The Skopje valley belongs to the Vardar geotectonic region, the subsoil of which is formed of Neogene and Quaternary deposits. The substratum is made of Pliocene deposits including sandstone, marl and various conglomerates. It is covered by a first layer of Quaternary sands and silt, which is between 70 and 90 m deep. The layer is topped by a much smaller layer of clay, sand, silt and gravel, carried by the Vardar river. It is between 1.5 and 5.2 m deep.