Greinar (Vísindafélag Íslendinga) - 01.01.1977, Síða 38

36
Still farther west, and between Vatnajökull and the Sprengi-
sandur area in the south, and Mývatn, Einarsstaðir, Grenjaðar-
staður and Hvammsheiði in the north, the basalts of our main
normally polarized zonal group of untilted dolerites are widely
exposed, and show the characteristic N-S valley system. This
drainage system must have created a large “Mývatn” before
a later connection with the Laxárdalur was created at Hellu-
vað. We see at many places, for instance at the head of Reykja-
dalur, that here we have a member of the old N-S generation
of valleys, created by clear, non-glacial water. The original
Laxárdalur must also have belonged to this generation. It had
proceeded so far to the south by headward erosion, that when
our story begins, its head was separated from the Mývatn de-
pression by a basaltic threshold at the present Helluvað. The
Mývatn depression is to be understood relatively, because on
its north side we have extensive uplifted blocks, including part
of the normally polarized dolerite basalts.
The large old Mývatn of the pre-glacial time of the N-S
valley generation, may have had an outlet over the basalt
threshold at Helluvað into the Laxárdalur, and that was very
likely the case. But we might also keep in mind the possibility
of an outlet towards the east over roughly horizontal ground,
which however, at present is cut into many steps and also very
extensively covered by postglacial lavas. This state prohibits
clear knowledge of this surface at the time of the old clear
Mývatn, and a possible outlet here. For this reason an outlet
at Helluvað is at present our only choice.
But clear water, still further purified in a big lake, would
have difficulty in cutting down the threshold at Helluvað. At
most a narrow canyon could be expected, due to the headward
erosion of the now strong Laxá at a place of steep gradient.
But for the excavation of a 1 km wide channel at Helluvað,
I think we need glacial water. This would only be available in
the early and late stages of the Pleistocene glaciations, when
the source of the glacial water was the Vatnajökull area, and
Mývatn turned into a lake with “white” water, such as seen
in the Hvítárvatn today. The erosive capacity of such water is