Land+forms

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1. A **valley** is a hollow or surface depression of the earth bounded by hills or mountains, a natural trough (ditch) in the earth's surface, that slopes down to a stream, lake or the ocean, formed by water and/or ice erosion. Erosion by rivers is a main valley-forming process. Other processes, such as movement of the earth's crust and glaciers, also have an important part in some cases. 2. A **plateau** is a large highland area of fairly level land separated from surrounding land by steep slopes. Some plateaus, like the plateau of Tibet, lie between mountain ranges. Others are higher than surrounding land. Plateaus are widespread, and together with enclosed basins they cover about 45 percent of the Earth's land surface. Some plateaus, such as the Deccan of India and the Columbia Plateau of the United States, are basaltic and were formed as the result of many lava flows covering hundreds of thousands of square kilometers that built up the land surface. 3. **Hills** are elevations of the earth's surface that have distinct summits (The highest points), but are lower in elevation than mountains. Hills may be formed by a build up of rock debris or sand deposited by glaciers and wind. Hills are also formed by deep erosion of areas that were raised by disturbances in the earth's crust. Erosion forms hills by carrying away all of the soil on a mountain, causing a hill to be left behind. Humans also make hills by digging soil up and dumping it in a giant pile.  Today a large part of **Australia** is arid or semi-arid. Sand dunes are mostly longitudinal (Placed lengthwise), following the dominant wind directions of a high pressure cell. The dunes are mostly fixed now. Stony deserts or gibber plains (covered with small stones or 'gibbers') are areas without a sand cover and occupy a larger area than the dune fields. Salt lakes occur in many low positions, in places following lines of ancient drainage. They are often associated with lunettes [1], dunes formed on the downwind side of lakes. Many important finds of Aboriginal pre-history have been made in lunettes. Despite the prevalence (occurance) of arid conditions today, real aridity seems to be geologically young, with no dunes or salt lakes older than a million years. The past few million years were notable for the Quaternary ice age. There were many glacial and interglacial periods (over 20) during this time, the last glacial about 20,000 years ago. In Tasmania there is evidence of three different glaciations-the last glaciation, one sometime in the Quaternary, and one in the Tertiary. In mainland Australia, there is evidence of only the last glaciation, and the ice then covered only 25 square kilometres, in the vicinity of Mt Kosciuszko. The broad shape of Australia is caused by earth movements, but most of the detail is carved by river erosion. Many of Australia's rivers drain inland, and while they may be eroding their valleys near their highland sources, their lower courses are filling up with alluvium [2] and the rivers often end in salt lakes which are dry for most of the time. Other rivers reach the sea, and have dissected a broad near-coast region into plateaus, hills and valleys. Many of the features of the drainage pattern of Australia have a very long history, and some individual valleys have maintained their position for hundreds of millions of years. The salt lakes of the Yilgarn Plateau in Western Australia are the remnants (bits and pieces) of a drainage pattern that was active before continental drift separated Australia from Antarctica.

[1] A broad, typically crescent-shaped mound (heap) of sandy or loamy (a mixture of moist clay and sand, and often straw, used especially in making bricks) matter that is formed by the wind, especially along the windward side of a lake basin.

[2] Sediment deposited by flowing water, as in a riverbed, flood plain, or delta is called //alluvium//.