Jökull - 01.12.1999, Page 66

Figure 6. Trace of the glacier push from 1992, on
the proximal side of an end moraine, probably from
1955. — Framskriðsfar frá árinu 1992 í jökulgarði
sem sennilegast myndaðist árið 1955.
Forms linked with glacier movement
This group of forms includes end moraine ridges
which have been traversed by the glacier as a result
of its transgression, drumlinized ground moraines,
drumlinized, dome-like ground moraines, and drum-
lins. Drumlinized ground míoraines form ridges,
sometimes exceeding 100 m in length, perpendicular
to the glacier snout, with a width of 3-5 m and a height
of 0.5-1.0 m. These ridges and depressions parallel to
them form a rhythmic relief clearly visible on aerial
photographs.
Drumlinized, dome-like ground moraines are
characterized by gentle surface distensions, cov-
ered by ground moraine sediments on top of mou-
tonized volcanic rocks or damaged cores of older end
moraines. This type of ground moraine is found in the
northern part of the Höfðabrekkujökull forefield.
Drumlins were observed in two places within the
backfield of the 1890-1900's end moraine ranges,
where they are arranged in the form of small lobes
and situated on fragments of a flat ground moraine.
Their length reaches 80 m, their width 30 m, and their
height 1-2 m. These drumlins have elliptical shapes,
and like the drumlinized ground moraine, they were
formed under subglacial conditions, thus document-
ing the direction of movement of the icemass.
Forms linked with accumulative activity of melt-
waters
The outwash plains
Outwash plains (sandar), formed by melt waters
within several drainage systems, occupy the great-
est expanses on the Höfðabrekkujökull forefield. Al-
though it is extremely difficult to determine their
age and thus correlate individual outwash plains de-
pending on their elevation, we were able to distin-
guish 10 levels of glacier drainage across the Sandvatn
(Múlakvísl) river basin (Figure 7). The highest ter-
race, adjacent to the Hafursey volcanic massif (level
I, Figure 7), dips southwards from about 190 m a.s.l.
to about 100 m a.s.l. over a distance of barely 6 km.
This easternmost terrace is about 30 m higher than the
Sandvatn river basin and is made up of sandy-gravelly
glaciofluvial sediments which have been subjected to
intensive eolian activity. This part of the outwash
plain predates thejökulhlaup associated with the 1918
Katla eruption (Maizels, 1992). Lower levels have
been preserved in three distinct fragments of differ-
ent sizes. Level II lies about 12-15 m lower than level
I. It has a large surface area, extending south of the
Hafursey massif. A fragment of this level is situated
between individual Sandvatn river channels. Small
depressions (rimmed kettles) surrounded by gravel-
boulderrims are common there. According to Maizels
(1991; 1992) these kettles were formed by melting of
iceblocks which stranded there during the 1918 jökul-
hlaup from Katla.
The lower drainage surfaces are younger. Like
level II, level VI, which lies about 8-9 m above the
Sandvatn basin, has been generated during a recent
jökulhlaup. It has well exposed walls where it is pos-
sible to observe two series of glaciofluvial sediments
which, according to Maizels (1991; 1992), can be
counted as belonging to the last sedimentation phase
of a jökulhlaup. The lower series, which are close to
3 m thick, are made up of trough bedded gravel which
has filled up migrating channels whereas the 1.2-1.5
m thick, higher series are made of horizontally lay-
ered sand and gravel. These sediments are clearly de-
posited under variable sheet-flow conditions, charac-
teristic for the final catastrophic phase of jökulhlaup
deposits. The kettles located in its proximal part (see
64 JÖKULL, No. 47