From the 18th century, trade, conquests and colonialism spread religious such as Islam in Africa in North Africa. Islam completely replaced traditional religions which often included the worship of ancestors. The regional climate of the northern region, the Sahara's and Sahel area, is a broad band of dry, duchy land that is starved of rain. By contrast in the steamy equatorial forests more than 38mm (1.5 in) of rain falls every day in places. The south experiences the monsoon season between May and October. The landscape varies according to its distance from the Equator, much of the region is rolling hills and valleys with craggy mountains in the north and east. The arid Sahara desert and Sahel cover the extreme north. Farther, south is the vast equatorial basin of the River Congo, surrounded by some unspoiled tropical rain forest. The hot humid basic of the River Congo is Africa's largest remaining region of tropical rain forest. Competing for light, a wide variety of trees grows tall, forming a protective canopy that teams with both, plant and wild life.