Náttúrufræðingurinn - 1969, Síða 75

NÁTTÚRUFRÆÐIN GURINN
117
cannot lie termed loess according to the definitions of those authors, wlio
require certain cliemical properties of it. It is proposed to use the term
volcanic loess about tlie Icelandic loessial soil and otlier aeolian sediments
of volcanic origin. Haukadalsheidi, which was selected as a field of studies
for these investigations, is situated about 75—100 km inlánd from the SW
coast of Iceland. It reaches from tlie coastal plain in the south up to the
Langjökull glacier in the north, and it gives by this a good section through
the zone of the most severe soil erosion. Of course, the results of the investiga-
tions are restricted to that area, but they are expected to be of great value
for otlier parts of tlie country.
The rate of accumulation of the loessial soil was fouml by measuring soil
proliles (Figs. 2 and 3), where known tephralayers were used to find its
thickening in different periods of time and on different jdaces (Table II,
Figs. 4 and 5). The grain size compositions of 36 soil samples from different
depth of the soil cover were measured (Table III, Fig. 6) and some studies
were performed on the shape and aspect of the grains. X-ray analysis of two
samples showcd that 90—95% of the grains were made by amorphous volcanic
glass. l'he following results were gained by the studies on accumulation and
texture of the loessial soil: Its accumulation depends on tlie vegetation covcr
and the loess will be accumulated everywhere il there is a continuous vegeta-
tion cover. The main constituent of thc loessial soil is a redeposit tephra,
which lias its origin in the unvegetated interior of Iceland, but the volcanism
has frequently provided that part ot the country with tremendous quantity
of tephra during the whole of the postglacial time. Glaciofluvial deposits,
and the product of physical weathering of the palagonite tuffs and breccia
and other bedrocks contribute only a minor part of the wind-blown material,
but some other authors belive this material to be the main bulk of the loess.
The thickening rate of the loessial soil in differcnt periods of time has been
depending on two factors, the quantity of accessible tephra and a deflation
of jirior accumulated loessial soil, but its constitution lias changed very little
through the time, except where violent soil erosion is in process.
The structure of loessial soil is cxtremely weakly expressed, because of its
relatively low clay fraction, grain size composition and a slow decomposition
of the organic fraction, and it is easily eroded by the erosive forces, water
ancl wind. Therefore the loessiál soil cover has never reached a fair equi-
librium and some soil erosion has always been in process, especially at the
upper limit of the vegetation cover. The soil erosion is an erosion cycle, but
it can be divicled into three different types, depending on the position of
the groundwater and the permeability of the substrata:
1. Winderosion, where the substrata is permeable and the groundwater
table is permanent far below the loessial soil cover, then irregular
barkans migrate across the vegetation cover leaving their trakcs de-
vastated.
2. Watererosion, whcre tlie substrata is impervious and the percolating
groundwater must escape through the bottom layers of the loessial soil;