Jökull - 01.12.1982, Page 60

Fig. 7. The cemented upper
part of the Preboreal sand,
which is beautifully eroded by
the Trjávidarlækur. Note the
cross bedding.
Mynd 7. Trjáviðarlœkur hefur
rofið sigfallega niður í efsta hluta
Prebóreala sandsins, sem er vel
samlímdur. Víxllögun sandsins sést
greinilega.
changing the birchwood overnight into a gloomy
desert characterized by dunes ofH4 tephra. Then
another Plinian eruption dumped down the H3
tephra layer 2,800 years ago, and for the next 1,600
years or so the H3 tephra drifted about. Scattered
remains of plants in the reworked H3 tephra must
have been transported there together with tephra.
Complete lack of organic soil also seems to indicate
that the plant rests are not in situ.
The observations discussed above seem to indi-
cate strongly that the Trjáviðarlækur basin íloor
was a desert for about 2,800 years, from 4,000 BP to
about 1,200 years BP, when loessial soil formation
started shortly before the Settlement tephra layer
fell (Profile 1 in Fig. 4). During this period parts of
the H3 cover was eroded away, leaving an uneven
topograhy of the surface, as can be seen in the
channel banks. Possibly about 1,200 years BP, the
basin was covered with vegetation again. No plant
remains are found in the sandy loessial soil, but it is
very likely that birch made a comeback to the basin.
"l'his description applies mainly to the basin fioor.
The steeper mountain slopes may have been cover-
ed with vegetation more or less during the Holoc-
ene. As can be seen in many areas in Iceland where
soil erosion is going on, the drifting sand is raised by
wind action to a certain height above which the
vegatation survives.The same goes for vegetation in
sheltered places, along river banks and around
springs. After the H, tephra fell, permanent water
supplies must liave been poorin theTrjáviðarlækur
basin as the tephraof H3 and H3 is coarseand highly
permeable. This is clearly demonstrated in the
channel where springs issue at the contact between
H t and the much denser peat (Fig. 9). This has had
a draining efiect on.the ground and has increased
the drifting of the sand.
The Hekla eruption in 1104 AD led to desolation
of farms in Pjórsárdalur valley because of the deteri-
oration of vegetation. In the next decades, the veg-
etation seems to have recovered, and for many cent-
uries the valley»was an important source ofwood for
farmers in South-Iceland. The wood utilization cul-
minated in the 16th and 17th centuries (Thorarinsson
Fig. 8. A birch log, 10 cm in diameter, inside the
tephra layer H5.
Mynd 8. Birkistofn, um 10 cm í þvermál, í gjóskulaginu
H5.
56 JÖKULL 32. ÁR