Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 145

143
Hátindur plateau, but there is a great difference in age. The im-
mediate proof of this is the fact that this Young porphyritic breccia
overlies the Hengill Basal Layer at the head of the Nesjavellir de-
pression, Fig. 10, D, whereas the Old porphyritic breccia on the
Hátindtu- plateau was found above to underlie the same layer.
Further evidence of age difference will soon be pointed out.
The Young breccia overlooks the Nesjavellir depression in a
vertical wall and, contrary to (16) this is a fault: the wall is coated
with vertically layered dislocation breccia made of the same ma-
terial. This is best seen in D of Fig. 10; in C it serves rather to
show the length and nature of the wall. (Skarðsmýrarfjall consists
of a very similar fresh breccia; it was formed in a subsided groove
in the Hengill-Sleggja rocks, then cut by an erosional plain and
finally uplifted with the Hengill-Sleggja blocks, cf. below. This
breccia is rather hkely to correspond in time to our Young por-
phyritic breccia).
On the east side of the Nesjavellir depression the Old porphyritic
breccia reappears and rises in fault steps (section C). On the high-
est step it is covered by the Stangarháls unporphyritic breccia
which originally-formed an extensive layer, reaching all the way
to the foot of the mountains Mcelifell and Sandfell. The long
Stangarháls and the Lómatjarnarháls are the main remnants of
this layer.
The two members of the southern Stangarháls, just mentioned,
continue southwest to Köldulaugargil (K, section D). At the mouth
of this gully tíie Old porphyritic breccia is seen to cross the gully,
to be overlain on the west side by the Hengill Basal Layer (H,
section D). This important detail, as well as significant facts foimd
at the mouth of Hvanngil are lost in (16 Fig. 15) because that
section crosses the heads of both gullys.
The Old porphyritic breccia forms an unbroken exposed ero-
sional slope from the floor of Hvanngil up to a clear erosional sm--
face with some gravel at 300 m, where it is superposed by the
unporphyritic Stangarháls breccia.
The mouth of Hvanngil is filled with hard tillite (not shown
in D) through which the river has now cut a deep and very narrow
canyon with vertical walls. There is no doubt that even such a
narrow gully as Hvanngil is older than the Last Glaciation (the