<P> The distinction between North Africa and Sub-Saharan Africa is historically and ecologically significant because of the effective barrier created by the Sahara Desert for much of modern history . From 3500 BC, following the abrupt desertification of the Sahara due to gradual changes in the Earth's orbit, this barrier has culturally separated the North from the rest of the continent . As the seafaring civilizations of the Phoenicians, Greeks, Romans, Muslims and others facilitated communication and migration across the Mediterranean Sea, the cultures of North Africa became much more closely tied to Southwestern Asia and Europe than Sub-Saharan Africa . The Islamic influence in the area is also significant, and North Africa is a major part of the Muslim world . </P> <P> Some researchers have postulated that North Africa rather than East Africa served as the exit point for the modern humans who first trekked out of the continent in the Out of Africa migration . </P> <P> The Atlas Mountains extend across much of Morocco, northern Algeria and Tunisia, are part of the fold mountain system that also runs through much of Southern Europe . They recede to the south and east, becoming a steppe landscape before meeting the Sahara desert, which covers more than 75 percent of the region . The sediments of the Sahara overlie an ancient plateau of crystalline rock, some of which is more than four billion years old . </P> <P> Sheltered valleys in the Atlas Mountains, the Nile Valley and Delta, and the Mediterranean coast are the main sources of fertile farming land . A wide variety of valuable crops including cereals, rice and cotton, and woods such as cedar and cork, are grown . Typical Mediterranean crops, such as olives, figs, dates and citrus fruits, also thrive in these areas . The Nile Valley is particularly fertile, and most of the population in Egypt live close to the river . Elsewhere, irrigation is essential to improve crop yields on the desert margins . </P>

What major desert encompasses most of northern africa