The Sahara Desert covers large parts of Africa. The desert is covered with sand dunes or sand seas. The desert also has several deeply dissected mountains and mountain ranges along with many volcanic mountains. Most of the rivers and streams that are found in Sahara are seasonal or intermittent, except the Nile river, which crosses the desert from its origins in central Africa to empty into Mediterranean.
The central part of the Sahara is very dry, with little vegetation. The northern and southern reaches of the desert , along with the highlands, have areas of sparse grasslands and desert shrub, with trees and taller shrubs at places where moisture collects.
“The desert is covered by sand dunes or sand seas”. What is the meaning of sand sea in the paragraph?