who presented a detailed description of the geomorphology and geology of the Lower Central Plain, the topography and soils of the plain reflected the fact that it was covered by a shallow marine sea in the Holocene, during which a soft clay was deposited in shallow near shore waters. During the past 3000 years the sea has retreated from the plain, leaving behind a veneer of soft marine clay that now mantles 13,800 km2 of the Plain. The thickness of the soft clay in the city area is generally about 12–16 m, but in few areas it may exceed 20 m.