Jökull - 01.12.1985, Síða 78

uninterrupted sequence of 13 tephra layers spanning the
period 1300 to 1947 A.D. The „Landnam“ layer
appeared to be absent from the lowest layers of this soil.
It was, however, found to be present immediately above
the outermost margins of Hrossatunga, suggesting that
Sólheimajökull reached a late Holocene maximum
between about 600 and 750 A.D. (Dugmore, op. cit.) A
glacial limit that extended as far south as these high
lateral moraines could be associated, in its latter stages
or in subsequent readvances, with the formation of the
Holar proximal outwash deposits, which lie only a short
distance upvalley of the moraine limit on Eystriheiði.
The Holar deposits therefore appear to date from
between late thirteenth century and 1704 A.D.
The most likely sequence of events in the Sólheima-
jökull valley therefore appears to commence with early
Neoglacial outwash (dating from at least 3100—3500
B.P.; Dugmore, in prep.) followed by a glacier ad-
vance in the 7th to 8th centuries which produced the
high sandur deposits of area A and their associated
flood channels. The glacier subsequently retreated so
that its meltwaters were no longer channeled through
Gorges 1 or 2, while subsequent ice readvances were
associated initially with the accumulation and reworking
of the area B outwash deposit, and subsequently of the
Holar deposits. The development of the Holar deposits
themselves could have occurred at any time between c.
late thirteenth century A.D. (from the moraine tephra
evidence) and the late 1600’s A.D. (from the historical
evidence).
CONCLUSION
Lichenometric methods have proved successful in the
relative and absolute dating of sandur deposits that have
accumulated since the mid-nineteenth century. The
main terrace surfaces on the floor of the Jökulsá valley
have been dated by lichenometric means to c. 1840,
1880, 1935 and post-1969, while the age of intermoraine
outwash deposits conform closely to dates proposed by
Jaksch (1970, 1975) for the period 1915 to 1950.
Lichenometric dating has not, however, proved feasible
on the higher, older sandur deposits largely because of
high rates of rock weathering and disintegration, frost
shattering and frost heave, eolian abrasion, and soil
stripping. The largest lichens in these areas appear to
date from c. 1840, suggesting that soil stripping may
have commenced at about this time, and that the pro-
cesses of rock breakdown have not been sufficiently
rapid and severe in all areas to preclude fairly con-
tinuous thallus growth. The maximum thallus diameters
observed in the whole proglacial area reached 110 mm,
suggesting that lichens are unable to exceed this size,
either because they have reached their ecological limit,
or because rock breakdown finally prevents further
thallus growth. The 110 mm size limit may well, how-
ever, represent early nineteenth century lichen col-
onization, and hence lichenometric dating should prove
feasible in this area for all sandur and moraine deposits
up to 140—150 years old.
The dating of the older surfaces, however, has only
been possible using tephrochronologic evidence, evi-
dence from 14C dating of a buried soil horizon, and
historical records. These sources of evidence suggest
that the oldest sandur deposits of Zone X (Sólheima-
sandur), probably date from the early Neoglaciai;
Skógasandur deposits (area A) date from between the
7th and lOth centuries; deposits of intermediate age
(Sólheimasandur, area B, excluding Zone X) appear to
relate to a period of ice readvance that occurred
between the 8th and 14th centuries; and the younger
deposits in this group (the Holar deposits, Zone XI)
appear to date from some time between the late thir-
teenth century and 1704 A.D.
ACKN O WLED GEMENTS
The authors would like to thank Dr. David Sugden, John
Sugden, and J. Keith Williams for valued assistance in
thefield, and also Ric Gard and all members ofboth the
Aberdeen University Department of Geography Iceland
Expedition 1983 and the St. John Rigby Sixth Form
College Iceland Expedition 1983 for their support and
interest in this study. Fieldwork was carried out under
National Research Council Permit 29/83. A.J. Dugmore
gratefully acknowledges the support of an N.E.R.C.
Studentship.
Ágrip
ALDURSGREINING Á SETLÖGUM VIÐ
SÓLHEIMAJÖKUL MEÐ ÖSKULÖGUM OG
FLÉTTUM
Judith H. Maizels og Andrew J. Dugmore
Aberdeen háskóla
Rætt er um aldursgreiningu setlaga framan við
Sólheimajökul. Fléttur eru taldar nothæfar við aldursá-
kvörðun s.l. 150 ár, en veðrun kemur í veg fyrir það
þegar setlög verða eldri. Höfundar benda á setlög sem
þeir telja frá 1840, 1883, 1935 og 1969. Með gjóskulög-
um aldursgreina þeir lög á Skógasandi frá um 600 e. Kr.
og á Sólheimasandi frá um 1400 e. Kr.
76 JÖKULL 35. ÁR