The Botany of Iceland - 01.12.1914, Side 62

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often alternate with layers of wind-polished stones, gravel, scoriæ
or pumice, sometimes with clay. Where the blown sand is conti-
nuously moving, no vegetation can thrive, but when the fine dust
and sand has blown away as far down as to the coarse gravel,
Icelanders say that the sand is “örfoka,” i. e. it cannot drift any
longer (see Fig. 14). Tlien plants are again able to take root and
new soil is gradually formed — until that also is blown away. The
phenomenon of alternating periods of sand-drift and of vegetation,
which has Iasted through centuries, is nowhere so distinctly trace-
able as in Rangárvellir. Here the substratum is exclusively formed
by “móhella,” the thickness of which is unknown, but it must be
considerable, probably 100 metres or more. Here the Iowland plain
abuts on the lava-fields of Hekla, whence quantities of volcanic
ashes are blown down into the cultivated land. The Iowland plain
is intersected by deep, branching valleys, w7hich are usually dry,
but during the thav's of winter and spring large cjuantities of water
have an outlet through these channels. From the plain a series of
small terraces leads dowTn to the bottom of these valleys, which
often consists of a grass-covered, level stretch of land. The valle}7-
sides oífer favourable opportunities for studying the composition of
the móhella: fine bluish-grey layers of sand alternate wdlh reddish
sand-layers penetrated by compounds of iron, and the embedded
stones of varying sizes bear testimony to the strong erosive action of
blown sand. In sorne layers soil and remains of plants occur, also
clay-tubes formed around haulms of grasses. Here and there layers
of pumice and scoriæ are also seen. No inhabited district at the
present time is so exposed to being attacked and overwhelmed by
blow'n sand as Landsveit in the southern lowdands. Here, during the
nineteenth century, large stretches of grassland and many farmsteads
W'ere overwhelmed by drifting sand, especially in the years 1836 and
1880—1881. The substratum consists of old lava w'hich formerlv had
a covering of móhella and greenswrard, now to a grest extent torn up
and destroyed by the masses of blovm sand from the north-east.
Sand storms cause deep channels and furrows in the soil, which
constantly enlarge and by combining with others, gradually destroy
the entire layer of soil, so that only a fewr massive fragments of
móhella with hollowed sides and covered with greensw7ard traversed
by the fibres of plants, are left behind until they also succumb to
the universal destruction. In large stretches of tliis district all
greensward and soil have been torn oíf down to the naked lava-rock.