The Botany of Iceland - 01.12.1914, Síða 34

218
THORODDSEN
In the Pliocene period Iceland was fissured transverselj' by nume-
rous lines of fracture which caused violent volcanic action, by which
tlie tuffs and breccias of the palagonite formation were produced.
Along the same lines the volcanoes of the Glacial period and of
the present day appeared. From the end of tlie Pliocene period to
the present time the coast-line has been subject to considerable
changes the boundary-values of wliich appear to be a positive dis-
placement of 150 metres and a negative displacement of 250 metres
of the position of the sea-surface relative to the land. At the end
of the Pliocene, or during the earliest part of the Glacial period,
the coast-line sank about 250 metres below the present level, and
in the broad coastal platform which thus became dry land, erosion-
grooves were formed leading off from each main valley. Now, each
fjord and bay is continued out to the edge of this submarine plat-
form by submarine fjords, as has been proved by the soundings
taken of late years by the Danish Marine Departinent.
The marks left by a negative displacement of the coast-line in
post-GIacial times occur around the whole coast, but are especially
well-developed round the north-western peninsula. Everywhere along
the rocky coast are found marine terraces of gravel, coast-lines and
surf-terraces marked on the solid rock; in several places remains
of shells are also found and sometimes bones of wliale and wal-
ruses, also old drift-wood; sometimes far from the present line of
coast. Round the north-western peninsula occur distinct and well-
developed coast-lines and surf-terraces at two levels (70—80 metres
and 30—40 metres above sea-Ievel); such are also found on other
parts of the coast, especially the lower line; the upper line is rarely
as distinct as the lower one; in some places on tlie main land there
appear to be indistinct marks of a water-level up to 100—150 metres.
In South Iceland caves, hollowed out by the surf during the time
of a higher water-Ievel, are rather common, and marine clay-forma-
tions occur upon all tbe low land, the latter having been submerged
during the frnal part of the Glacial period. In the clay-deposits in
south-western Iceland, Yoldia arctica and other High Arctic molluscs
are found at a level which corresponds with the higher coast-line
(70—80 metres); other shell-mounds, with a fauna which resembles
the present one, correspond with the level of the lower coast-line
(30—40 metres), Saxicava is especially characteristic of these shell-
mounds. Since the Glacial period the coast-line has retreated, but
with some oscillations, and even now several indications may be