The geology of the Netherlands describes the geological sequence of the Netherlands. Large parts of the Netherlands today are below sea level and have in the past been covered by the sea or flooded at regular intervals. The modern Netherlands formed as a result of the interplay of the four main rivers (Rhine, Meuse, Schelde and IJssel) and the influence of the North Sea and glaciers during ice-ages. The Netherlands is mostly composed of deltaic, coastal and eolian derived sediments during the Pleistocene glacial and interglacial periods. Fairly all of the west Netherlands is composed of the Rhine-Meuse river estuary, but human intervention greatly modified the natural processes at work. Most of the western Netherlands is below sea level due to the human watermanagement, lowering the waterlevel by windmills leading to (mostly peaty) soil-subsidence leading to lowering the waterlevel. In the east of the Netherlands, remains are found of the Saale glaciation, which ended approximately 130,000 years ago. As the continental ice sheet moved in from the north, it pushed moraine forward. The ice sheet halted as it covered the eastern half of the Netherlands. After the ice age ended, the moraine remained in the form a long hill-line. The cities of Arnhem and Nijmegen are built upon these hills. Sedimentary succession Paleozoic Era Rocks from the Carboniferous period (360 - 299 mya) outcrop in the southeastern province of Limburg, e.g. in the Heimans quarry. Younger sediments (Permian in age) are not exposed at surface. The subsurface of the Netherlands (including the Dutch sectors of the North Sea) is geologically far more significant especially with respect to the occurrence of oil and gas resources. The Slochteren reservoir rock in Groningen is an eolian sandstone from the Rotliegend (lower Permian). This reservoir has excellent porosity and permeability characteristics and supplies the vast majority of the Dutch natural gas reserves. The lateral equivalent of this reservoir comes at surface near the German town of Bad Bentheim. The latest Permian deposits in the area of northwestern Europe was characterised by the laying down of thick successions of rocksalt and anhydrite. This Zechstein stage also comprises dolomites and limestones. The salt is mined in the east of the Netherlands (in villages as Boekelo) and forms an excellent seal over the deeper North Sea reservoirs. Mesozoic Era Mesozoic rocks are exposed in the Netherlands in the quarry near Winterswijk (Triassic Muschelkalk limestones). The Jurassic Posidonia