Jökull - 01.12.1985, Page 77

by the mid-nineteenth century the sandur sediments
themselves became exposed to lichen colonization.
Similarly, the absolute dates predicted for the formation
of the sandur deposits in area B suggest lichen coloni-
zation took place between c. 1850 (c. 1858, curve 2) for
the upper terrace surfaces, and c. 1880 (c. 1884, curve 2)
for the lowest surface. As in area A, the predicted dates
may well represent the period of initiation of soil ero-
sion, probably associated with heavy grazing by sheep.
(2) Tephrochronologic evidence
The substantial age of the high terrace surfaces of
area A, (although apparently colonized by Iichens
between c. 1843 and 1898), appears to be confirmed by
a 14C date for a soil buried directly beneath the Skóga-
sandur deposits and exposed on its northern margin.
(Larsen, 1978; Einarsson et al. 1980; Thorarinsson,
pers. comm.). (See note added in proof).
At this site, the soil horizon lies above tephra layer
„Y“, and 14C analysis of this soil indicates a date of
approximately 600 A.D. In addition, the Landnámabók
(Lodmund the Old, pp. 115—116; and see Sigbjarnar-
son, 1973) refers to a massive flood that occurred during
the tenth century, perhaps related to a Katla eruption at
c. 930 A.D. (Einarsson et al. 1980). Hence the sedi-
ments of Skógasandur appear to date from shortly after
c. 600 A.D., (See Hofsá section Dd 117 by Larsen, in
Einarsson et al. 1980), while the flood channels of zones
IV, V and VI may date from between c. 600 A.D. and
the tenth century. The large northern flood channel
which emerges from Thurragil (Gorge 1) post-dates the
Skógasandur deposits and its associated flood channels,
and hence appears to date from about the tenth century
(Landnámabók). After this date jökulhlaups from Mýr-
dalsjökull were confined to the east of Skógasandur.
The 1357 A.D. jökulhlaup from Sólheimajökull, for
example, was channeled through the present Jökulsá
valley, and the Yztagil and Holsargil gorges to the east
(Einarsson et al, 1980), while most subsequent jökul-
hlaups extended across Mýrdalssandur.
In area B there are three relict soil patches directly
overlying the Sólheimasandur surface (Fig. 1). The
deepest of these soil patches (2 m thick) which lies on
the lowest terrace, comprises up to ten tephra layers.
The oldest tephra layer probably dates from the early
fifteenth century (Katla 1416?). The Katla tephra from
1357 is absent at the base of the section. Hence the
tephrochronologic data suggest that the higher sandur
deposits accumulated between 1357 and 1416 (?) A.D.
Nevertheless, there is a possibility that these sandur
deposits were formed earlier than this, possibly in
association with the Eystriheidi limit (Dugmore, in
prep.), i.e. between c. 600 and 750 A.D. These sedi-
ments were subsequently eroded or reworked to pro-
duce the observed terrace sequence (Maizels, in prep.),
and only finally abandoned after the 1357 A.D. jökul-
hlaup. The lower deposits of area B (i.e. Zone X) appear
to have been formed well before the accumulation of the
Skogasandur deposits, and hence pre-date 600 A.D.
They are so severely weathered that they may well date
back to early Neoglacial times, and were only exhumed
during fourteenth century. They subsequently developed
a soil and vegetation cover that in turn has been gra-
dually removed over the past century or so, re-exposing
the oldest surface in the area.
With one exception (Zone XI, see below), the out-
wash deposits of areas C and D appear to be reasonably
accurately dated using the lichen growth curves of Fig.
2, since these dates are also closely supported by mor-
phologic, historical and field evidence of former ice
limits. The Holar (Zone XI) deposit, however, which
has been dated to the early nineteenth century by
lichenometric methods, appears to be very much older
than this, and according to other sources of evidence, is
likely to date from before the mid-fourteenth century.
Jaksch (pers. comm.) suggests that the Holar deposit
may bear some resemblance to the Storalda moraine
infront of Svínafellsjökull which, although it also
exhibits lichen sizes of up to 110 mm, is covered by a
Ioessial soil which contains tephra from the 1362 A.D.
Öraefajökull eruption (Thorarinsson, 1956). Hence
Jaksch’s conclusion suggests that the Holar deposit may
date from pre-1362 A.D. The possibility of this date
applying to the Holar deposit is supported in the
Sólheimajökull area by several historical documents
(e.g. see Bardarson, 1936; Magnusson, 1705 in Thor-
arinsson, 1940) which indicate that Sólheimajökull did
not extend beyond the 1890 limit (c.1.2 km upvalley
from the Holar deposit; see Fig. 1). at any time during
the previous 200 years, (that is, at no time between the
periods 1704 and 1890). Hence the Holar deposit is
likely to date from well before 1704, and possibly from
pre-1362. According to Thorarinsson (1956, 1964) pre-
Little Ice Age moraines (i.e. pre-1600), such as Stor-
alda, may represent a glacial advance to a Holocene
maximum in the early Subatlantic.
More recently, Dugmore (in prep.) has obtained
additional tephrochronologic evidence from the lateral
moraine sequence located on the western margin of
Eystriheiði (Fig. 2). Under the outermost ridge Dug-
more found the pre-historic tephra layer „Y“ (notation
after Einarsson et al 1980) suggesting that this moraine
dates from post-600 A.D. A soil pit dug through to the
underlying till between this outermost moraine ridge
and the next inner moraine ridge revealed a complete
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