These rocks called "Mexican hat" are concretions originate in the deep sea.
There are generated magnetic fields which attract the iron oxide particles. These particles are grouped in the form of spheres formed by water currents. They were then covered by sediments. Then came the Andean orogeny. Were exposed by erosion these strange rocks, which being of ferrous metals do not suffer erosion. When erosion destroys the sediment containing the hat, rock falls down. They look great cannonballs.
They are very rare in the world. They are in El Calafate, Argentina
photo by: Ana Pisani