The Botany of Iceland - 01.12.1914, Side 76

THORODDSEN
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cracks, the circumstances are somewhat different. During spring the
ground is partially thawed; it freezes in the night and thaws during
the day. Tlie sub-surface ice forms a downward limit whicli does
not permit the water to drain away, and uniform circulation and
evaporation at the surface are prevented in a “rudemark” by ice-
formation in the many cracks, originally full of water, which to-
gether with the sub-surface ice as a base form a vascular- or cell-
system over the entire flat, and this system lasts at least for soine
time. The water from the melting snow and ice on the surface col-
lects mainly in the cracks and depressions, where it freezes during
the night; this is best observed on a knolly flat, which during the
thaw of spring produces an entire network of small water-canals.
The water cannot penetrate downwards on account of the ice in
the cracks or, if they should be free from ice, it will yet remain
for a long time in them, for as the water-layer is thicker there than
upon the polvgonal cakes, the evaporalion is slower. The heating
of the sun and consequent evaporation of water is therefore greater
on the cakes, so that the wet from below, l'rom the slowly thawing
parts of the ground and the ice of the subsoil is drawn up into the
centre of the cakes. A clayey soil with particles of a certain size
has great capillary |>ower and can absorb water and draw it up in
great quantities.1 The power of absorption of tlie clay-soil is greatly
increased when it is covered by soil, humus and plant remains.
During spring, frost and thaw alternate constantly and daily. The
“rudemark” freezes in the night, at least partially, and thaws in the
morning; in tlie course of the day the water rises in the individual
clay-prisms owing to the rapid evaporation from the surface, but
in the night it freezes, expands and raises the central part of the
cake. This occasions a constant wandering of the particles of the
clay soil upwards into each clay-prism, and by the constant pres-
sure exerted by the frozen soil throughout a long period, the heavier
particles are sorted out, and as they are less rnobile, they are left
behind or pushed to the sides. This sorting-out of the coarser ma-
terial is the most characteristic feature of the “rudemark.” The
enormous pressure due to the freezing process is well known. As
will be mentioned again later on, in several places in the Iceland
mountain-hogs there are opportunities of observing how the frozen
1 The air contained in sandy clays in a dry condition may amount to 40 %>
of their volume, and by iníiltration, as large a volume of water may replace the
air. See A. G. Högbom in Geol. Förhandl., Stockholm, 1905, XXVII. p. 22.