Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 146

144
river in Hvanngil runs finally into a broad erosional valley soon
to be mentioned).
The east side of Hvanngil is in D formed by the Old porphyritic
breccia which, however, is cut by an erosional level at 230 m,
especially clear on the north side of the narrow canyon, and on
this level rests the erosional remnant of a lava flow. The 230 m
level is no doubt a certain stage in the formation of the broad valley
just mentioned (section C and D). The east side of Hvanngil lies
tectonically a few metres higher than the west side; it was this
slight displacement which, after the full development of the main
valley, so changed fhe drainage as to cause the erosion of Hvanngil,
this strangely rough stroke of the last interglacial period in the far
older smooth morphological picture.
Crossing our main erosional valley, and mounting the east slope,
we come to an extensive erosional plain at 280-290 m. (The por-
phyritic breccia can be traced without interruption right across the
head of this broad valley). On this plain rests the pyramid-formed
remnant Skolli of a much younger “tuff”. A few small remnants
of this tuff are also found at the edge of the valley, 500 m north
of Skolli, and here it is seen most clearly to overlie the edge and
extend from the plain down the slope for a short distance, which
demonstrates a clear age difference between the two rocks. The
“tuff” is mineralogically very similar to the underlying Old por-
phyritic breccia. It is mainly tuff, and thus of explosive origin,
but there are also coarser parts. The Skolli tuff can be traced south-
west across the old valley and up to Sandklettar (Brúnkollublettir)
where it rests with the expected erosional unconformity on the Old
porphyritic breccia, separated from it by a tillite, as earlier shown
in (16, Fig. 15). Erosional features or unconformities, so important
here as elsewhere in the area, are not mentioned in (16), so that
a further comparison is not possible.
The Old porphyritic breccia reaches in the “Skolli erosional plain”
the foot of Mælifell, and is here seen to be an extension of an abra-
sion bench surrounding Mælifell and Sandfell (Fig. 10, EB sect. C).
At the western side of Mælifell the height of the bench is 280-
290 m, and the same height is seen on the north side of Sandfell.
Bæjarháls is a 250-300 m broad erosional remnant of the abrasion
bench with a maximinn height of 200 m. A little farther southwest,