A large part of the substratum of Saint Honorat is formed of dolomite (a sedimentary carbonate rock composed of calcium magnesium carbonate) dating from the Early Jurassic period. These rocks form parallel layers laid out in approximately horizontal slabs, but are often irregular and fractured.

These rocks are extremely hard and impermeable providing effective resistance to the onslaught of the waves. Although more or less waterproof, the rock contains many fissures and is interspersed with numerous more or less hollow layers of marl.

They are thus completely impermeable only in certain areas, in basins formed from large horizontal slabs.

Most of the dolomite dates from the Hettangian Age. This is characterised by its ash-grey colour, its arrangement in large beds, often in parallelepiped form, and by the reseda-green clay intercalations.

On the southern side, there are also grey limestone rocks with facies of large dolomites dating from the Bathonian age as well as light grey to reddish limestone rock from the Bajocian age.

The tectonics of Saint Honorat are also peculiar to the island – the island has a ridged surface similar to a that of “corrugated iron”, so that if one were to “flatten out” the surface of the island, it would have a width three times than that of the neighbouring island Sainte Marguerite, although in reality it is only half as wide.

The island St Honorat is crossed from west to east by four anticlines, the dip is never less than 75°, which results in a very tight fold.

The fact that the axes of the anticlines have significantly different dips according to their orientation on the island makes the structure even more complicated. In 1964 the geologist PALAUSI made the observation that the island is of “exceptional tectonic trituration”.

He hypothesises that the limestone covering detached from the liasic mantle in the north was carried by alpine movement, sliding south over the solid mass of California to butt up against a resistant mass, causing significant folds on St Honorat.