Sometimes, the sandstones from the Eocene marine in the district of Bages contain balls in the range of 2-15 cm diameter, from the same rock nature. In Geology, they are known as cannonballs. The outcrop of the sandstone can show them either in positive or convex relief in the so-called noisette shape, or in negative or concave relief in the so-called gruyere shape, if they have been washed out.
The image displays cannonballs in a layer of sandstone in Les Brucardes (Sant Fruitós de Bages). Similar cannonballs are found in the wall of the shelter of Les Brucardes with cave paintings where one diametrically cut cannonball frames an old painting that is understood as the sun. The pictures 2nd and 3rd display cannonballs into sandstone in El Malbalç, near to the industrial area of Bufalvent (Manresa). There are cannonballs in the sandstone of the natural shelter of Els Polvorers (Manresa) as well.
In different geological formations in the world, cannonballs can be larger and more spectacular and more consistent. The Bowling Ball Beach in California really hosts a pile of sandstone cannonballs, same size as the ones in a bowling. In the Iberian Peninsula, the most beautiful ones are inserted in the sandstones of Jaizkibel formation, in the coastal flysch in Gipuzkoa.
The origin of the cannonballs relates to the sand sedimentation and to further cementation process to become a consolidated rock in a continental shelf marine environment. It is likely that cannonballs of varying textures and sizes from different geological formations also have different origins.
We speculate the cannonballs found in the Eocene marine sandstone of the Bages district as the result of Posidonia seagrass debris or Neptune’s balls presence during the sedimentation period on the seabed, like those we find today washed ashore on Mediterranean beaches near underwater meadows of this plant. Pushed by the currents, the hairy balls of Posidonia roll across the sandy seabed trapping sand particles, just as a scouring pad traps dirt. When the Posidonia balls finally settle in the sediment, their organic matter decomposes and disappears; they don’t fossilize. However, their spherical shape made by the grains of sand they incorporated remains. If this understanding is right, cannonballs belong to the category of ichnites, these are the traces of the activity of living organisms on the sedimentary rocks. Cannonballs from the Anthropocene, this is present time, will be identified in the future by their microplastic content, in addition to sand.
[photos Jordi Badia]