The wind whipped across my face as I stood on the edge of a vast plateau in remote northern Portugal. Lavender-blue haze stretched across the horizon, and I could just make out the outline of a steep waterfall as it cascaded down the mountain below. Delicate yellow flowers created swathes of colour in the otherwise murky landscape. Looking down on the mountains spread out before me like an undulating carpet, I felt as if I was standing at the edge of the world, and I could see why these were called the Montanhas Mágicas (Magic Mountains).
I was in the Arouca Unesco Global Geopark, an open-air geological museum that has long been an attraction for the scientific community due to the sheer number of geological marvels it contains: 41 separate geosites in total. These include some of the world’s biggest trilobite fossils – arthropods that lived more than 500 million years ago – as well as geological fault lines and otherworldly rock formations sculpted by water over millions of years.
But I had not come to see any of these – admittedly interesting – natural exhibits. I had come for something even more fascinating: one of the world’s most baffling geological phenomena.
Pulling myself away from the mountain view, I continued towards my destination: the tiny village of Castanheira. Nestled into the side of the Freita Mountain (one of the Magic Mountains located within the geopark), the village is home to the Pedras Parideiras, or “Birthing Stones” in English – 300-million-year-old rocks that seem to “give birth” to smaller “baby” stones. They are unlike any other rock on the planet.
The 300-million-year-old Pedras Parideiras, or “Birthing Stones”, are unlike any other rock on the planet (Credit: Biosphoto/Alamy)
It was the height of summer, but as I drove nearer, a low-hanging mist shrouded the village and the sky became dark. Soon, I could no longer see the mountains that I knew surrounded me. As I stepped out of the car, I felt a chill in the air. I heard someone calling out through the fog and, moments later, the clang of bells echoed throughout the mountains and a herd of cattle appeared, traipsing back to the village from out on the pasture.
I hurried across to Castanheira’s small interpretation centre, which was built around this mystifying outcrop of granite and its “babies” in 2012 to help visitors better understand what they were looking at. Large rocks filled the covered forecourt, each one dotted with small black circles as if it were wearing a polka dot dress. They looked like nothing I’d ever seen before.
Inside were more of these rocks. Looking more closely, I could see that the black dots were not just flat markings; they were three-dimensional stones sitting within the larger rock.
The big granite ‘mother’ rocks seem to push out these ‘baby’ nodules
“We estimate that both these rocks and the smaller stones inside them were created around 320 [to] 310 million years ago,” said Alexandra Paz, a geologist at the Arouca Geopark.
“The big granite ‘mother’ rocks seem to push out these ‘baby’ nodules made of biotite [a common rock-forming mineral] inside them. These are the smaller black stones you can see,” Paz explained, pointing out the nodules, “hence the name ‘Pedras Parideiras’.”
Paz went behind a desk and came back holding a shiny black stone in the palm of her hand. “This is one of the biotite stones that has completely detached, becoming an entirely new stone,” she said, handing it to me.
Castanheira’s interpretation centre was built around the granite outcrop to help visitors understand the phenomenon (Credit: Dan Convey)
The black stone had a circular appearance when inside the mother rock, but now, holding one in my hand I could see that it was actually oval-shaped and slightly flattened, like a giant almond.
Paz explained that these nodules are more resistant to the weathering agents than the larger granite rock, and after being battered by the winds and the rain, contracting and expanding with the heat in the summer and frost in the winter, they are finally forced out. At some point in the distant future, all the small stones will be pushed out and the mother rock will become just like any other slab of granite. And while thousands of these baby stones can be seen near the surface of the rocks, no one knows exactly how many more are located deep down inside the mother stone, waiting to be “born” hundreds of years from now.
Through the information and a 3D video in the centre, I learned how incredibly rare this phenomenon is. While orbicular granite (granite rock with spherical shapes inside), like the Pedras Parideiras, is found in a handful of countries, it is only in Arouca where the rock seems to “give birth” to these unique biconvex-shaped discs.
Located in rural northern Portugal, Arouca Geopark is a massive, open-air geological museum (Credit: LuisPinaPhotography/Getty Images)
That’s because of the exact conditions that arose to create them in the first place. The geopark is located on top of a fault system, and at the time the Pedras Parideiras were created more than 300 million years ago, the landscape here was volatile. Tectonic plates crashed into each other, forcing the Earth upwards in the creation of the mighty Magic Mountains. Earthquakes caused fissures in the land, and built-up pressure caused lava to spew to the surface and magma to flow deep underground.
While geologists still don’t completely understand why these smaller baby nodules are produced,
Artur Agostinho de Abreu e Sá, scientific coordinator at the Arouca Geopark and associate professor of geodynamics at the University of Trás-os-Montes and Alto Douro, said that thephenomenon is associated with the chemical composition of magma and its interaction with the rock, as well as the tectonic movement that existed when it formed.
Current theories note that either the molten rock encased these smaller stones before it eventually hardened eight million years later; or that the pressure – “similar to that of shaking a closed Champagne bottle,” Sá said – caused small bubble-like fractures in the rock, which became the baby stones.
He added that these baby nodules are unique because they were slightly flattened due to pressure from Earth’s tectonic plates. “When looking at an outcrop of the Pedras Parideiras, it is as if we are admiring a kind of photograph that tells the story about the history of rock,” Sá said. “This is because during the crystallisation process of the rock, it preserved the direction of the tectonic forces that were in play when it was created.”
The geopark is located on top of a fault system, and the ancient, volatile landscape created a wealth of natural wonders (Credit: Cro Magnon/Alamy)
To get a better look at these geological marvels, I headed outside to the wooden walkway that runs along the top of Freita Mountain. Looking down onto the main outcrop of Birthing Stones, I could see just how far the Pedras Parideiras stretched: huge slabs of granite mother rocks were spread over an area the size of 10 football pitches. Even though the mist still hung low in the air, I could see the black shine on the baby stones.
Suddenly, a hunched old woman in a black cloak approached me through the fog. Half concealing her hands behind the folds of material, she opened them and discreetly showed me one of the prized black nodules. “You buy,” she whispered. “Bring baby,” she added, as she moved her hands in a circular motion over her stomach.
I’d heard there were local myths surrounding the stones, but I remembered Paz’s warning not to buy or take any home with me as they are collected by the park and used for scientific research into the origins of the area.
According to urban myth, if you sleep with one of the baby rocks under your pillow, it’ll help you conceive a child.
I thanked the woman and shook my head, continuing up the walkway, while she slunk back into the mist.
Later, I asked Sá for more information on the legends surrounding these rocks. He explained that many locals believe that the Pedras Parideiras have the power to help couples with fertility issues. According to urban myth, if you sleep with one of the baby rocks under your pillow, it’ll help you conceive a child.
Local legends say that the stones have the power to help couples with fertility issues (Credit: Sérgio Nogueira/Alamy)
The theory spread so widely that, by the 1990s, mother rocks in Castanheira had become almost completely devoid of baby nodules on the surface level, as people from across Portugal and even abroad travelled here in the hope of obtaining a much-desired “fertility stone”.
“While there obviously isn’t any scientific proof that these stones help couples conceive, it’s curious to note several success stories from those who have obtained these nodules and placed it under their pillows at night,” Sá told me, adding that he was once contacted by a Brazilian woman who reportedly became pregnant after taking a stone from Arouca.
In fact, the Geopark currently has an experimental programme to “rent a Pedra Parideira” for a defined period to couples wanting a baby (interested couples can contact the Geopark to enquire). Although the main reason behind it is to stop people stealing the stones, its introduction has perpetuated the fertility myth.
As I turned to leave the Magic Mountains, the image of the woman in the black cloak came back to me. Whether or not the stories are true, it seems that the mysterious power of the stones continues to live on in this remote region of Portugal.