Among the lithological types making up the northern Appennines, an important
part is played by the (occasionally schistose) clays which make up the main
bulk of an enormous nappe (“liguride,” according to Stille) which has
slipped from the Ligurian region towards the Po valley. These clays are
particularly well developed in the mid-mountain region, and can, because of
their plasticity, insinuate into the more recent formations.
Within the argillaceous nappe, frequent nuclei of basic and ultrabasic rocks
(gabbros, diabases, serpentines) are observed, of size ranging from that of
a pebble to masses of many thousands of cubic meters (Monte Penna, Sasso
Tignoso), which appear immersed in the clay mass without having produced in
it the well-known contact phenomena.