The Botany of Iceland - 01.12.1914, Blaðsíða 73

PHYSICAL GEOGRAPHY
257
downward-flowing water, during thaw, tunnels below the ice-layer
and forms subterranean channels, causing the upper layer to col-
lapse and fall down into these hollows. In this way, after the vol-
canic eruptions of Askja in 1875, great damage was done to the
soil on Jökuldalur; its enormous moraine-terraces were covered
with pumice-gravel which froze into a thick layer that melted but
slowly, because the white, glistening gravel reflected the rays of the
sun; below this layer, the water dug out channels, 20—30 metres
deep, tlnough earth, sand and gravel, and caused catastrophes such
as tliat mentioned above.
In this connection may be mentioned the influence often exerted
ujion the soil hy the numerous avalanches and rock-slips ol' different
kinds, and the very slow, creeping movement wliich may be ob-
served in connection with gravel, stones and rocky hlocks upon the
mountain sides whicli, in the course of years, may become of very
great morphological importance. During earthquakes it may happen
thal mountain-sides clad with grass and coppice are suddenly de-
nuded of their surface soil, which slides down into the lowland
plain. During the earthquake of 1896 a piece of swampy soil, 10,000
sq. metres in area and 2—3 metres thick, at Thjórsá in the neigh-
bourhood of Krókur, thus slid down, being thrown into wavy folds
and hummocks, although the slojie oi' the ground was only 1—2°.
The mountain of Skarðsijall, wliich rises 227 metres above the plain,
had, before the earthquake, a thick coating of soil, and was grass-
covered to its verge; but after the earthquake it resembled a fruit
which has been jieeled. Thirteen landslijis descended on the western
side, leaving behind them large surface depressions, and strewing
helow mighty mounds of soil, clay, gravel and stones together with
larger and smaller fragments of torn greensward. These landslips
must, in the course of time, have had an enormous influence upon
the soil and the plant-growth of many districts where earthquakes
are very frequent. All over the island are often seen, along the
mountain-sides, marks of ancient huge earth- and rock-slips that
could only have occurred during earthquakes.
Level tracts witli their surfaces cracked into polygonal cakes
(rudemarks) are extremely common in Iceland, and fine specimens
of such may be seen as, for instanee, in the neighbouring districts
of Reykjavík. They liave a peculiar effect upon j>lant-distrii)ution
on the rocky flat. “Rudemarks” are usually formed only on flat
land where the soil consists of gravelly clay — especially clay inter-