This cutting off was caused largely by subsidence, though partly by marine action.
Probably before this period the movements of subsidence had set in which faulted the basalt plateaus, lowered them to form the basin of Lough Neagh, and broke up the continuity of the volcanic land of the North Atlantic area.
There is no doubt that at this epoch various movements of elevation and subsidence affected the north-west of Europe, and modern Ireland may have had extensions into warmer regions on the west and south, while the area now left to us was almost buried under ice.
In post-Glacial times, a subsidence admitted the sea into the Lagan valley and across the eastern shore in several places; but elevation, in the days of early human occupation, brought these last marine deposits to light, and raised the beaches and shore-terraces some 10 to 20 ft.
At the time of the last great subsidence, in glacial times, an arm of the sea extended across Sweden, submerging a great part of the littoral up to the Gulf of Bothnia, and including the Plies period the this of themnorthe and Baltic were s fg ficiently salt for oysters to flourish.