are
found gnd they indicate that a glacial lake
larger than the present one was situated there.
The glacial striations indicate that the ice-
flow was in a NW direction. The bulk of the
soils in the Park are formed from moraine
and these are often clearly revealed by ero-
sion.
Heiðmörk is bounded on all sides, except
the north-west, by lava-fields. The western
boundary falls on lava which flowed from
Búrfell about 7200 years ago, Búrfellshraun,
(Kjartansson 1972). To the south there is a
continuous lava-field, Húsfellsbruni, stretching
from Búrfell east to Bláfjöll in which at least
18 different lava flows have beerr distinguished.
The Hólmshraun field forms the eastern boun-
dary and have run over older lava flow. Leita-
hraun, which has been found to be about 4630
years old by the C14 method. At least 5 flows
of the Hólmshraun have been identified.
As mentioned earlier the area is heavily
fractured by faults and tectonic fissures. The
major fault-line (Hjallamisgengi) runs from
Vífilsstaðahlíð to Elliðavatn, a distance of 5
km with a maximum vertical displacement of
65 m. That it is older than 7000 years is shown
by tlie Búrfellshraun lava, which has flowed
over the fault and itself been since displaced
by 12—15 m. The fault-line is still active and
has a mean annual displacement of 2.8 mm
(Tryggvason, 1968). The fissuring of the bed-
rock has considerably influenced the ground-
water flow. There are no water courses or
springs in the Park except in the north-east,
Elliðavatn, Silungapollur and Myllulækur all
of which are very obviously connected to the
fault system. Finally it should be mentioned
that a considerable area of faulting is covered
by recent lavas, two of which could perhaps
have flowed during historical times.
Heimildir
Einarsson, Þ. (1968) Jarðfræði, saga bergs og
lands. Mál og menning, Reykjavík.
Jónsson, J. (1958) Sjá Tryggvason og Jónsson.
Jónsson, J. (1960) Jökulberg í nágrenni Reykja-
víkur. Náttúrufr. 30. árg. Reykjavík.
Jónsson, J. (1965) Bergsprungur og misgengi
í nágrenni Reykjavíkur. Náttúrufr. 35.
árg. Reykjavík.
Jónsson J. (1971) Hraun í nágrenni Reykja-
víkur I. Leitahraun. Náttúrufr. 41. árg.
Reykjavík.
Jónsson, J. (1972a) Spricktektonik och grund-
vattenströmmar i omrádet öster om
Reykjavík. Vannet í Norden Nr. 3 1972
IHD-nytt.
Jónsson, J. (1972b) Hraun í nágrenni Reykja-
víkur II. Hólmshraunin fimm. Náttúrufr.
42. árg. Reykjavík.
Jónsson, J. (1972c) Grágrýtið. Náttúrufr. 42.
árg. Reykjavík.
Keilhack, K. (1925) Die geologischen Ver-
háltnisse der Umgebung von Reykjavík
und Hafnarfjörður in Súdwest-Island.
Zeítschr. d. Deutschen Geol. Gesellsch. Bd.
77.
Kjartansson, G. (1972) Aldur Búrfellshrauna
við Hafnarfjörð. Náttúrufr. Reykjavík.
Rosenbusch, H. (1907) Mikroskopische Physio-
graphie der Massigen Gesteine. Schweizer-
bart. Stuttgart.
Tryggvason, T., Jónsson, J. (1968) Jarðfræði-
kort af nágrenni Reykjavíkur. Atv.deild
Iðnaðard. Háskólans, og bæjarverkfr.
Reykjavík.
Tryggvason, E. (1968) Measurement of surface
deformations in Iceland by precision leve-
ling. Journ. of. Geophys. Res. 73.
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