You are in one of the most emblematic geosites of the Arouca Geopark. Here, you’ll be surprised by a unique phenomenon in the world: Pedras Parideiras. Geologically designated as “nodular granite of Castanheira”, the outcrop of Pedras Parideiras extends throughout an area of 1000 meters by 600 meters. The granite presents a medium grain, with two micas and a rare amount of biotite nodules, with a discoid and biconvex shape, markedly aligned. Quartz, orthoclase, albite, biotite and muscovite are its essential minerals. Zircon, apatite, rutile, titanite-leucoesfena, clorite, ficrolite and sillimannite are the accessory minerals. By the action of the erosion, the nodules are released and accumulated in the soil, leaving in the granite a cavity whose walls are covered in a biotite layer.
This is why the inhabitants of the Castanheira village call this “a rock that delivers stone”, commonly known as “Pedras Parideiras”. These nodules present dimensions that vary between 1 to 12 centimeters diameter and, although they are externally composed by biotite only, the nucleus is quartz-feldspathic. They have a very flat shape, and the body is granitic, with a well determined orientation. As they are more resistant to the weathering agents than the fitting rock, they stand out more easily from the “mother rock”. The most recent dates point to K-Ar ages (obtained in muscovites and biotites of the nodules) between 320 and 310 million years old. Thus, you are in a unique geosite in the world.
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