Geology
In pre-glacial times a river, which the Baltic Sea basin then contained, must have passed through the region. So also did the Eemian Sea, just prior to the last glaciation, which covered the entire region with ice thousands of metres (many more thousands of feet) thick. Today's topography is totally post-glacial. The Great Belt was eroded into existence by streams passing between the Baltic sea basin and the Kattegat. Currently it is a drowned channel.
It is possible to speak of northern and southern zones beneath the surface. The northern one consists of two v-shaped cuts more than 50 m (160 ft) deep. The southern one has a relatively shallow bottom, 30 m (100 ft) deep, showing the tops of riverine and lacustrine sediments. This configuration gives evidence that for most of its life the Great Belt hosted an outward, downhill flow.
The northern zone is located in the sea off the north coast of Zeeland. The southern zone is just south of Langeland, leading into the Kieler Bucht, or Bay of Kiel. The Fehmarn Belt then connects the Kieler Bucht to the Lübecker Bucht, or Bay of Lübeck, to the south of Lolland. The Bay of Lübeck is open to the Baltic sea.
The current channel of the Great Belt was created by a relatively high fresh water phase of the rising Yoldia Sea breaking through to the lower Kattegat levels at about 10,000 BP. At that time the exposed northern zone was a valley less than 1 km (0.62 mi) wide.
The Yoldia sea continued to drain and levels in the Kattegat continued to rise. By 9500 BC the outward flow stopped and the sea proper began to penetrate the enlarged Great Belt, turning it brackish very slowly. During the Ancylus Lake phase, 9500-8000 BP, the Great Belt was an extension of the Kattegat. At the end of that time rising Kattegat levels broke into the Ancylus lake, creating the Littorina Sea.
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