Earth Science: Origins and Geology – Deposition
The surface of the Earth undergoes constant change. In this never-ending process, forces created by wind, water, or glacial ice erode the land in one place and deposit it elsewhere due to the pull of gravity.
Rivers transport loose particles and debris, and new land is created where these fragments are deposited. At river bends, material is carried away from so- called undercut slopes, while sediment creates sandbanks at slip-off slopes. In the underflow, coarse material gathers in the form of stream banks that continue onward. If the speed of the flow decreases, erosion is no longer possible and large fields of rubble build up. Finer material is often sedimented far from the mouth of the river. In some cases, sediment creates flat river deltas.
At the coast, erosion rubble is transported either by the surf or waves that contact the shore at an angle. The material deposits subsequently cause the shoreline to shift. If this debris aggregates in the ocean, it becomes a spit, which is a narrow ridge of sand. As the spit extends, it can develop into a bar. A bay that is cut off by this formation can become a backwater or lagoon; however, sand dunes are the most typical deposition landform in flat coastal areas.
The wind collects fine material from dry areas, transports it—sometimes across long stretches-and deposits it. Dust from the Sahara can reach central Europe in this way, and occasionally even the Brazilian rain forest. For most grains of sand, however, the trip ends much sooner. As soon as they encounter small obstacles, such as tufts of grass, they are de- posited and become dunes. The wind also transports fine dust. Loess, a relic of the ice age found in central Europe and North America, is often deposited in vast layers. In China, loess drifts coming from dry, high-altitude areas still occur today.
Most of the material moved by glaciers is deposited gradually as the glacier melts. The characteristic rubble formations formed by the melting glacier are referred to as medial, lateral, internal, or ground moraines depending on where the material was located within or along the surface of the glacier. Deposits at the glacier tongue are known as end moraines.
LAND RECLAMATION IN THE NETHERLANDS
In the Netherlands, coastal protection has a long tradition, especially since more than one-fourth of the country lies below sea level. Dikes were built to separate the coastal shallows from the North Sea. The land was drained with a system of canals and pumps driven by wind power, creating the productive polder landscape.
One of the largest projects was draining the Zuider- zee. This bay was separated from the sea in 1932 by an enclosing dam and drained to produce about 637 square miles (1,650 km 2) of dry land. The remaining expanse of water became the Ijsselmeer.