Piana Crixia 'Mushroom'
Near the country hamlet Borgo, along the bank of a meander of Bormida di Spigno, you can see the famous "Mushroom" of Piana Crixia, a unique form of erosion for Liguria and the surrounding areas. Similar forms can be found in some alpine areas, where erosion shaped ancient morainic or river-glacial deposits creating picturesque earth pyramids.
The fifteen-meter high "Mushroom" is formed by a giant boulder of ophiolitic rock supported by a conglomerate column (sedimentary rock made by pebbles of different size joined by thinner material). The boulder protected the underlying conglomerate column by washing out, while the surrounding ground was progressively swept away by erosion.
It seems impossible that the high column of cemented pebbles can bear the weight of the giant stone above it, but we must consider that conglomerate is a solid rock, despite its detritus aspect.
However, the ongoing process of erosion slowly takes away material, and in the future the stone is bound to collapse and the conglomerate column which will be no longer protected will be gradually swept away by the washing out of the rain.
