The gypsum outcrops

Rocks formed when the Mediterranean dried out, six million years ago. Soft enough to dig a house into.

The story

How it forms

Gypsum forms as sea water evaporates, bringing about sedimentation: when the shallow water of an enclosed sea evaporates, the dissolved salts grow concentrated and settle on the sea floor. The gypsum of this area formed about six to five million years ago, at a time when the sill of Gibraltar cut the Mediterranean off from the Atlantic.

Some gypsum rock formed on the spot, and is called autochthonous. The gypsum mass on which the rock-hewn centre of Gessopalena stands, on the other hand, was carried by sea currents and laid down on the clays, like most of the outcrops along the Aventino: in that case the rock is described as allochthonous.

Selenite crystals

Gypsum is a salt known as calcium sulphate dihydrate: a rock made of calcium, sulphur, oxygen and two molecules of water. It often consists of large crystals of a single mineral, which takes the name of selenite. These crystals are either wholly transparent or coloured from grey to black or yellow by impurities. Selenite, which can take the shape of a spearhead or a swallow's tail, reflects sunlight: it is for this reason that the people of Gessopalena call their village “Pretalucente”.

On the Mohs scale of mineral hardness, gypsum comes second behind talc: both are soft stones and can be scratched with a fingernail. Another rock found along the Aventino, above all at Casoli near Lake Sant'Angelo, is gypsum alabaster, made of very small crystals and pure white in colour.

Where they are

Along the river Aventino the largest outcrops are found in the areas of Taranta Peligna, Lama dei Peligni, Civitella Messer Raimondo, Casoli, Pennapiedimonte and above all Gessopalena. Locally, gypsum is called “prete de jsse” when the pieces are small; for more substantial rock the word “murrecene” is used. Only for the largest outcrops is the word “monte”, mountain, used: Monte pe' la Terra is the mass on which the medieval settlement was built.

Erosion, caves and brigands

Rainwater dissolves gypsum, as it does limestone. The outcrops therefore show the typical karst erosion forms: deep grooves (karren), clearly visible in the Sant'Agata area of Gessopalena; candle forms; dolines, swallow holes and caves.

The gypsum is often heavily fissured and contains cavities and small caves, especially at Gessopalena, formed by collapse or by karst processes. Here, tradition says, brigands and deserters from the Bourbon army took shelter in the years after Italian unification.

A difficult habitat

Even at this latitude, gypsum is a difficult habitat: little water, a strong presence of salts, easy erosion. In the Aventino valley rare plants are found such as Athamanta sicula and Aurinia sinuata, and rarer ones still such as Allium moschatum, Elaeoselinum asclepium, Lavatera punctata and rosemary.