Geology

Brittany is divided into three geological sectors which are the North, the Central and the South Armorican areas. Along the edge of each is a fault line extending from the West to the East. The fault line separating the North and the Central sectors is called the "North Armorican Shear Zone". The other fault line, dividing the Central and the Southern geological areas is named : "the South Armorican Shear Zone"

Near Saint Brieuc, the North Armorican shear zone has resulted into two oblique secondary faults which meet up along the coast. The main one extends from Quessoy, up to Saint Cast. Soka’s primary deposit is located at the edge of this fault, which forms a large mass of country rocks made of granite and black shales.