mergelgrottenMarl is a soft sedimentary rock made ​​of clay and finely divided lime.
Marl is created by sunk and compacted small animals with a calcareous skeleton.
Blocks are cut out to build walls. (Churches) It is used for liming fields and burned to make cement.

In the Dutch - South Limburg St. Peter and the Belgian- South Limburg municipalities Riemst, Kanne and Vroenhoven are mines, ”marl caves” where the stones were quarried as a building material. Wet limestone is freezing.

The "Limburg marl 'is actually limestone containing + 70% lime and used as building material.
 Marl contains more clay (ea, phosphor, ...) and was for centuries mined as 'fertilizer' for farmland.