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

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Michael Leach said... April 3, 2014 at 11:51 PM

There is a place in Utah called Mexican Hat. I am wondering if these same types of formations are found near there, or does the name have a different origin for the Utah town?

North said... July 9, 2014 at 10:08 PM

Recently I came across concretions on the Southern English Coast in East Sussex called ironstone, used since the Roman until 18th century for early iron production.