Then the country came once more under the sea, and the debris of the previous formations, mixed with fragments from the volcanoes then situated in West Finland, formed the so-called Bothnian series.
New masses of granites protruded next from underneath, and the Bothnian deposits underwent foldings in their turn, while denudation was again at work on a grand scale.
In Norrland the deposits at Gellivara have long been worked, with the assistance of a railway to the Bothnian port of Lulea, but in 1903 the northern railway was completed across the Norwegian frontier to Narvik on Ofoten Fjord, and the vast deposits at the hills of Kirunavara and Luossavara began to be worked.
On the Bothnian coast there is a port at or near the mouth of each great river, where the timber floated down from the interior is both worked and exported.
North of the Aland Haf a ridge defines the southern edge of another depression, the Bothnian Sea, which in turn is separated from the most northerly division, the Gulf of Bothnia, by a ridge across the narrow Quarken or Kvarken Strait.