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In the recent geological past (1 million years ago or less) the Mèrcure basin was occupied by a lake, at the bottom of which sediments brought by streams from the surrounding mountains were deposited. Today the lake is no longer there but the sediments remain, thick banks of soft conglomerate that appear between 300 and 500 m above sea level. The conglomerate is generally an excellent rock for gorges, because it is easily eroded by torrents. However, this conglomerate is so young and tender that the walls collapse and the gorges soon become large canyons occupied by luxuriant vegetation, generally not interesting for canyoning.
At Vallone Gavaretto the collapses have obstructed the canyon in many points, forcing the stream to carve a path under huge conglomerate boulders. So the descent of Gavaretto features numerous underground passages which make it unusual and interesting.
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All the valleys carved in this conglomerate formation have point where the stream flows under collapsed boulders, but usually they are easily bypassable or too short. None of them has a concentration of tunnels comparable to Gavaretto.
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