Jökull - 01.12.1986, Síða 47

tephra layers from Katla are most prominent, ranging
in thickness from a few mm to tens of cm. The most
coarse grained tephra layer has a maximum grain size
of 25 mm but submillimetre material is dominant in
most Katla layers. Tephra from more distant sources
is restricted to the submillimetre grain size and dif-
ferent chemical composition is reflected by different
hue of colour, which is however often difficult to dis-
cern in the wet sediment of the bog. The tephra layers
constitute roughly 27% of the total thickness of the
sequence presented on figure 5 but this percentage is
likely to vary within the bog.
The most important tephra layers for the present
studies are the landnám layer and the
K—1357 tephra layer. The former originated in the
Vatnaöldur—Hrafntinnuhraun crater row some 70km
to the north (Larsen, 1978; 1984), and is thought to
have been deposited around A.D.900, near coinci-
dently with Norse settlement in Iceland (Einarsson,
1963; Thórarinsson, 1967). This Landnám-tephra is a
most valuable isochrone which divides the deposits
into pre-Landnám and post-Landnám and greatly
facilitates the observation of changes caused by the
Settlement. The tephra from the Katla eruption of
— 1357 forms the thickest tephra layer deposited in
the Mýrdalur district during historical time. The
Ketilsstadir bog lies within the lOOmm isopach but
locally thickness may exceed 200mm. This was
probably the worst catastrophe that has hit the area
smce it was settled and damage to the vegetation must
have been extensive. The tephra fall and abandoning
°f farms as a result of it are only vaguely mentioned in
old annals (Einarsson et al., 1980).
Other dated tephra layers identified at Ketilsstadir
are from the Hekla eruptions in 1341 and 1597 and
the Katla eruption of 1580 (Larsen, 1978). No
attempt was made to identify younger tephra layers
because the topmost part of the soil at the Ketilsstadir
bog has been disturbed by ploughing, prior to reseed-
ing.
geomorphology
The basin is almost completely enclosed, one of a
series of bogs that drop in a staircase from Oddnýjar-
tjörn at 92m a.s.l. to sea level (fig. 2). The stream
which drains from the lake and the upper basins now
flows to the west in a steep-sided valley and avoids the
main bog at Ketilsstadir, which is drained over a rock
lip to the south and on to the lowest of the bogs, close
to sea level. The basin appears to have been glacially
overdeepened and much of the sediment brought into
the basin during the Holocene has been trapped.
Fig. 6. Ketilsstadir: variation in inorganic content of
peat at section BRl. — Mynd 6. Breytingar á magni
ólífrœns efnis í mó úr sniði BR I í mýri við Ketilsstaði.
Material moving off the encircling steep slopes and
any aeolian sediment has also been caught. Sections
through the bog thus provide information on the evo-
lution of the area, perhaps since the end of the last
glaciation, although present exposures only cover the
Iast two thousand years.
A section, at about the centre of the basin, shows an
intercalation of tephra layers with peat and organic
silts (fig. 5). Close to the surface, the peat is extremely
fibrous and of a reddish-brown colour but, at greater
depths, it is more compressed and humified. Towards
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