The quartzite and the ripple marks

The rock through which the tunnel has been cut is a quartzite, a metamorphic, siliceous rock which is very hard and formed by grains of quartz welded together. It originates from sand deposited 230 million years ago in the still-shallow waters of the nascent Alpine ocean, and brought here by the erosion of the surrounding topography. In places, undulations can be seen on the tunnel wall: these are ripple marks, "fossilised" creased created by water flowing over the sand.