The Botany of Iceland - 01.12.1914, Blaðsíða 78

262
THORODDSKN
of Ihe clay soil, which projects into them like a kernel and forms
the main part of their volume. Tlie knolls have usually a diameter
of x/2—2 metres and a heiglit of V*—V2 metre; sometimes they are
somewhat smaller; or they may be larger. The form varies, but is
usually oblong. When the knolls are large the channels (kargatthyfi)
between are but narrow. The thiclcer soil-layer together with the
greensward-covering and the vegetation, are intersected by a net-
work of cracks similar to tliat in the “rudemarks:” These cracks
divide the ground into numerous pieces of various shapes which
behave, as regards the sub-surface ice and the moisture, as the
“rudemark.” The knolls are dependent on the crack-systems of the
substratum. In the knolls the ascending stream of clay and humus
particles must be stronger than in the cakes of the rudemark as the
greensward, rich in humus, has a very great capillary attraction and
an enormous water-capacity — it can ahsorb water to the extent of
50—60 % of its volume; therefore during the evaporation from the
surface it absorbs water vigorously, not only from the thawing ground
below, but also laterally from the channels between the knolls fdled
with water during the spring thaw: in spring the greensward upon
tlie smaller knolls is as saturated with water as a sponge. Bands of
volcanic ashes, which were present in the ground or in the greensward
before knoll-formation began, become bent upwards in curves according
to the form of the knolls — a fact which, among others, is a proof
of the local pressure from below in each knoll. In spring, during the
melting of the snow, the channels between the knolls are often half
filled with water which cannot escape, while the tops of the knolls are
dry, sometimes even very dry owing to evaporation, so that tlie vege-
tation upon them is totally different from that upon the sides of tlie
knoll. Sometimes large knolls liave a kernel of ice far into the
summer. The formation of knolls does great damage in the peasants’
home-fields and great trouble is laken in Ievelling them, but they
may reappear comparatively quickly if the ground is not thoroughly
drained, so that the surface-water and the water from the melting
ice of the subsoil are immediately carried away. In this connection
it should be mentioned that where a snow-covering during spring
protects the ground for a long time against the action of frost and
thaw, no knolls are formed.
Knolls of the kind described above occur in thousands also in
uncultivated grasslands with clayey móhella-soil, 011 heather moors
and 011 grass “móar”, and here, also exclusively upon somewhat flat