From Wikipedia:
Several thousand years ago, the river bed was blocked by a lava flow from Mount Etna. As the lava was cooled much more quickly by the water than it would have done otherwise, it crystallised in the form of columns. Over the next millennia, the river naturally eroded a channel through these columns, resulting in impressive gorges and ravines such as the "Gole dell'Alcantara".
Spectacular place - not only for geologists.
