geopark

Giants' p ot-holes of Mt. Sumbra
geomorphology

geology

a little story...

Traditional hypothesis of pot-hole formation in stream bed (Panizza)

Anguillaja and Fatonero’s pot-holes have been moulded directly into the rock and their dimensions may vary from a few centimetres until reaching a diameter of 6.6 m and a deepness of 1.6 m.

Quantifying them is not easy, not least because sometimes smaller pot-holes are contained in larger ones.

Considering only the latter, thirty is probably the total number in both streams. Giants’ pot-holes are often formed as a consequence of the abrasive action of rotating pebbles.

Many factors, not least the stream whirling movement and a consistent and homogeneous rock substrate (as marble in Mt. Sumbra’s southern side), contribute to their development.

Anguillaja and Fatonero’s pot-holes seem to follow this traditional model, as presumably demonstrated by the regular signs left by mechanical erosion on the internal borders of a number of these pot-shaped cavities.

In certain cases, their formation might have been favoured at first by chemical leaching (superficial karst phenomena), but then the hydrodynamic action was either prevalent or exclusive.

However, other Authors hypothesised that the pot-holes formed as a consequence of water forcedly flowing through subglacial tunnels, which were probably found under the glaciers formed during the last Glaciation.

Indeed, Sumbra and Fiocca’s southern side is characterised by spread forms of glacial, glaciokarst and cryonival origin, which testify to the presence of glacial masses during Würm and of small ephemeral ice and snow masses during Late-Glacial and Post-Glacial.

Pot-hole wall with evident traces (scallops)
of water flow under pressure

sub-riverbed water flow
along Tùrrite Secca stream

Antonio Stoppani (1824-1891) – Italian geologist and palaeontologist – was here on 13th June 1872 to estimate the value of Val d’Arni marble (later called “the White Valley”).

On that occasion, the famous scientist observed and understood the phenomenon of sub-riverbed water flow along Tùrrite Secca stream.

You can read the original description in Il Bel Paese (“The beautiful Country”), published in 1876. Stoppani wrote: “the riverbed instead of collecting the tributary streams, becomes the roof covering all of them…”.

The Tùrrite Secca water resurfaces from Pollaccia spring, near Isola Santa village, about one kilometre further down, as the geologist tell us in his book.

The karst absorption makes dry the riverbed, but water suddenly floods during the severe thunderstorms.

You can walk safely along this natural “cobble street” on days without rain.

In the 19th century, Tùrrite Secca dry riverbed was the only trail linking Val d’Arni and Castelnuovo di Garfagnana.

On the left: Tùrrite Secca dry riverbed

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