http://knowledge.allianz.com/climate/impacts/?621/green-sahara-how-climate-change-transformed-the-desert
Climate change was the reason why the Desert today is more made up of sand then having any form of grasslands. The Earth's change in orbit around the sun helped cause this during the past 20,000 years roughly. The Earth's orbit around the sun and climate shifts such as areas becoming warmer and rain being shifted south paved the way of how The Sahara Desert looks today.
The Sahara Desert now takes up 3,500,000 square miles of all of Northern Africa. In comparison think of the distance from Los Angeles to New York City! Below is a conformal map and satellite view courtesy of NASA.
http://www.godsgeography.com/africa/africa1.html
The Sahara desert is made up of numerous of landforms but
here are two that take up a huge amount of The Sahara Desert. The first being
Ergs in which in other words are "Sand Seas" that constantly shift
and are fossilized. They cover 20% of Sahara and can rise up to 1,000 feet in
height.
http://www.britannica.com/EBchecked/topic/191273/erg
Second, are the
Plateau Mountains of the Sahara Desert in which look to me more like Intrusive
volcanic rock, Crypto volcanic rock to be precise. The Plateaus seem to have
more of a form of faulting landform than folding due to the different shapes of
rock. The sand and mountains seem to be more apart than connected to form a
folding type of land.
