The Botany of Iceland - 01.12.1914, Síða 75

PHYSICÁL GEOGRAPHY
259
where l'or some reason or otlier no ice-layer is formed in the ground,
neither “rudemarks” nor knolls seem to occur.
The depth at which this ice is present in spring diífers greatly
in the diíYerent parts of the island, and — according to the weatlier
— in the different years. In the northernmost districts this ice may
remain throughout the summer for years during cold and damp
periods. As a rule, in the first half of June, frozen ground is met
with at a depth of 1—172 metres over the greater part of the island;
on the plateau a thicker or thinner layer of lliis sub-surface ice is
no doubt always present throughout the summer, and there, in
several places, it gives rise to the formation of swamps, bogs and
lakes, as the melting snow and ice cannot drain off. In some districts
with very warm ground, where hot springs or other secondary effects
of volcanoes occur, tlie ground is never frozen.
As is well-known clay and clay-soil develop, by contraction,
numerous intersecting clefts; such cracks are also formed during
winter by the action of frost, and in severe winters loud cracks are
constantly being heard, announcing the rending of the ground. Tlie
surface layer ol' soil is therefore traversed hv a network of numerous
rents and cracks which divide the clay-soil into irregular frag-
ments or a number of prisms. On closer investigation these cracks
are nol only found to occur on gravelly and clayey flats, poor in
jilants, but also in the clay-humus of the home-fields; some of them
are as fine as hairs, others have a breadth of 2—3 cm. Both tlíese
factors, the sub-surface ice and the cracks in the ground, are neces-
sary to the formation of “rudemarks” and knolls, and to these should
he added two other very important factors, viz. frost and unequal
surface-evaporation.
When, in spring, the snow melts on the cracked and netted
surface of a flat consisting either of clayey gravel or of plant-covered
clav-soil, and the flat itself thaws at the surface, the water percolates
through the ground and the cracks, but cannot escape on account
of the sub-surface ice so that the entire layer of soil becomes
saturated with water. Where the flat consisls exclusively ol' sand
and gravel, without anjr mixture of clay, the entire soil-layer is
evenlj’’ saturated and the surface of tlie water can sometimes rise
to a level with that of the ground. Evaporation then takes place
evenly over tlie entire surface, and when the sub-surface ice melts,
the surface-water drains away or evaporates, and nothing further
happens. But on a clayey gravel-flat intersected by a network of