Their white color is due by presence of Sulfates (i.e. Sodium) which are released as the rains penetrate into the soil. The following exposure to sun, causes its drift towards the surface and salts are consequently depositating becasue of evaporation, in a cycle which self substain endlessly.
In Tuscany we have further examples of this in Val Orcia and Siena Crete area; in Umbria we find such examples north of Orvieto.
[From: Wikipedia]
