The calcareous tuffs :
a particularity of the St-Benoît spring

The Geological Park’s administrative buildings and museum are settled on a particular rock headland.
These deposit rocks are formed at the St-Benoît spring’s exit and are called calcareous tuffs.

What are the calcareous tuffs ?

The calcareous tuffs are rocks deposed at the exit from a spring or river which is not very deep and has little waterfalls.
The tuffs are light, insulating (for buildings, houses…) and easy to carve.
There exist other sorts of tuffs, like volcanic tuffs, but in this case the genesis is different.

How are the tuffs from the "Musée-Promenade" formed ?

The tuffs from the Geopark are formed at the St-Benoît spring’s exit.
The ground water is charged in minerals which settle when the water spouts out in the open air.

The tuffs’ formation cycle

When rainwater falls, it captures atmospheric carbon dioxide (CO2). While infiltrating in the ground, this water is again charged in CO2, which come from the vegetation’s activity (and decomposition).

Its CO2 concentration makes the water erosive. Then, when the rainwater penetrates the ground in depth it dissolves the calcareous rocks while circulating in the aquifer. Thus the water carries dissolved calcium ions (Ca2+).

When the ground water arrives to the surface, in the geological museum’s park, it has a higher CO2 concentration than the atmosphere. The water releases then its CO2 in gaseous state in the ambient air, in order to find its balance.

The calcium ion content in the water is always proportional to the dissolved CO2 content.
When the CO2 content decreases, the capacity of the water to transport calcium ions also falls.
The minerals are then going to precipitate: the dissolved calcium in the water is transformed in calcium crystals
in the air. Little by little, the rock is bent.

This process is observed by the big waterfall from the St-Benoît spring, where the tuffs are continuously shaped.
The calcium precipitates on the vegetation, mainly mosses, and recovers it. When the plant support dies and disappears,
it leaves empty spaces, which explain the cavernous or alveolated aspect of the rock.

Calcium is settling on the moses.

Tufs after the vegetation's disappearing.                    

Factors supporting the formation of tuffs…

Some vegetation take part to the tuffs’ elaboration.

The presence of plants near a source is essential because it provides refuges where the calcite crystals can settle to form the rock.
  • The  green algae and cyanobacteria 

  • The mosses

  • The microscopic mushrooms

Others factors control the formation of the tuffs

Waters rich in calcium carbonates do not form tuffs inevitably. In order to have this rock built, other factors must be present :

The water run-off plays a fundamental part : if water is agitated and not very deep, the contact between the air and water is more important, which supports the formation of the tuffs.

A cold water will facilitate the formation of the tuffs. Indeed a chemical property of CO2 is to dissolve more easily if water is cold.

Lastly, it seems that the presence of rocks rich in sulphates at the spring's exit, as here the gypsum, plays a determining role in the formation of tuffs.
Indeed, the St-Benoit spring forms tuffs because the water which spout out the ground comes in contact with sulphated rocks : the gypsum, whereas other sources whose water is also rich in carbonates do not form travertines, like the Fontchaude spring, in Barles (near Digne-les-Bains).

Where do we find the tuffs in the "Musée-Promenade" ?

The administrative buildings and the museum are bent on a natural headland formed by old tuffs.

Furthermore, the tuffs have been used to build the walls from the park.

Finally, the big St-Benoît waterfall is an active site of formation of tuffs.