Cannonballs in sandstone

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 origin of the cannonballs relates to the sand sedimentation and to further cementation process to become a consolidated rock in a delta or a pre-delta marine environment. At the sedimentation time, one living organism as one polychaete worm or one bivalve, or just the hole that it left, was buried into the sand at the point where nowadays is the centre of the ball, developing one gradient of conditions -oxygen flow, red-ox potential, dissolution or precipitation of calcium carbonate, pH… As a result of the regular gradient around, the cementation made one clear cut ball. The circumstances and the details are lost. Therefore, to some extent the cannonballs belong to the category of ichnites, these are the traces of the activity of living organisms on the sedimentary rocks.

The cannonballs may be entirely released by erosion and left on the ground, ready to roll. In different geological formations in the world, cannonballs can be larger and more spectacular. 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 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).

[photos Jordi Badia]