Greinar (Vísindafélag Íslendinga) - 01.01.1976, Síða 134

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generally of light colour and often rather coarse, and are tradition-
ally called dolerite (or Reykjavík dolerite).
These dolerite lavas are younger than Mosfell: there is a clear
proof that the dolerites leaned in fluid state against the scree-
covered westem side of that volcano ruin. These lavas understand-
ably also overflowed the strandflat on the west side of Úlfarsfell
corresponding to the 50-100 m valley baselevel (cf. the age of
Thverfell). But whether the lavas are younger than Hvalfell can-
not be shown. Again, the dolerites flowed to the foot of Gagnheidi
(in Sláttugil, km SW of farm Svartagil), where a clear contact
proves that at that time the southern slope of the 500-600 m high
Gagnheidi existed in its presently much denuded form.
The lavas reached the previously mentioned coastal area soon
after a glaciation, as seen in the Gelgjutangi section at Ellidavogur
(13, 14). Their total thickness was up to about 100 m in the
Reykjavík-Hafnarfjördur area. This sheet is now heavily eroded
and dissected by well developed and graded valleys, now partly
as sea lanes (vogur). This landscape was fully developed before
the last glaciation of the area, for on the floor of one of the valleys
rest the well-known interglacial marine sediments of Fossvogur.
Also an interglacial strandflat at present sea-level was eroded into
the otherwise morphologically fully developed dolerites in the
Reykjavík area (15, p. 196). These facts conceming the stage of
erosion have always suggested to the author that the dolerites must
be at least several hundred thousand years old, roughly about
*/2 My. An upper limit of 0.69 My was taken to be set by their
normal magnetic polarity. But we now know that they are older,
probably about 1 My.
In the area of Kleppur-Sundahöfn the lavas overflowed an ero-
sional plain on reverse, mainly intrusive and brecciated finegrained
basalt.
These may be the same “old” dolerites which we traced in A) in
stepwisely rising blocks up to the edge of the Bláfjöll ridge
On the east side of Thingvellir Lake, the flat Lyngdalsheidi
shieldvolcano, about 13 km from the Thingvellir axis, seems to
correspond reasonably in age to the Mosfellsheidi. Its extensive
lavas have reached the present Hvítá river in the east, and are
traceable to the river Villingavatnsá and the base of Vidihlíd in