In the Freita mountain range in northern Portugal, close to a village called Castanheira, is a huge block of granite that periodically ejects small pebble-sized stones. This rare geological phenomenon is locally known as Pedras Parideiras, which translates into English as “the rock that gives birth.”

The “mother-rock” is a granitic outcrop measuring roughly 1,000 meters by 600 meters. The rock’s surface is incrusted with small nodules shaped like biconvex discs that are between 2 and 12 cm. Due to thermal weathering or erosion, these nodules become detached from the mother stone, leaving dark reliefs on the surface. These nodules or “baby stone” are made up of the same mineral elements of granite as the mother stone is, but its outer layer is composed of biotite—a kind of mica that has very little mechanical resistance. Rain or dew water seeps into the cracks in the mica, and when winter comes it freezes. As water expands when frozen, the ice acts like a wedge that’s driven deeper and deeper into the biotite each winter until the nodules are forced loose from the granite rock. It takes many hundred winters for this to happen.

Photo credit: Cláudio Franco/Flickr

For the local population, Pedras Parideiras symbolizes fertility. They believe that a women wanting to get pregnant can increase her chances by placing one of the small rocks under the sleeping pillow.

Taking these rocks away from the site—which is now part of the Arouca Geopark and is recognized by UNESCO for its geological importance—is prohibited, but despite warnings visitors still take samples with them.

A similar phenomenon is said to occur near Saint Petersburg in Russia, but I wasn’t able to find any information about this place.

Photo credit: Cssantos/Wikimedia

Photo credit: Cssantos/Wikimedia

Photo credit: Cssantos/Wikimedia

Photo credit: zwigmar/Flickr

Not too long ago, there was a story in the media about an “egg laying cliff” in China. Like Pedras Parideiras, there is a cliff that expels large egg-shaped stones every 30 years or so, according to local residents. The “eggs”, believed to be concretions of some sort, are much bigger and heavier than the Portuguese “baby-stones”—they can vary between 30 to 60 centimeters in diameter and weight as much as 300 kg. Scientists have a few theories but the exact origin of these stone eggs is still not known.

Similar to the Portuguese stones, the locals believe these rocks bring good luck and help pregnant women give birth to baby boys, so families always take them home whenever new eggs are found. There are about 70 eggs still attached to the rock face ready to pop out in a few decades.

Sources: Earth Story / amateriadotempo.blogspot.com / Metro