Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 150

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From Sandfell the Old breccia extends to Selhóll (Fig. 10, B).
Only a 250 m wide gap in the bed of the Thverá-ölfusvatnsá river
bed separates Selhóll from the easternmost exposure of the Old
feldspar-porphyritic breccia, i.e. in Yídihlíd. The latter is cut by a
wide erosional plain at the 200-230 m level. In the north and east
of Vídihlíd, the eastem dolerites crop out at the base (16, p. 79);
in Fig. 10, B they are shown in Stekkás farther NE.
But between the breccia and the dolerite, unporphyritic rocks
did exist, just as in the west part of the zone. This is evidenced by
unporphyritic dykes, older than the Vídihlíd breccia, but these
unporphyritic rocks were lost by erosion before this breccia was
formed.
Exposures in the Thverá-ölfusvatnsá valley are here of interest
(Fig. 10, B, and Fig. 12). At the base, we have here rather well
consolidated blue-grey sandstone and siltstone (1, Fig. 10, B), cut
by a 0.5 m thick unporphyritic dyke (2, in the same figure).
This dyke does not extend up into the late-glacial loose sand and
gravel layer of the 150 m terrace of (17), found in the same sec-
tion. Farther up the river, which flows along the vertical wall, a
Fig. 12: Exposure along ölfusvatnsá on the west side of Vídihlíd. Bottom: Vertical
wall of blue-grey lake sediments (younger than Stapafell) is cut hy a basalt dyke,
correspónding to a lost group of unporphyritic basalts. Between these sediments
and the Vídihlíd erosional remnant of the Old porphyritic tuff-breccia, is a 10 cm
thick remnant of a hard tillite. A little further up the eastern branch of the river,
the Vídihlíd breccia overlies non-glacial debris, and is seen to have formed here
in a preexisting river cutting. The 150 m terrace is late- to postglacial.