Náttúrufræðingurinn - 2011, Side 25

129
Tímarit Hins íslenska náttúrufræðifélags
fastalandið árið 1780.34 Telur Skúli
Magnússon 35 í lýsingu Gullbringu-
og Kjósarsýslu frá 1785 að landbrot
hafi átt stóran þátt í afdrifum kaup-
staðarins, en allur jarðvegur var
horfinn á Hólmi í hans tíð. Þetta
er nefnt hér til að sýna að landbrot
hefur verið viðvarandi við innan-
verðan Faxaflóa um langa tíð. Flest
bendir til að sjór hafi tekið að flæða
að einhverju marki inn í mýrina
neðan við Sjávarhóla á 17. öld.
Hækkun sjávarborðsins við
ströndina hjá Sjávarhólum er meiri
en annars staðar hefur mælst.
Ástæðan kann að vera að nokkru
leyti staðbundið landsig sem stafar
af berghlaupinu. Staðbundið sig í
berghlaupum, ekki síst þeim sem
sjór brýtur framan af, er þekkt
annars staðar á landinu, svo sem í
Almenningum vestan við Siglufjörð
og í Víkurhólum við Eyjafjörð.36
Í þessari athugun hefur áherslan
verið á aldur Sjávarhólaframhlaups-
ins en lítið verið hugað að mynd-
unarsögu fjörumósins og umhverfis-
breytingum á Kjalarnesi. Mun ýtar-
legri og margþættari rannsóknir
þarf til að fá skýrari mynd af þeim
þáttum.
Summary
Tephrochronological dating of a
submerged peat and a landslide
in Kjalarnes, SW-Iceland
The Sjávarhólar rockslide in Kjalarnes,
SW-Iceland, originates in the southwest
slopes of Mt. Esja and covers the low-
land around the farms Sjávarhólar and
Skrauthólar, reaching several hun-
dreds meters into the sea beyound the
present shore. It covers an area of ap-
proximately 1.7 km² (including the
submarine part). Its length is 2900 m
and the vertical drop 725 m. The vol-
ume is poorly known but has been es-
timated 15–20 million cubic meters,
incl. 4 million covered by sea. The
rockslide scar is easily seen high up in
the Esja slopes near the peak. The rock-
slide deposit covers the slopes below
the starting zone but in the lowland
most of it is buried beneath younger
screes, alluvial fans and thick soil. In
the littoral zone part of the rockslide is
covered by peat layers containing tree
trunks and roots. The peat is suggested
to have formed in a bog or a wood near
the shore. A continuous subsidence in
SW-Iceland has caused a transgression
of the sea that submerged the peat. An
investigation was made on tephra
layers in the peat in order to estimate
the age of the rockslide and the rate of
relative sea-level change in the area.
Several tephra layers form useful
marker horizons in the Reykjavík area.
They are listed in Table 1. The youngest
tephra layer in the submerged peat is
Katla~1500. This is the thickest histori-
cal tephra layer in the area. The
Settlement layer from c. 870 AD is also
a prominent tephra layer. The oldest
tephra layer originates from an erup-
tion in sea off Reykjanes, 6000–6100
years BP. Just above it is a 6000 years
old Hekla tephra layer. The Saksunar-
vatn tephra (10,200 BP) has not been
found in the peat. This layer is well
known in soil profiles in Reykjavík and
on Kjalarnes, west of the Sjávarhólar
rockslide. Calculations based on soil
accumulation rate indicate that the for-
mation of the peat started 9,600–9,800
years BP. The rockslide itself might be
several hundred years older, possibly
around 10,000 years BP or slightly
younger than the Saksunarvatn tephra.
During this period the sea level in
Faxaflói bay was near its minimum or
around 30 m lower than today. The
shoreline was consequently farther out
and the rockslide has most likely not
reached it.
81_3-4_loka_271211.indd 129 12/28/11 9:13:57 AM