Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 142

140
Fig. 9: The eastern summit of the Hengill mountain, formed by three south-dip-
ping unporphyritic basalt lavas 1, intercalated by tuff t. This is a small part of
an extensive tuff-lava series which was, first, tectonically disturbed, then subjected
to planation close to (a low) erosional base level and, finally uplifted in blocks to
form the eastemmost % of the present surface of Hengill, as discussed in the
main text.
tic tuffs and lavas (B in Fig. 10, E), which can be followed up
to the eastem summit of the mountain at 767 m, Fig. 9. This series
had been faulted, tilted and denuded to a plain prior to the forma-
tion of the mountain by uplift. It is seen on the whole eastem
% parts of the mountain surface, and is directly covered by the
topmost series of the mountain (T in Fig. 10, E), left in the west-
em Yí of Hengill (cf. also 16, p. 55). This plain is no doubt part
of a more extensive plain of denudation formed at a low level.
The same erosional plain is perfectly clear on the narrow ridge
Sleggja, a SW-outlier of Hengill, at the 600 m level. Here the
plain is covered by, first, dark-grey siltstone at 620 m, and then
the same series of stratified “tuff”, formed of a very slightly worn
material, as forms the just mentioned topmost member of Hengill.
The thickness is 60 m on Sleggja, and here some lavas are inter-
calated. On the east side of Sleggja, on a lower block, the surface
of this layer is at 570 m, and here a direct jump of it up to Hengill
takes place in a vertical wall coated with dislocation breccia. A
short distance away, on the SW-side of Hengill, the same layer is
seen to proceed in steep flow forms from the edge of Hengill down
to the Sleggja level. It were just these unmistakable flow forms
which the present author observed here in 1945, after having seen
such forms also in a number of other tuff-breccia block mountains
(19). He mistakenly interpreted them as “sand flows” (cf. the
Katmai eruption of 1912), a conception he later abandoned, as
stated 1962 in (2, pp. 185). Yet five years later, it was thought
more worth while in (16) to re-refute the sand flow conception in