Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 130

128
to the Thingvellir axis without gaps that could justify the intro-
duction of the concept “spreading”.
II, Sa means Sandfell which rests on a flat and relatively
extensive erosional surface at the 260 m level. That this volcano
was formed at the time of the 250-300 m hase level is not only
indicated by this position of a tuff-breccia volcano remnant, but
also by the diversion of later drainage which the volcano heap
seems to have caused: the present deep valley makes a bend around
Sandfell (9, p. 110).
III, Þv means Thverfell at the head of Lundarreykjadalur. This
volcano was formed on a relatively broad flat and graded valley
floo'" a clear remnant of which is now seen in an area aroimd the
farm England at 170 m above sea-level. This floor very likely
corrpsponded to a 100 m base level at the mouth of this relatively
very long valley. The volcano formed an effective dam which pro-
duced the lake Reydarvatn. The dam diverts much of the earlier
drainage to an ungraded valley farther north (it is full of water-
falLj naturally the ungraded state of that valley is in part due to
the lack of eroding material in the lake runoff). The decreased
drainage to the old valley left parts of the old floor intact. A pro-
longation of the valley axis through Thverfell strikes the crater of
Skjaldbreidur, the implication of which will be considered under c)
below.
IV, M and Hv mean Mosfell and Hvalfell, respectively, which
were formed in valleys graded to present sea-level. Fig. 5 shows
Fig. 5: Cross section of Botnsdalur through Hvalfell (H:L = 1). g is an inferred
surface of a valley glacier, based on tillite occurrances on the slope of Hvalfell.
The table-like form of the mountain is certainly due to erosion, mainly by such
valley glaciers (2, p. 66-70).