Rit (Vísindafélag Íslendinga) - 01.07.1962, Blaðsíða 86

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in the fluid state) and lava spats. Then there are tuff-breccia
layers of various grades and finally lava flows. The struc-
ture is very instructive, cf. Fig. 42. At the bottom of the sec-
tion we have stratified tuff-breccia deposited on and in front
of a slope of 33—34° dip. The slope was formed by alter-
nating deposition of tuff and thin tongues of lava. On this
surface (a-a) was deposited, with indistinct stratification,
tuff and flattened bombs, ending with the surface (b-b).
Now a thick series of thin horizontal lavas with scarse tuff
layers was formed, starting at the foot of the slope and gra-
dually filling up the lower ground. Obviously these lavas did
not flow down the pre-existing slope (as did the lava tongues
below a-a); they originated elsewhere and were piled up
against the slope.
Possibly the steepness of the slope a-a was the result of an
Fig. 43. Hrafnabjörg, edge of westem wall.
Explanation in the text.
earlier creep and slip in the underlying mass but this cannot
be seen because of scree.
Along the edge of the main west wall of Hrafnabjörg si-
milar condition are often seen, Fig. 43. The lavas have gra-
dually encroached upon a slope until finally they overflowed
the tuff-breccia mound.
The lavas in the southern section have a total thickness of
over 120 m. They are often incoherent and merge into pil-
low lavas and primary breccia. Dykes are numerous, and
many cut the whole lava series. From bottom to top the ba-
saltic material is of the same light-grey fine-porphyritic type
and this would rather suggest that the tuff-breccia socle and
the lava cap belong to the same volcanic cycle.
The surface of the lava cap forms a number of erosional
mounds and hummocks and is covered by weathering debris.