The geology of Brunei is primarily made up of Cenozoic rocks overlying deeper rock units on the island of Borneo. Rocks from the Oligocene through the Holocene are up to 15 kilometers thick.(Main article: Geological history of Borneo) The oldest rocks are part of the Meligan Formation and Temburong Formation in the east and are the deformed remnants of floodplain sediments. An unconformity separates the older rocks from younger Middle Miocene and Pliocene rocks. Among Neogene sediments, geologists distinguish the Setap, Belait, Miri, Seria and Liang Formations. The Setap Formation is marine shale, while the Belait Formation includes thick sandstones and some reef limestone. Both the Miri and Seria Formations include sandstones and shales and the Liang Formation preserves brackish water sediments and cobbles, together with gastropod and crustacean fossils. Faults that developed in these thick sedimentary rocks help to trap oil and gas. (en)