Salt in underground
The limestone of the Caves of Mønsted dates back to the same period as the limestone of Bulbjerg and the upper parts of Stevns Klint. That the limestone of Mønsted reaches the surface is due to a deep-seated salt dome that lifts the limestone to the surface.
It all began 250 million years ago as the present central and northern parts of Jutland were covered by sea.The climate was warm and dry – and on the sea floor a salt layer with a thickness of several kilometres was deposited. In the following period a thick layer of clay and sand from the Scandinavian rocks and later chalk and limestone were deposited. The salt is under enormous pressure. As the specific gravity of salt is about 10% lower as the gravity of the layers above, the salt and the heavier sediments above will tend to change places.The salt structures at several places in the underground of Northern Jutland move upwards pushing aside the layers above. These movements of the salt are probably still ongoing.
The existing form and thickness of the salt layer vary and and at some places there is a salt horst with a width of severeal kilometers reaching the surface of the earth. This is the case in the underground of Mønsted where the salt surface stops at a depth of 300 metres. The salt stops at this depth because the movements are so slow that the ground water dissolves the salt before it is getting nearer to the surface of the earth.
On top of the salt horst of Mønsted there is a 100 metre thick cap rock consisting of the indissuloble salt minerals, a substantial part of these being gypsum. The thickness of the cap rock shows that through the years enormous amounts of salt have been dissolved.