Total thickness of sediment below Kolkata is nearly 7,500 m (24,600 ft) above the crystalline basement; of these the top 350–450 m (1,150–1,480 ft) is Quaternary, followed by 4,500–5,500 m (14,760–18,040 ft) of Tertiary sediments, 500–700 m (1,640–2,300 ft) trap wash of Cretaceous trap and 600–800 m (1,970–2,620 ft) Permian-Carboniferous Gondwana rocks.[64] The quaternary sediments consist of clay, silt and several grades of sand and gravel.