Flora and fauna previously widespread retreat northwards to the Atlas Mountains , southwards into West Africa , or eastwards into the Nile Valley and thence either south-east to the Ethiopian Highlands and Kenya or north-east across the Sinai into Asia . Flora and fauna previously widespread as a result retreat northwards to the Atlas Mountains , southwards into West Africa , or eastwards into the Nile Valley and thence either south-east to the Ethiopian Highlands and Kenya or north-east across the Sinai into Asia .