Rit (Vísindafélag Íslendinga) - 01.07.1962, Side 165

163
there is a 5—10 km broad erosion terrace which carries the
names Kirkjubæjarheiði, Merkurheiði and Geirlandsheiði. It
lies hetween heights of 200 and 300 m and is remarkably even.
Close to the southern rim there are heights near 200 m and
here is mostly a steep wall down to the very broad coastal
plain of 20—40 m height and less.
Some parts of the erosion terrace lie between very narrow
height limits. This is true for the 8 km long and 2—3 km
broad part stretching SE from Eintúnaháls; it is 260—300 m
in the north and 240—250 m at the southern end.
In the west two ridges rise above the general level of the
terrace: Steinaheiði (423 m) and Skálarheiði (500 m). Be-
tween them is the Holtsdalur, now incised but at its mouth
there are clear remnants of a smooth floor at the 300 m level.
The rocks of which the mountains of Síða and Fljótshverfi
are built have largely horizontal stratification and there are
alternately layers of tuff-breccias and kubbaberg in many
variations, and thick lava flows. One might therefore suspect
that erosional plains and terraces in this landscape are main-
ly of structural nature. But this is not the case. The lavas
do not mark speciel terraces. In addition we remark: Each
layer or unit in the rocks is horizontally very variable, chang-
ing from a compact lava at one place to a fine, relatively
loose breccia at another. The erosion levels are independent
of this horizontal variation in structure, as I have observed
in many instances.
Thus, it is quite natural to understand the main erosion
terrace here discussed as being originally at a base level near-
ly 300 m above the present one.
Even more convincing is this interpretation if we look at
the broad and deep valleys of the rivers Hverfisfljót and
Brunná. The valley of the former has now an unnaturally
narrow mouth; this is because on one side there is a 2 km
broad terrace left over from an older stage. The height of this
terrace is mostly about 250 m but individual hills reach the
heights 267, 281, and 291 m. At terrace level the form of the