Page 156 - SST Class 07

natural or human-made surfaces, such as build- Ancient people carved these petroglyphs into
ings. Stones that have become polished and desert varnish near Canyonlands National Park
faceted due to abrasion by sand particles are in Utah.
called ventifacts. Wind Deposition

As wind blows from different direction, polished The main features deposited by wind are called
flat surfaces create a ventifact. sand dunes. Loess are wind deposits of finer
Desert Varnish sediments.
Exposed rocks in desert areas often develop a Sand Dunes
dark brown or black coating called desert var- Deserts and seashores sometimes have sand
nish. Wind transports clay-sized particles that dunes. Beach dunes are usually made of quartz
chemically react with other substances at high because quartz is what’s left in humid areas as
temperatures. The coating is formed of iron and other minerals weather into clays. Sand dunes
manganese oxides. may be composed of calcium carbonate in
tropical areas. But, in deserts, sand dunes are
composed of a variety of minerals because there
is little weathering.
Sand dunes are usually very uniform in size and
shape. Larger particles are too heavy for the
wind to transport by suspension and smaller
particles can’t be picked up. Particles are
rounded, since rounded grains roll more easily
than angular grains.

Desert Varnish

Terminology

Elevation : the action or fact of raising or being raised to a higher or more important level, state or
position
Hydroelectricity : electricity produced from generators driven by turbines that convert the potential
energy of falling or fast-flowing water into mechanical energy
Accumulates : gather together or acquire an increasing number or quantity
Deflation : the action or process of deflating or being deflated
Suspension : the action of suspending someone or something or the condition of being suspended

Inside Story
Inside Story
g Weathering, water, elevation, sinking and erosion of the soil are constantly shaping the
surface of the earth.
g The place where the river originates is called the source.
g When it rains, some water seeps underground, some of it evaporates and the rest forms
small streams.
g When the river leaves the mountains and enters the plains, the flow is no longer swift.

Social Science-7 156