Náttúrufræðingurinn - 2004, Page 51

Tímarit Hins íslenska náttúrufræðifélags
9. mynd. Berggangur í Vatnsdalshólum. Horft til norðurs út Vatnsdal. - A dyke
enclosed in theformation of Vatnsdalshólar, view to thc north. Ljósm./Photo: Höskuldur
Búi Jónsson.
berghlaupsins hægði á sér, leitaði
miðhlutinn til hliðanna, og skýrir
það skorumar um miðbik hólanna.
Líklegt er að keilulögun stakra hóla
sé orðin til vegna molnunar og
veðrunar þeirra eftir að hlaupið féll,
en þó má búast við að bergflykkin
hafi molnað að hluta til við sjálft
hlaupið.
Vatnsdalshólar eru að öllum
líkindum myndaðir eftir að sjór
hvarf úr Vatnsdal fyrir um 9500
ámm og áður en súra öskulagið H5
féll fyrir 6100 ámm. Ekki er ljóst
hvort austustu hólamir hafi mynd-
ast um leið og meginþyrpingin eða
hvort þeir hafi myndast seinna, en
þeir eru ekki samhangandi við
meginhólana og berggerð þeirra
nokkuð önnur.
Það er niðurstaða þessarar athug-
unar að Vatnsdalshólar séu hvorki
hluti af óhreyfðum berggmnni né
mótaðir eða myndaðir af jöklum eða
jafnvel hvort tveggja, eins og Agúst
Guðmundsson10,11 hefur haldið
fram. Ljóst er að hugmyndir Jakobs
H. Líndals3 um að berghlaup hafi
myndað hólana hafa staðist tímans
tönn og þó svo að þær skýringar
sem hér em settar fram séu að ein-
hverju leyti frábmgðnar skýringum
Jakobs, em niðurstöðumar að mestu
leyti samhljóða. Niðurstaðan undir-
strikar orð Sigurðar Þórarinssonar í
formála bókarinnar Með huga og
hamri, sem m.a. geymir safn jarð-
fræðidagbóka Jakobs H. Líndals, en
þar segir: „... em þær óbrotgjam
minnisvarði glöggskyggni, skarp-
skyggni og vísindalegum hugsunar-
hætti þessa hógværa húnvetnska
bónda."4
SUMMARY
Was it a Rock Avalanche that
formed Vatnsdalshólar in
Northem Iceland?
In this paper we discuss the formation of
Vatnsdalshólar, which is a hummocky
deposit in Vatnsdalur, northern Iceland
(Fig. 1). It has been widely accepted that
it was formed in a large rockslide (rock
avalanche - bergsturz - sturzstrom).3,4'5
By comparing rock types in Vatnsdals-
hólar with those exposed in Mt. Vatns-
dalsfjall east of the Vatnsdalshólar form-
ation, Jakob H. Líndal3,4 found a good
correlation between what can be found
in the northem part of Vatnsdalshólar
and the northem part of the mountain.
The same can be applied to the southem
part (Fig. 2). This evidence, combined
with the fact that glacially striated
bedrock is exposed within the formation
- stratigraphically undemeath it, and
with the lack of sediment of glacial
origin, prompted Líndal to suggest that
Vatnsdalshólar were formed by a large
rockslide. This has been contradicted in
a recent study that suggests that Vatns-
dalshólar are a part of the bedrock, with
. .hummocky push moraines, modified
by ablation of interior ice".11 The present
work confirms Líndals suggestions.'4
The geology of the Vatnsdalur area is
greatly influenced by an ancient volcanic
centre. A map was made based on the
studies of Annells,12 which shows the
geology surrounding Vatnsdalshólar
and Vatnsdalsfjall mountain (Fig. 3), a
possible position of the main scarp of the
rockslide has been included on the map.
The distribution of rock types in Vatns-
dalshólar has a striking pattem (Fig. 4)
with basalt in the outer part and rhyolite
in the middle. It is not likely that such a
pattem can be of a glacial origin. In two
places within Vatnsdalshólar glacially
striated bedrock, that underlies the
Vatnsdalshólar debris, shows that the
Vatnsdalshólar formation is of post-
glacial age. The surrounding hummocky
terrain is made up of a deposit and not
bedrock, since a glacier would have
eroded the mass of the bedrock easily
(Fig. 5). It is suggested that the pattem of
rock types within the formation shows
details of the original rock mass, where
the rockslide originated, with rhyolite at
the top and basalt at the bottom creating
a pattem (Fig. 6). The morphology of
Vatnsdalshólar is peculiar. In the outer-
most part (south, west and north), there
is a series of small hills (hummocks) and
in the middle it has tumed into a more
continuous mass (Fig. 7), with low val-
leys in between. The reason for these
divergent forms can be because of a dif-
ference in the thickness, position of dif-
ferent parts of rock types and the com-
pressive and extensive forces within the
moving mass (cf. Fig. 6). When individ-
ual hills of the formation are studied, it
can be seen that they are usually made
up of one rock type, but when comprised
of more rock types, layering is found
(Fig. 8). This suggests tliat Vatnsdalshólar
is not made up of glacier deposit, since
the hills would be made up of a mixture
of all the rock types and not in layers.
Within the Vatnsdalshólar formation,
there are several dykes (Fig. 9). The dis-
tribution of these dykes is random,
which suggests that they are not part of
undisturbed bedrock and since they are
present in Vatnsdalshólar it mles out the
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