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.
On the surface, the rock is only visible in certain places. It is often covered with a reddish clay silt of about 10–15cm thick. This silt is capable both of absorbing and losing a lot of water. In dry periods, a crust forms in certain places, retracts, hardens and cracks. When it rains, the clay swells up and makes a sticky muddy substance which provides a surface barrier against significant water penetration.
In this way we see that the soil and substratum work together to retain only a part of the annual rainfall.
In addition, the monks’ agricultural methods are chosen to further improve the quality of the soil.
The island has a classical Mediterranean climate: hot and temperate (due to the latitude, altitude and influence of the sea) with long, hot, dry summers; rain and sometimes violent downpours in spring and especially in autumn. Frosts are rare, and the island benefits from an exceptional luminosity due to the long hours of sunshine; at night-time the air is laden with humidity.
The island of Saint Honorat also benefits from special climatic conditions as a direct result of its southerly position and the uninterrupted belt of wooded hills which form a protective screen.
As a result of the unspoilt natural environment, Saint Honorat is home to a wide variety of Mediterranean vegetation, ranging from plants typical of seaside locations, to trees and plants traditionally found in wooded areas: luxuriant plants, pines, cypresses, eucalyptus, palms, olive-trees, oaks…
There are also certain plants that grow here outside of their natural environment, such as the tree heath (Erica arborea), or the arbutus.
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