The Living Cave Paste
Deep inside the limestone caves of the Garraf Massif, the walls are coated in a peculiar white, creamy substance known as moonmilk. When wet, it has a soft texture often compared to cream cheese; when dry, it becomes a crumbly powder. For centuries, its origin was a mystery, with some even speculating it was petrified moonlight. Modern analysis shows it is primarily composed of fine crystals of carbonate minerals, usually calcite. While moonmilk exists in caves worldwide, the deposits in Garraf are notable for the specific biochemical process that creates them.
The formation is not a simple geological process. It is the direct result of microbial life. The actors are chemolithotrophic bacteria, organisms that derive their energy from chemical reactions with inorganic materials, not from sunlight or organic matter. In the Garraf caves, these bacteria perform a specific reaction: they oxidize manganese. This chemical process releases the energy the bacteria need to live. As a waste product of their metabolism, they precipitate calcite, effectively building a mineral coating around themselves. Over generations, these microscopic mineral deposits accumulate to form the thick, paste-like layers of moonmilk seen on the cave walls.