At this stop, a roadcut exposes a section of pillow lavas from the Balagne Nappe. These rocks formed on the seafloor of an ancient ocean by submarine volcanism. Therefore, the outcrop offers to study processes associated with this type of volcanism, for example, the interaction of molten rock and cold seawater.
In this outcrop, we found a dark, fine-grained basalt, a volcanic rock. It forms decimetre-sized roundish structures in cross-section. Since these structures resemble pillows, this rock is called a pillow-basalt. The pillows are fractured by radial cracks or veins that are filled by very light-coloured, milky white calcite and some dark green minerals, pumpellyite and prehnite. Basalt itself is such a fine-grained rock that no minerals can be distinguished by the naked eye. The outer rims of the basalt pillows appear even more fine-grained or even glassy. These observations indicate that the lava extruded under water: The outside parts of the outflowing lava came in direct contact with the water, so that it cooled and solidified rapidly on the outside, but could stay molten on the inside. If the lava breaks through its own solid crust again, it forms new tube- or pillow-like shapes, and the process repeats again. In cross-section, these tube-like flows appear as the roundish structures visible in this outcrop. Going further along the road, we found the contact between the pillow lavas to a rock consisting of smaller blocks of brecciated pillows.