The distinction between North Africa and Sub-Saharan Africa is historically and ecologically significant because of the general barrier created by the Sahara Desert for much of modern history. The Sahara is the dominant feature of the North African landscape, and stretches across the southern part of the region. The Sahara serves as a geographical boundary between North Africa and sub-Saharan Africa and marks a transition zone from the largely Arab identifying population of North Africa to black Africa of the south. From 3500 BC, following the rapid desertification of the Sahara due to gradual changes in the Earth's orbit, this barrier has partially culturally separated the North from the rest of the continent. The overwhelming majority of the North African population is concentrated along the Mediterranean and Atlantic coastlines and the Nile river, while the Sahara desert is one of the most sparsely populated places on Earth.
The Sahara desert has therefore played an important role in the history of North Africa. As the seafaring civilizations of the Phoenicians, Greeks, Romans, Arabs 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.
North Africa has three main geographic features: the Sahara desert in the south, the Atlas Mountains in the west, and the Nile River and delta in the east. The Atlas Mountains extend across much of northern Algeria, Morocco, and Tunisia. These mountains 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 tallest peaks are in the High Atlas range in south-central Morocco, which has many snowcapped peaks.
South of the Atlas Mountains is the dry and barren expanse of the Sahara desert, which is the largest sand desert in the world. In places the desert is cut by irregular watercourses called wadis—streams that flow only after rainfalls but are usually dry. The Sahara’s major landforms include ergs, large seas of sand that sometimes form into huge dunes; the hammada, a level rocky plateau without soil or sand; and the reg, a level plain of gravel or small stones. The Sahara covers the southern part of Algeria, Morocco and Tunisia, and most of Libya. Only two regions of Libya are outside the desert: Tripolitania in the northwest and Cyrenaica in the northeast. Most of Egypt is also desert, with the exception of the Nile River and the irrigated land along its banks. The Nile Valley forms a narrow fertile thread that runs along the length of the country.