The photograph you see here is the "Cascade Pétrifiante de St Pierre de Livron". The river that feeds the waterfall emerges from a cave approximately a kilometre upstream. The petrification on the waterfall and more noticeably in the foreground on the riverbed itself is the deposit of calcium carbonate or calcite and it is this process of petrification that requires a little scientific explanation.
When rain falls it absorbs carbon dioxide from the air and when it passes through the ground it also absorbs carbon dioxide from organic matter and this forms a weak carbonic acid. This carbonic acid dissolves calcium carbonate, which is the main composition of limestone that is found in this region.
The dissolution of the limestone by the water as it passes underground makes the water supersaturated with carbon dioxide. When the river emerges (from the cave) the excess carbon dioxide in the water needs to reach an equilibrium with air and the carbon dioxide is off-gassed in the same way as a fizzy drink which leaves the precipitate of calcium carbonate behind.
So, back to the picture of the waterfall and rock pools. The precipitate of calcium carbonate will deposit itself in the same way as hard water in kettle (lime-scale). At a waterfall the deposition of calcium carbonate over time creates a flowstone of calcite over the original rock, unfortunately not so visible here, but it will be in summer when the river is not flowing. The rock pools in the foreground are rimstone dams or gours and these are formed when the water tumbling over an edge leaves deposits of calcite.
Nev
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