Jökull - 01.12.1986, Blaðsíða 48

the margins of the bog, the sequence of peat and
tephra horizons is interrupted in places by lenses of
gravel, sand and silt. The sedimentology suggests a
bog whose surface is intermittently covered with flood
water, with small streams that move in an irregular
pattern across its surface. These streams, probably
ephemeral flow from snow melt, may have brought in
material from the sides of the basin which was then
deposited because of the low energy associated with
the gentler angles of the bog surface and because the
sediment was trapped by vegetation. Aeolian material
has also been deposited on the surface, contributing to
the much higher inorganic content of the upper part
of the stratigraphy.
The semi-enclosed nature of the basin allows the
inorganic content to be used as an indicator of the
amount of material being brought into the basin by
the various geomorphological processes. At Ketils-
stadir, the Landnám tephra in the lower part of the
section provides a marker for pre- and post-Settle-
ment organic deposition and this content can be com-
pared with the inorganic. The evidence from other
areas (Thórarinsson, 1961; 1981) is one of increasing
erosion and consequent deposition in basins since
Landnám. A series of loss on ignition tests on samples
through the organic parts of the succession was some-
what inconclusive (fig. 6). Valid conclusions are diffi-
cult to make because of variability, both spatially and
temporally, in inorganic inputs and the sampling
problems this poses; aeolian sediment may have been
reworked during the spring melts before becoming in-
corporated in the peat and organic silts.
In the limited depth of stratigraphy which could be
observed, the situation pre-Landnám appears to have
been fairly stable, apart from one abnormally high
value in the loss on ignition tests (fig. 6). There is a
slight increase in inorganic content immediately after
Landnám and then a gentle decline until the tephra
layer associated with the —1357 eruption of Katla is
reached. This is by far the thickest tephra layer and
the sample from the peat immediately below may
have been contaminated, contributing to the high
inorganic content. This thickness of tephra may have
instigated local erosion by killing the vegetation as
well as itself being weathered and moved by geo-
morphological processes. This is tentatively inferred
from the higher inorganic content in the peat immedi-
ately above the tephra.
The modern drainage of the basin seems largely to
have stabilised the former bog surface. Although some
erosion continues on the rofbards of the surrounding
overgrazed slopes, both screes and other surfaces show
some evidence of revegetation and the major changes
of the last one thousand years may be giving way to a
new stasis.
MODERN FLORA AND FAUNA
A limited survey of modern flora and fauna was
carried out so as to place in context the interpretation
of the fossil remains. A survey of the area was made
during two summers (D.R.S.) and the plant species
recorded were assigned to five habitat zones. Within
each of these, Domin cover values for all species in
randomly located quadrats were recorded. These were
converted to a percentage scale (Bannister, 1966) and
analysed using the CLUSTAN computer package
(Wishart, 1978). The resulting dendrograms support
the subjective delineation of five habitat zones sur-
rounding the Ketilsstadir sampling area:
1. Hayfield on the former bog surface, now drained
and reseeded, dominated by the grasses Phleum
pratense, Festuca spp. and Poa spp., with wetland
species confined to ditches.
2. Improved pasture, formerly scythed hayfield;
diverse, highly anthropogenic flora with many species
typical of disturbed and broken ground (e.g. Rumex
acetosa).
3. Unimproved pasture, lightly grazed by sheep;
intermediate zone dominated by grasses with much
Equisetum arvense, Luzula multiflora and Alchemilla
filicaulis.
4. Upper slopes, discontinuous eroded pasture;
grasses, mosses and much bare ground with extensive
growth of arctic/alpine types (e.g. Erigeron boreale,
Alchemilla alpina and Potentilla crantzií).
5. Summit, heavily grazed and nearly bare;
cushions of Silene acaulis and Thymus praecox, with
scattered Cardaminopsis petraea, Galium normanii
and Koenigia islandica.
Collection of invertebrates was restricted to the
Coleoptera (beetles), the group which provides the
most frequent identifiable fossils. A transect from the
ridge to the bog surface was searched and swept.
Although variations in the physical environment in-
fluence capture and hence numbers of individuals, the
resulting faunas may be compared with the fossil
ones. The ground beetle Notiophilus biguttatus
seemed the most eurytopic species, whilst Nebria
gyllenhali, Bembidion grapei, Amara quenseli, Lath-
robium fulvipenne, Hypnoidus riparius and Otiorhyn-
chus arcticus are characteristic of the bare ridge crest
(vegetation zone 5). N. gyllenhali was also found in
open, unvegetated parts of the peaty ditch banks. In
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