Among the lithological types making up the northern Appennines, an importantpart is played by the (occasionally schistose) clays which make up the mainbulk of an enormous nappe (“liguride,” according to Stille) which hasslipped from the Ligurian region towards the Po valley. These clays areparticularly well developed in the mid-mountain region, and can, because oftheir 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 ofa pebble to masses of many thousands of cubic meters (Monte Penna, SassoTignoso), which appear immersed in the clay mass without having produced init the well-known contact phenomena.