Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 138

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canic activity accompanied the beginning of faulting. This faulting
in Jórukleif was on a small scale, for else the breccia would not
only be found at the edge and the foot (5) of the scarp, but it would
also constitute the slope in between. Instead we find here a small
mass of plagioclase-porphyritic volcanic breccia (4a) cut by a
strandline (4) about 40 m below the edge of the scarp, and covered
with well rounded pebbles up to 6-7 cm in diam. It seems clear
that the porphyritic breccia was formed at this stage of the water
level.
The full, and presently final displacement at Jórukleif is best
seen along Símonarbrekkur, where the top of the dolerite crops out
8—15 m above the present lake, and is covered by the unporphyritic
breccia and strand sediments reaohing a maximum height of 50-
60 m above the lake.
Proceeding east from the coast at Jórukleif (i.e. from the base of
Fig. 7), dolerite layers occur in eastward dipping low ridges across
the head of the bay Hestvík, and then disappear under younger,
partly much younger volcanic rocks, and terrigenous sediments.
These consolidated sediments (belonging to the Nesjaskógur series
in (16)) first rise southwards with a gentle tilt, until (at the road)
they jump to a height of over 300 m in a steep, partly vertical wall,
where they are flatlying. The sediments contain only the unpor-
phyritic dark variety of basaltic material (cf. 3, Fig. 7), and are
occasionally intercalated by unporphyritic basaltic lava flows. Here,
in the edge just east of Hátindur, the sediments consist of cross-
bedded, in detail south dipping strata of silt, sand, and fine gravel.
The pebbles of basalt are not much worn but of an inhomogeneity
suggesting various sources, while þebbles of pumice are well round-
ed. This material has been transported by a river or rivulets com-
ing from the north, that is from the present area of Thingvellir
Lake and -depression, forming a thin delta into shallow water. This
sediment(P) is shown in a section about 1 km farther southwest in
(16, Fig. 9) as “ 2 = 0-1 m finegrained sandstone” and “3 and
3a = finely brecciated tholeiitic hyaloclastites of the Háhryggur
series . . . at 3a crudely bedded”. Cross-bedding is not mentioned,
nor the tilt and then sudden rise of the sediments from Hestvík to
the horizontal position at about 300-360 m.
These horizontal beds form a plateau, here called the Hátindur