Geologically speaking one can easily severn the left side and
the right side of the Trebbia Valley in respect to its main watercourse.
To the left of the Trebbia River only one sedimentary rocky
formation can be observed, called "the Mount Antola limestone ".
Such formation appears in a vast part of the middle oriental Liguria, from the
Polcevera Valley to the provinces of Alexandria and Piacenza northwards, and
eastwards to Chiavari.
The name "limestone" is given to rocks formed by calcium carbonate
(CaCO3) precipitations; nevertheless, the rock also
origins from the deposition of small fragments of preexisting
rocks, such as sand and clay.
In such case speaking about marly limestone is more precise.
Millions of years ago, probably when Liguria started assuming
its actual structure, such material, located on the edge
of the continental slope, "slipped" into
the oceanic bed, by a process known either as "pulp-ore current " or "flysch."
Overall this rocky formation is well stratified, where the
calcareous, sandy or clayey component prevails depending
on its layers. The most calcareous
layers show a clear grey colour; the ones more arenaceous, where the sand prevails,
appear yellowish or clear brown, and they are rough to the touch; the clayey
ones, which are the thinnest ones, are dark grey or brown. Arenaceous and
calcareous layers are more resistant to erosion, instead clayey
ones are very friable.
Between one layer and another enigmatic fossils called "elmintoid" can
“Elmintoidi” originate from animals of which we know nothing about
as these animals did not possess any fossilized part,
neither shell nor skeleton.
Even more interesting is the geologic situation of the slopes situated on the
right side of the Trebbia River.
One should imagine an ocean bed, nowadays not existing anymore, situated between
Africa and Europe, where a remarkable volcanic activity spread basaltic lava
across the oceanic crust and mantle, formed by rocks called peridotites.
While subsequently the biggest part of the oceanic floor
disappeared, swallowed by the two continents, some parts
of the ancient ocean were pushed upwards, and eventually
they emerged outcroping on the the Alpine and Appennine chains
: the "ophiolites" form
this way , rocks so called because of their typical dark
Such magmatic rocks have changed and transformed later ,
giving birth to serpentinites, basalts not much metamorphic,
ophiolitic breccias: one can observe them above all in Roccabruna’s area, Pietranera, Mount Castello del Fante and
Nearby sedimentary rocks prevail, due to deposit of different materials: they
could have sufferred transformation processes and metamorphism.
Also among sedimentary rocks there are the sliceous ones (jaspers), calcareous
ones (“calpionelle” limestones), the ones due to ooze deposition
(“arginiti”, “argilloscisti”) or
Jaspers, that outcrop on the surface along with ophiolites, are thin layers by
reddish colour, sometimes grey or greenish .
“Calpionelle” limestones which the name of the Pietre Bianche
(White Stones) resort is related to, are typically clear.
“Arginiti” and “argilloscisti” often alternate with sandstones:
while the first ones, by dark grey colour, are easy to wear away, sandstones
are clearer and more compact and resistant to the erosion.
Important important sandstones are found in Casanova.
(Article based on the publication "Naturalistic Itineraries" by
the Comunità Montana Alta Val Trebbia – High
Trebbia Valley Mountainous Community).