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50
I. PHYSICAL GEOGRAPHY OF SWEDEN.
careous (clayey marl). Such is the case in Uppland (where it comes
from a Silurian district at the bottom of the Gulf of Gefle),
Östergötland, and other places.
The strait across Nerike became shallower and shallower as the
country rose again out of the glacial sea, and the Baltic was by and
by completely cut off from the ocean and transformed into a freshwater
lake. This lake was larger than the present Baltic, for it extended,
besides, over the provinces of Uppland and Södermanland, by far the
largest part of Nerike and Östergötland, parts of Gotland and Öland,
and quite a broad strip of the coast of Norrland. At this period a
clay, not very thick, was deposited, which in many places covers the
glacial clay. In this clay and in contemporaneous shore deposits a
little mollusc, Ancylus fluviatilis, has been noticed, which lives only in
fresh water. The great freshwater basin above described, has
therefore been called the Ancylus-lake and the age during which it existed,
the Ancylus-period. When, in consequence of the rising of the land,
the strait across Nerike was closed, the waters of the Ancylus-lake
cut new outlets at the places where now the Belts and the Öresund
are situated, and when, at the end of the Ancylus-period, a slow
submergence took place of the region around the southern part of the
Baltic and the German Sea, the waters of the latter broke into the
Ancylus-lake, and the Baltic thus became an inland sea with
brackish water.
The communication thus opened was wider than at present, and
the percentage of salt in the Baltic a good deal higher. In the
deposits of that time a mollusc, Litorina litorea, has been found, which
does not now live in the central or northern parts of the Baltic. These
Post-glacial deposits have been called the deposits of the Litorina-period.
Almost the whole of Uppland, a large part of the provinces round
lakes Mälaren and Hjelmaren, and the shores all the way from Skåne
northwards to Haparanda were then to a greater or smaller extent
covered by the Litorina-sea. In Uppland the water rose 60 meters
higher than at present. During this period was deposited the grey,
unstratified clay, which, within the districts mentioned, covers the
glacial clay and other older quaternary deposits to a depth of one or
two meters, and forms the larger part of our cultivated land.
In general, the marine deposits constitute the best and most easily
cultivated soil of Sweden, and in consequence they are almost entirely
claimed for agricultural purposes. Therefore, the best cultivated and
most densely populated districts of the country coincide, on the whole,
with the territories that in glacial and post-glacial times were
submerged under the sea.
Sweden is very rich in lakes. Besides the larger lakes Venern,
Vettern, Mälaren, Hjelmaren, Siljan, Storsjön, Hornafvan, and others,
innumerable smaller lakes are found in the different parts of the country.
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