Jökull - 01.12.1990, Blaðsíða 43

advancing glacier terminated there (Fig. 9). The ma-
rine limit at 55-60 m a.s.l., the 50-55 m shore level
on Höfði and Borgarholt near Keldnaholt (Tryggva-
son and Jónsson, 1958), and the 40-45 m level on
Öskjuhlíð in Reykjavík (Thorkelsson, 1935; Hjartar-
son, 1989) are probably synchronous features (Fig. 9).
A lower set of beach-ridges and a raised delta is found
at Mosfellsdalur and close to the 40 m level.
Subfossil marine molluscs, collected from the bot-
tomset units of the ice-contact delta at Varmá just south
of Mosfellsdalur, have been 14C dated to 9,815 ± 150
B.P. (U-2817). A 14C date of another sample com-
posed of shells that have apparently been weathered
out of the sediments, has yielded an age of 10,415 ±
110 B.P. (U-2898) (Halldór Torfason; personal com-
munication 1991). It is not possible at this moment
to decide whether this date is less reliable or if it ac-
tually dates an older phase of sea-level changes in
Southwest Iceland. The formation of the ice-contact
delta, and the termination of a glacier in the mouth of
Mosfellsdalur occurred most likely in early Preboreal
time. This again indicates that the glaciers in South-
west Iceland may have been much more extensive in
early Preboreal and Younger Dryas times, than was
Previously indicated by the DAD-model.
discussion and conclusions
Our ideas conceming the mode of deglaciation in
Iceland have developed from the earliest model of ap-
Parently continuous deglaciation that was randomly
mterrupted by standstills, towards a single advance
deglaciation (SAD) model and later a double advance
deglaciation (DAD) model (Fig. 1). The most impor-
tant features of the DAD-model were, firstly, that it
accounted for two regional and simultaneous read-
vances of the Icelandic inland ice sheet, and sec-
°udly, that concurrently with the glacier advances
Ihe land subsided due to increased glacier overburden
load, and relative sea-level transgressed and reached
a temporary maximum elevation. With regard to the
number of Weichselian glacier readvances, the DAD-
utodel remained mostly unchanged until Norðdahl
(1981, 1983) put forward a multi advance deglacia-
tion (MAD) model for North Iceland, a model that
contained at least nine advances (stadials) in the pe-
riod between the Weichselian maximum glaciation and
the beginning of the Holocene, i.e. between 18,000
B.P. and 9,650 B.P. (Norðdahl, 1990).
The simultaneous changes in the extent of the in-
land ice sheet and in the elevation of relative sea-level
indicate that these changes had a common cause in
climatological variations and in altered mass-balance
of the ice sheet, but were not caused by inconsistently
dispersed factors such as local topography or glacier
surges. Correlations between glacier readvances and
marine transgressions in different parts of the country
- based on morphology and deglaciation pattem only -
have never been but presumptive and reliable correla-
tions must be based on stratigraphical correlations and
absolute dates such as 14C dates.
Until the early 1980’s approximately 16 14C dates
conceming the history of deglaciation and sea-level
changes in Iceland had been published. Their number
has quadrupled in the last 10 years, and today between
50 and 60 such stratigraphically controlled 14C dates
covering the time span between the Bplling and Pre-
boreal Chronozones (Table I and Fig. 10), have been
published. The increased number of 14C dates has en-
abled both correlation and separation of events in the
deglaciation history of Iceland.
An early Preboreal glacier readvance has now been
dated at four different localities in Iceland (Fig. 10). In
North Iceland local cirque and valley glaciers probably
advanced and extended out of their confining valleys
shortly before 9,650B.P. (Norðdahl, 1979), and in the
Vopnafjörður district in Northeast Iceland, the glaciers
probably advanced just before 9,700 B.P. (Norðdahl
and Hjort, 1987). In South Iceland the Búði moraine
was formed during a glacier advance that reached into
the seaat 9,670 B.P. (Hjartarson and Ingólfsson, 1988).
Approximately at the same time (9,815 B.P.), a glacier
advance was terminated in the mouth of Mosfellsdalur
northeast of Reykjavík (Fig. 10).
A number of 14C dates in West Iceland bracket
a Younger Dryas glacier advance, the Skorholtsmelar
event which culminated at about 10,600 B.P. (Fig. 10)
(Ingólfsson, 1988). In North Iceland a glacier ad-
vance, the Fomhólar stage of the Fnjóskadalur Se-
quence, has indirectly been dated to 10,600 B.P. (Norð-
dahl, 1990; Norðdahl and Hafliðason, 1990). A
JÖKULL,No. 40, 1990 41