The Botany of Iceland - 01.12.1914, Síða 18

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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.