Náttúrufræðingurinn - 1969, Blaðsíða 76

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NÁTTÚ RU FRÆ.Ð I NGURINN
then the soil erosion either takes place as channeling, land slides or
mud flow.
3. Combined water- and winderosion, where the groundwater table is un-
stable. Fig. 11 A, B, C and D shows this kind of the erosion cycle,
which is the most common one on Haukadalsheidi and in rnost other
places.
The soil erosion has only been a slow process most of the preliistoric timc
and the devastated areas liave regrown soon again, but three events can
change this process into catastrophic soil destruction. They are: A great
tephra-producing volcanic eruption, climatic depression and last but not least
the presence of man and his domestic animals. No one of these tliree factors
alone seem powerful enough to bring about a serious destruction, but il two
or three of them join lorces the result is a violent soil destruction and great
areas are devastated, as happened in the 12tli and 13th centuries and again
in the 17th, 18tli and 19th centuries (Figs. 4 and 5), the result of which is
that most of Haukadalsheidi is at the present a devastated land (Fig. 1). Thc
remainders of the loessial soil there show that it was covered with vegetated
loessial soil about one meter thick, when the settlers arrived about 1100 years
ago. The soil erosion there in historical tinte has thus been about 220 million
cubic meters, but this corresponds to the fact that the wind and water have
every year carried away about 800 cubic meters from eaclx square kilometer
during these 1100 years of settlement.