The Botany of Iceland - 01.12.1914, Blaðsíða 77

PHYSICAL GEOGRAPHY
261
water inside the knolls can rupture a greensward, 10—20 cm. thick,
and traversed with plant-roots. This pressure from below, repeated
for years in a “rudemark,” must gradually push and force the gravel
aside so that it lodges at last in the cracks which, while they are
filled with ice, form a kind ol' wall around each clay-prism. Thus
the stones are placed in the neutral territory between the small
centres of power, and form a boundarv to each cake, the upper
edge of which boundary appears upon the surface while the lower
reaches down to the ice in the subsoil. Below the level ol’ this ice
the gravel is irregularly dispersed in the clay; it is regularly arranged
only in the surface-layer above the ice. In the summer, when the
soil has thawed and the sub-surface ice melted, the wnter drains
off, and the “rudemark” dries. Everybody who has travelled in
Iceland during spring knows wliat an enormous difference there is
between the clayey gravel-ílats in which the Iiorses sink deej) down
while the ice of the subsoil still hinders the draining off of the
water, and tlie same tlats in summer when they are dry, so tliat
liorses can gallop across them. During summer the clay-polygons be-
come somewhat depressed. Many of them are however slightly arched
during tlie summer also and retain for a long time a considerable
amount of wet in their interior. Clay which easily absorbs water
and expands is well known lo Swedish geologists1 who call it
“jaslera,” and recently it has been connected with “rudemarks.”2
In the neighhourhood ol' Reykjavík (Melar) some well-defined “rude-
marks” have developed in clay soil where a water-containing layer
at a depth of about l1/* metres rests on a thick “móhella” through
wThich water can penetrate only with difficulty, and which therefore
freezes in winter into a plate of sub-surface ice. Where the ground
consists of clayless sand no “rudemarks” are developed, nor wrhere
the subsoil is so porous that water cannot accumulate and form
sub-surface ice proper.
In my opinion the knolls which are of such connnon occur-
rence in the home-fields of llie farmsteads (see Fig. 17) are developed
in a similar manner. These knolls are usually larger or smaller
elevations of earth which occur together in numbers: the surface-
layer consists of humus and plant-remains, but the interior is formed
1 A. G. Högbom: Om s. k. jiislera och om villkoren för dets bildning (Geol.
Fören. Förhandl., Stockhoim, XXVII, 1905, pp. 19—36).
2 E. Bergström: En márklig form af rutmark frán barrskogsregionen i Lapp-
land. Geol. Fören. Förh., XXXIV, 1912, pp. 339—340.