Fold mountains are caused by the pressure of two plates compressing upwards and downwards to create mountain ranges such as the Himalayas. There are also layers of folded sediments like the Jura or rock such at the Alps. The part which goes down is called a geosyncline while sediments as they are lighter go upwards to form the mountains.
Features trade winds produce
West coast deserts
These mainly occur in the trade wind belt on the western side of continents where the winds are offshore. On shore local winds do blow across these coasts but they rarely, if ever, bring rain because they have to cross cool currents which parallel the coasts in these latitudes. The cool currents cause condensation to take place in the on shore wind which produces mist, fog or light rains. By the time they reach the coasts the winds are dry. E.g.: Sahara desert Africa
These occur in the interior of continents where the winds have already travelled a considerable distance across the land and in doing so, have lost much of their moisture as happens when winds blow over a dry land or even high mountains. The day temperature of these continental areas are very high because they are so far from the moderating influence of the sea. The night temperatures are low because the absence of clouds causes these areas to lose their heat rapidly. Deserts of this type occur mainly in west tropical latitudes but they do extend into the temperate zones. Sometimes these deserts occur in intermontane zones plateau regions. The deserts of Arizona and Nevada in the Rockies are of this type and they are sometimes known as mountain deserts. A desert often occurs because of a combination these factors e.g. the sahara and the Australian desert are partly of the west coast type and partly of the continent type.
Called erg in the Sahara and Koum in Turkestan. This is an undulating plain of sand produced by wind deposition.
Called reg in Algeria and serif in Libya and Egypt. The surface of this desert was covered with boulders and angular pebbles and gravels which have been produced by diurnal temperature changes.
Called hammada in the Sahara. The bare rock surface is formed by deflation which removes all the small loose rock particles. A part of the Sahara desert in Libya has extensive areas of this type of desert.
This is quite a different type of desert to the three just mentioned above in that it develops in semi desert regions mainly as a result of water erosion produced by violent rain storms. The land is broken by extensive gullies and ravines which are separated by steep ridges. Excellent examples of this type of desert occur in the region extending from Alberta to Arizona.
Features produced by wind Erosion
Wind abrasion attacks rock masses and sculptures them into fantastic shapes. Some of these, because of their shape are called rock pedestals.
Rocky mass formed of alternate rock layers of hard and soft rock. The abrasion is greatest at ground level.
Wind abrasion turns a desert a desert area which has a surface layer of hard rock underlain by a layer of soft rock into a ridge and furrow landscape. The ridges are called zeugens which may be as high as 30 meters. Ultimately the are undercut and gradually worn away.
Bands of hard and soft rocks which lie parallel to the prevailing winds in a desert region are turned into another type of ridge and furrow landscape by wind and abrasion. The belts of hard rock stand up as rocky ribs up to 15 meters high and they are fantastic shapes Yardangs are very common in the central Asian desert and in the Atacama desert.
Some depressions produced by wind deflation reach down to water bearing rocks. A swamp or an oasis then develops.
The Qattara depression is 122 meters (400 feet) below sea level. It has salt marshes and the sand excavated from it forms a zone of dunes on the leeside
Fault produced depressions
The formation of a depression may first be caused by faulting. The soft rocks thus exposed are excavated by wind action.
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