Shale is well developed but is buried deep in the West and under the North Sea. It acts as source rock for oil fields in the West Netherlands Basin. During late Jurassic times the Zuidwal volcano was active in a now submerged part of the Netherlands known as the Wadden Sea. During Cretaceous times, most of the Netherlands was covered by a warm shallow (inland) sea in which thick successions of chalk were deposited. This is in places chert-bearing; chalk is mined near the southern city of Maastricht and the Maastrichtian stage was named after this locality. Here a number of mosasaur-fossils have been excavated as well. The Schoonebeek oil-bearing sandstones are Early Cretaceous in age and underlain by hydrocarbon-rich Lower Cretaceous lacustrine Coevorden Formation which forms the source rock for the east Netherlands oil. Cenozoic Era The Cenozoic is characterised by a further shallowing and erosion of earlier sediments. In places a reasonable thickness of Paleocene and Eocene sediments is reached, but most of the subsurface of the Netherlands lack deposits of this age. The recent Netherlands is formed by Pleistocene and Holocene age sediments as result of (glacio)-fluvial, eolian and marine sedimentation. Eolian dunes characterise the North Sea coast, a horseshoe-shaped moraine forms the Utrecht Hill Ridge (Dutch: Utrechtse Heuvelrug) and the river influence is still visible all over the Netherlands. Tectonics and orogeny Although the Netherlands seems to be a tectonically quiet area, a number of fault activities have been shown in the past. The latest strong earthquake, the 1992 Roermond earthquake (5.4 on Richter scale), had its epicenter close to the Limburg city of Roermond. The south of the Netherlands is topographically higher and is linked to the geology of the Ardennes and the London-Brabant Massif. The Lower Rhine Graben, part of the European Cenozoic Rift System, which has been tectonically active since the Oligocene, affects the southwestern part of the Netherlands. See also Geology of southern North Sea References ; 2007: Geology of the Netherlands, KNAW, External links Geology of the Netherlands Geology map of Europe