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18
Matti Sauramo, The mode of the land upheaval in Fennoscandia.
limit of the tilted area has been limited by the great ice-lobes of the
first Salpausselkä stage, see Fig. 9. Inside this limit the load of the
stagnant and probably also increasing ice, caused according to Hyyppä
(1933, 1936) by the rising moisture during the last maximum of the
solar radiation (Milankovitch 1930, 1938, W. Köppen 1934, Sauramo
1938 b, Ahlmann 1938), seems to have put a brake on the uplift,
whereas the sudden thinning of the water-cover outside the same limit
may have facilitated the tilting, as in the case of Lake Bonneville in
North America.
ABNORMAL UPLIFT OF EASTERN CARELIA.
However, the deformed area has also a northern limit, to the east
of Joensuu. There the isobases split into two groups, as seen on the
map Fig. 5, the northern ones following the general direction to the
north-east, while the southern group turns to the east, describing a
wide bow, which probably comprises also the area of Lake Segosero
(Seesjärvi) and the northern end of Lake Onega in Eastern Carelia.
This form of the isobases is seen already on some earlier maps (S. B.
Jakowleva 1932). The course of the isobases suggests, that the area
in question has warped along a line running parallel with the axle
of the old Archaean mountain ridge, thereby uplifting the eastern
side of the area to an abnormal height in relation to the Leningrad
district.
In the basin of Lake Segosero (Seesjärvi) and also south of it
varved marine late-glacial sediments occur, containing i. a. fossil
shells of Yoldia (Djakonova-Saveljeva 1929, S. B. Jakowleva 1932).
Therefore the corresponding marine stage has been called the Yoldia
Sea. It has also been correlated with a stage of the same name in the
development of the Baltic Sea. With regard to this dating it must be
remembered that the Baltic Yoldia in our system is of finiglacial age,
and that at this stage the land-ice had already entirely withdrawn
into the Baltic basin and inside the Salpausselkä moraines. The area
east of this ice-border line again was freed from ice during the later
part of the gotiglacial time (BI) and the Onega district still earlier.
Consequently, the above-mentioned Yoldia-bearing sediments are of
gotiglacial age or still older, corresponding to the shore-lines g, h or,
perhaps, i in Tanner’s (1930) system. It would be of great interest to
know if the finiglacial Yoldia Sea also extended to Lake Segosero.
In that case it w^ould be possible to follow the progress of the
abnormal deformation of the area in question more in detail.
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