Jökull - 01.01.2016, Síða 64

Paweł Molewski and Leon Andrzejewski
roundness of the gravel and boulders. The sandy de-
posits separating them (Sl, SFl) may be interpreted
as the effect of sheetflows separating the subsequent
episodes of debris flow (Nemec et al., 1980; Zieliński
and Van Loon, 1996). This flow occurred at the snout
of the melting glacier, probably after the ice-dammed
lake had drained away.
The sediments building the kame terrace represent
two different sedimentary environments, i.e. glacila-
custrine (units A1, A2 and B1) and glacifluvial (units
A3 and B2, B3). The sediments in profile A repre-
sent a deeper part of the water body, while sediments
in profile B represent its shore zone. These probably
indicate differences in the height of the occurrence of
the dead ice layer in the analysed profile. Both studied
profiles have a similar altitude. The top of the dead ice
in profile B, located further away from Jökulgrindur,
is approximately 4 metres higher than in profile A.
After a slow draining of the lake, meltwaters probably
flowed along the glacier snout, between the snout and
Jökulgrindur. This is attested by a dry channel run-
ning in a south-westerly direction on the surface of
the kame terrace.
RECONSTRUCTION OF GLACIAL
EVENTS
The geomorphological and sedimentological research
clearly supports the existence of ice-dammed lakes of
various sizes in the northern part of the marginal zone
of the Tungnaárjökull, formed by the damming of the
outflow of meltwaters during successive glacier ad-
vances. Combining this study with published, archival
cartographic material and aerial photographs (Frey-
steinsson, 1972; Thorarinsson and Sigvaldason, 1972)
we were able to reconstruct a probable sequence of
glacial events in the northern part of the marginal zone
of Tungnaárjökull since the end of the LIA.
As evident from previous studies (e.g. Eythorsson,
1963; Thorarinsson, 1964) and research by the authors
(Andrzejewski and Molewski, 2000; Andrzejewski,
2002), Tungnaárjökull reached its maximum extent
in the late nineteenth century, i.e. 1880–1890 (Frey-
steinsson, 1968) (Figure 10A). This maximum limit is
clearly constrained by a series of arcuate end-moraine
in the central part of Tungnaárjökull’s marginal zone
(e.g. Andrzejewski, 2002). In the northern part of its
forefield, end moraines are located in different parts of
the Jökulgrindur ridge and its vicinity. In the research
areas the end moraines are situated on the glacier-
facing slope of the ridge, in its culmination or on
its western side (Figures 2A and 6). In the southern
part of the northern study area the snout of the glacier
blocked the meltwater flow in a south-westerly direc-
tion between Jökulgrindur and the glacier resulting in
the development of a large ice-dammed lake (Figure
10A). The length of time during which this lake ex-
isted is, however, unknown. It is known that the wa-
ter must have drained prior to 1939 because the to-
pographic map from that period (see Freysteinsson,
1972, p. 83) shows a lake whose size is similar to that
existing today (Figure 2A).
Tungnaárjökull advanced at least twice during
the first half of the twentieth century, in the years
1915–1920 and in 1945 (Thorarinsson, 1964; Frey-
steinsson, 1968). Earlier studies have shown that
these advances were probably surges (e.g. Andrze-
jewski and Molewski, 1999; Evans et al., 2009). The
1945 advance was documented in the northern part
of the glacier’s marginal zone, based on aerial pho-
tographs (see Thorarinsson and Sigvaldason, 1972,
p. 18). Their map indicates that the glacier blocked
meltwater flow, resulting in the formation of an elon-
gate, NE–SW-trending lake between the Jökulgrindur
bedrock ridge and the margin of Tungnaárjökull (Fig-
ure 10B). It cannot, however, be concluded whether
a similar situation occurred during the advance of
1915–1920, because of the lack of aerial photogra-
phy. It is likely that the deltas/ice-contact fans devel-
oped during the 1945 advance and represent points at
which meltwater flowing off the glacier entered the
lake. Its gradual disappearance is indicated by clear
traces of ancient shorelines, expressed in the form of
a series of small wave-cut platforms (Figures 2A and
3A). Since traces of shorelines on the slope of Jökul-
grindur lie approximately 15 m above the delta, it can
be assumed that the level of the lake, probably asso-
ciated with the LIA, was at least that high. The small
end moraine ridges on the glacier-facing slope of Jök-
ulgrindur are also probably associated with the LIA
64 JÖKULL No. 66, 2016