The Renon earth pyramids, near Bolzano, are among the highest and best formed in Europe. They are a phenomenon of erosion formed from glacial moraines. When dry, the moraine clay is as hard as stone, when dampened it slips downhill. This builds up steep cliffs, up to 15 meters in height. Wherever the moraine contains larger stones which protect the mud below, the protected areas remain hard forming the pyramid. With each rain a little more is eroded and when the protecting stone falls the pyramid rapidly disappears.