Written by Claus Møller (cmoller@stofanet.dk)
Geologically The Mønsted quarry began about 260 million years ago in the geological period called Perm. Denmark was at that time situated some 10 degrees north from equator. The area was covered by a shallow ocean which flooded and dried up several times, and in that process deposited a thick layer (up till 2000 meter) of chlorides and sulfates including NaCl and CaSO4·2H2O (salt and gypsum).
Over time the salt layers got buried under as much as 5 kilometers of marine deposits. Because of the low density of salt (2.2 g/cm3) versus that of the overlying sediments (2.5 g/cm3) and the high pressure of the overlaying sediments, the salt became plastic allowing it to break upwards and towards the earth surface where faults and weak spots allowed it to do so. These bend ups towards the surface, is known as a salt diapirs. The bending of the layers combined with ice age erosion has exposed layers usually buried much deeper below the surface.
The mining at Mønsted started about 6000 years ago during the late stone age. The desired material was not the limestone itself but the layers of flint with in. The reason for that was that the flint in the limestone has not been destroyed by frost as most flint at the surface, therefore it is much better for making tools. The flint became high in demand and together with amber; it became the first goods to export to foreign places. Danish flint has been found as far away as northern Norway. Later when humans in 11-12 century started to build churches on a larger scale they realized that the limestone from Mønsted was an ideal building material and easy accessible resulting in large scale mining and many churches in Denmark has limestone from Mønsted in their walls.
The limestone consists of small calcalcerous algae (Fig. 3) and bryozoans together with small macro fossils and make a rock type that can be so hard it is difficult to break even with a hammer.
The algae lived in a warm shallow ocean that covered all of Denmark and its surroundings 64 million years ago in a geological period called Danien
When the algae died they fell to the bottom of the ocean and slowly build the layer of limestone that make the Mønsted limestone mine.
Figure 3: A coccosphere made of coccoliths fused together These very small algae are the basic building blocks of limestone together with Diatoms and other small nanoplankton
The Golden Hour 