One of the main themes of the novel is how colonialism corrupts the soul. As well as depriving indigenous people of their land and forcing them to undergo a long and dangerous march across the Western Sahara, it also turns other African natives into accomplices to colonial exploitation. When the Sheikh's Tuareg warriors are massacred by French colonial troops, it's notable that the majority of those troops are Senegalese. This is a clear indication that these soldiers' souls have been corrupted. They're on the side of the colonial authorities instead of their own people.
A related theme is the vulnerability of traditional society to being swept aside by modernity. For quite some time, Lalla tries to put enough the inevitable confrontation with the forces of modernity—as represented by European colonialism—by living in a shanty town with her aunt and uncle. During this time, Lalla retreats into a world in which she develops a close connection with nature, to the dunes and the seabirds. But she cannot escape modernity for long, and so one day she heads off to France in search of work. The implication is that colonialism and its attendant modernity will at some point always force indigenous people to make some kind of compromise that involves diluting their traditional identity.