Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 116

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The relationship between the dolerites on Lyngdalsheiði
and those of Bíldsfell and Háafell—tJlfljótsvatnsfjall was first
described in 1948 (Report to the Direction of The Sog Hydro-
electric Power Development: Trausti Einarsson, Jarðsaga
Grímsness, Grafnings og Þingvallavatns. Pp. 1—18; unpub-
lished). It was later accepted by Tryggvason (1955) but it is
not shown in Kjartansson’s map of SW-Iceland (Museum of
Natural History, Reykjavík 1960).
Coming to Grafningsfjöll, the block Úlfljótsvatnsselfjall is
probably slightly uplifted relative to Háafell. But a main fault
is indicated by the straight line running WNW along Hlíðar-
fjall-Villingavatnsselfjall. The dolerites have here been up-
lifted to 350 m, as seen in Úlfljótsvatnsbotnar, and are co-
vered by a primary breccia as in Bíldsfell; the breccia attains
a thickness of about 100 m in Villingavatnsselfjall, Klóarfjall,
and Efjumýrarhryggur.
Summarizing, we find the following general picture. A
wide area was occupied by tuff-breccias, the flat surface of
which lay at a low altitude. Over this surface was spread
an extensive thin sheet of dolerite lavas. The flat surface of
the breccia was in part of erosional origin but in part the
topmost breccias were formed immediately before the dole-
rites and may then be considered as belonging to the same
volcanic period.
After a glaciation, the dolerites were covered with a new
thin layer of breccias, at least in Grafningur.
In the end the area was cut by a system of complementary
fractures running NE and WNW, and blocks were uplifted,
mainly in the Laugardalsfjöll and Ingólfsfjall—Grafningsfjöll
areas and less in the intervening Lyngdalsheiði area.
Hestfjall.
Like Laugarvatnsfjall Hestfjall has a cover of dolerites and
is a clear case of a relatively late uplift. It could be called an
unfinished table mountain. The north end was uplifted more