Jökull - 01.12.1967, Page 22

Fig. 8. Cliff of ice, about 30 m high, cut by the rockslide into the glacier Steinsholtsjökull. —
Photo G. Ivjartansson, May 14, 1967.
8. mynd. Brotsárið i Steinsholtsjökli, urn 30 m hátt.
than 25 m. Supposing that the surface of the
hlaup tilted evenly from right to left, its cross
section would have been approximately 22,000
m2 in this place. — On two cross sections
farther down-stream, levelled by Sigurjón Rist,
hydrologist of the National Energy Authority,
Reykjavík, the corresponding figures were as
follows: (2) 0.8 km down from the lake, right
side 42 m and left side (indistinct) about 20 m,
cross section approx. 12,000 m2; and (3) at the
mouth of the valley, or 2 km clown from the
lake, right side 20 m, left side 40 m, and cross
section approx. 4,000 m3.
This rapid decrease in cross section of the
advancing hlaup cannot be caused solely by the
flattening out of the flood wave. More likely
the hlaup, on its course down the valley, was
continuously decreasing in volume due to the
rapid escape of its air content.
On the flat alluvial cone in front of the
mouth of the Steinsholtsá valley the hlaup
spread out to become nearly 1 km across. Here
the escape of air from the hlaup mass was
completed, and at the same tirne what was left
of its content of huge boulders was cleposited.
These boulders, measuring up to 80 m3, have
travelled a distance of 5 km from the rockslicle
scar in Innstihaus. Farther down-stream in the
track left by the hlaup no boulders were ob-
served larger than 1—2 m3. Here the lilaup had
evidently changed into a mere flood of water,
although boulders of moderate size were still
transportecl by bottom traction for several kilo-
metres and lumps of ice still farther.
THIRD PHASE:
FLOOD IN THE MARKARFLJÓT
All the way from the mouth of the Steins-
holtsá valley down to the sea the hlaup ad-
256 JÖKULL 17. ÁR