How does the phenomenon of birthing stones occur?
The growth of calving stones is due to thermal oscillations and the action of erosion on biotitic nodules. They eventually break free from the parent rock and accumulate in the soil, leaving a cavity in the granite lined with a biotitic layer.
This geological process takes a long time, so it can take about 300 million years for the nodules to be released. These have dimensions that vary between 1 and 12 centimeters in diameter and have a core with quartz and feldspar minerals.
In popular parlance, this phenomenon came to be called birthing stones, as it refers to a stone that “stops” other stones.
Where can you see birthing stones?
The birthing stones can be seen in only two places in the world: in the city of Saint Petersburg, Russia and in Arouca, Portugal.
In Portugal, the calving stones are located in the village of Castanheira, in the heart of the Serra da Freita plateau, divided between the municipality of Arouca and Vale de Cambra. Amidst this idyllic landscape, divided by grazing activities, lies this famous natural and geological heritage, which extends over an area of about 1 km.two.
It was here that the Arouca Geopark, a geosite of extreme international relevance, was installed. The objective of this non-profit private law association is to conserve, promote and enhance its cultural, natural and geological heritage.