Jökull - 01.12.1980, Síða 33

sludge ice. A part of the crystals is brought by
the turbulence to the bottom where it sticks
and often forms a continuous layer. But his
layer only partly consists of ice as the inter-
stices between ice grains are filled with un-
frozen water.
A comprehensive description of anchor ice
is found in a paper by Rist (1962). In his paper
he also describes how sediments are trapped in
the ice. This takes place in two ways, first the
suspended sediment grains act as nuclei for
the ice crystals. This, he states, causes the
water to become clear in otherwise muddy
rivers as soon as sludge ice and anchor ice
formation is started. The other way is by
direct trapping of bedload into the anchor ice.
As can be seen on Fig. 9 air temperature
frequently fell below freezing during the rec-
eding phase of the jökulhlaup, creating con-
ditions for anchor ice to form. The columnar
diagram of sample SKÚ-4 in Fig. 8 is typical
for the grain size of this material; a major part
of it is coarse silt containing a small fraction of
sand but only very little of still finer material.
The formation of anchor ice is supposed to be
the reason for the deposition of this coarse silty
layer on the sandur. By comparing the grain
size distribution of layer 1 (the black deposit)
in the SKÚ-diagram to that of the SKÚ-4
diagram (Fig. 8) the similarity becomes ob-
vious, otherwise this layer is somewhat more
sandy.
A natural bottom for a stream of Skeiðará-
type would be somewhat similar to the SKG-
Fig. 9. Air temperature at
Fagurhólsmýri during Skeiðará
jökulhlaups in 1972 and 1976.
— Mynd 9. Lofthiti á Fagurhólsmýri
(hlaupdagana) 1972 og 1976.
H= Hómark hlaups/ Maximum discharge
°C
H

f!

16 I 8 20 ' 22 24 26 20 ‘
Dagar marsmónadar 1972
Days of March 1972
Dagar septembermdnaðar 1976
Doys of September 1976
JÖKULL 30. ÁR 31