As the drainage by cuniculi removed the moisture in the subsoil, so the drainage of the lakes by emissaria, outlet channels at a low level, prevented the permeable strata below the tufa from becoming impregnated with moisture which they would otherwise have derived from the lakes of the Alban Hills.
It rests on a subsoil of clay and marl.
Here the prevailing soils are grey and sandy with a subsoil of loam, but they are less fertile than those of the Lime Sink or Cotton Belts.
The subsoil is either of clay, of limestone, or mixed sand and clay, gravel, or of a peculiar kind of pudding stone which exists in a hard and a soft variety.
It is suitable for stiffish soils where the subsoil is fairly open, but is less successful in sand.