The Botany of Iceland - 01.12.1914, Qupperneq 18

202
THORODDSEN
fjords have been filled up even within hisloric times, and several
large parishes and districts have been destroyed. Öræfajökull has,
in the same manner, caused considerable destruction; while the
waterfloods of Skeiðarárjökull, which were especially frequent during
the 19th century, have done less material damage, as only unin-
habited sandy wastes were inundated. Minor glacier-bursts are also
occasionally due to lakes and rivers, which have been dammed by
glaciers, suddenly breaking through their barriers and inundating
the district. These glacier-bursts have a great effect upon the plant-
life, because no permanent vegetation can exist upon gravelly and
sandy flats which are constantly inundated by waterfloods carrying
large pieces of ice; therefore very large stretches of lowland in the
neighbourhood of such volcanoes and glaciers are destitute of plant-
growth, for what little vegetation appears in the period between the
glacier-bursts is quickly destroyed. The general physiographical con-
ditions pertaining to the glaciers of Iceland may be best seen by
a study of the table opposite.1
Snow-Iine. In Iceland it is not easy to determine the snow-
line, owing to the great variability of the climatic factors. Because
of the great annual and periodical variability of temperature and
of the circumstances connected with precipitation, the snow-line also
varies. In Iceland three kinds of height-levels connected with the
vertical distribution of the snow, and dependent on climatological
and orographical conditions, may be supposed to exist. The snow-
line proper, which signifies the lowest limit of the permanent, con-
tinuous snow-covering of the mountains, is not subject to very great
changes from year to year. Below this comes a zone of detached,
more or less closely placed, patches of névé and wreaths of snow;
these never melt entirely, but either are added to or else diminish
according to the character of the year. Below this zone comes the
most variable of the snow-coverings: scattered snow-drifts which
to a greater extent than the others are dependent on orographical
conditions. These snow-drifts may persist tlirough a series of
damp and cold years, but dwindle alinost to nothing or disappear
entirely in warm and dry years. The snow at Drangajökull upon
the north-western peninsula may serve as an example. Here, on
1 Here it should be noticed that many of the figures for the area of the
Jökulls are approximate and given as a rough estimate, because the maps of a
great part of Iceland, and especially of the plateau are still very imperfect.