Jökull - 01.12.1967, Page 18

m y. 5. m y.S.
Fig. 4. Section along A—B in Fig. 5. — a, scar lel't on the niountain side, removed rock; ax,
removecl ice, moraine and talus; b, heap of rock debris; c, sheet of debris, consisting of rock ancl
ice, covering the glacier; c,, wall of ice blocks marking the border of the hlaup track; cl, new
cliff formed by the rockslide; d,, slide plane; e, glacier; /, scree, deposited gradually after the
rockslide; g, the mountain slope before the rockslide; m y. s., m above sea level, true scafe.
4. mynd. Snið yfir Steinsholtsjökul milli Innstahauss (t. h.) og Skaratungna (t. v., A—B á 5.
mynd). — a: brotsár í bergi; a,: brotsár i jökulís, ruðningi og skriðu; b: hrunurð; c: hlaupurð,
grjót og is; c^: hrönn á hlaupmörkum; d: hrunstál; dj: skriðflötur; e: skriðjökull; f: skriðufót-
ur, myndaður eftir frarnhlaupið; g: fjallshliðin fyrir hrun.
rockslide the hiclden lower part of the slide
plane continues in nearly horizontal direction
at least 30 m below the surface of the glacier.
The overlying layer of ice was removed by the
rockslide and incorporated in the hlaup clebris
farther down.
The scar left in the glacier was first after
the rockslide bounded on the south side by a
vertical wall of ice, rising up to 30 m above
the rock debris, that concealed the bottoni of
the scar at the foot of the ice wall. During the
spring and summer of 1967 the ice wall became
gradually pushed forward by the moving gla-
cier and cleformed by its melting. The volume
of ice removed frorn the glacier is roughly
estimated as no less than 10° m3.
Most of the rock tlebris carne to rest at the
foot of the scarp on the mountain side, where
it forms a very rugged heap consisting of an-
gular rock fragments of all sizes up to tens of
metres across and covering an area of 280,000
m2. The front part of the heap, probably less
than half of it, rests on the glacier. The highest
ridges and peaks of the heap of debris rise
more than 50 m above the surface of the gla-
cier, but as there is insufficient indication of
the position of its base, its volurne cannot be
estimated with any degree of certainty. In all
probability it is considerably less than the vol-
ume of the scar on the mountain side.
Innstihaus ancl the rockslide debris, as well
as the whole of Steinsholt, consists of volcanic
rocks of late Pleistocene age. The most
common rock is móberg, or liyaloclastite, com-
posed of rather fine-grained, hardened volcanic
ashes and mud, possibly deposited in glacial
water. A rough stratification is discernible on
the new scarp formecl by the rockslide. The
layers dip about 8° north or almost at right
angle to the direction of the sliding. The sum-
mit of the mountain, however, consists of
heavily jointed crystalline basalt. In spite of its
relative hardness, this rock evidently disinte-
grated much more than did the móberg during
thc transport as all the largest boulders found
far down in the track of the hlaup consist of
the latter.
SECOND PHASE: FRAGMENTED ROCK
AND ICE MIXED WITH WATER,
LAUNCHED BY COMPRESSED AIR
A considerable part of the rock debris, bc-
sides all the ice removed front the surface of
252 JÖKULL 17. ÁR