Limestone

Limestone

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.

On top of the rock cap there is a 100 meter thick layer of chalk from the youngest Cretaceous and above that 75 metres of limestone from the Danien age, the oldest age of the Tertiary. The surface of the Danien limestone would normally be at a depth of 4-500 metres but the salt horst has pushed it up to the surface of the earth.In this Danien limestone the Caves of Mønsted were dug out.

The Danien limestone consists of the shells of one-celled algae which 60 million years ago were floating in the subtropical sea. When the organisms died their shells sank to the bottom of the sea building a thick layer of calcareous mud. As the layer of mud grew thicker the pressure built up pressing the water through the layer of mud.In this way the limestone originates as a compact and solid rock.In the limestone you will find few larger fossils. The reason is probably that the depositing of the algae happened so quickly that a thick layer of mud was built up which offered no good living conditions to the larger organisms living on the sea floor.

If you walk the tunnel leading East from the head entrance you will after  25 and 40 metres see fault lines in the limestone sloping 45° towards East. The fault lines are easy to find because they are filled with flint crossing the layered flint. Along those fault lines a displacement has taken place, the limestone east of the fault line having sunk 12 metres compared with limestone at the western side.

These movements were probably caused by the climate change at the end of  the Ice Age.During the Ice Age the water was periodically frozen not being able to dissolve the salt as the salt horst moved upwards. In the warmer climate after the Ice Age the ground water again started to dissolve the upper part of the salt horst causing the landscape to sink. The faults occurred when the limestone filled up the cavities created by the dissolved salt that was carried away by the ground water.