Indian Ocean trading network, bringing imports of Chinese pottery and Indian fabrics in exchange for gold, ivory, and slaves. Swahili Kingdoms created a prosperous trade empire, where occupied the territory of modern-day Kenya, Tanzania and Uganda. Swahili cities were important trading ports for trade with the Middle East and Far East. In the interior of Africa, trade was far more limited. Low population densities made profitable commerce difficult. The massive barrier of the Congo rainforests were more imposing than the Sahara, blocking trade through the center of the continent. It was the arrival of the Islamic armies that transformed the economies of much of Africa. Though Islam had comparatively little impact on North Africa where large cities, literacy, and centralized states had been the norm, Muslims were far more effective at penetrating the Sahara than Christians had been. This was largely due to the camel, which had carried the Arab expansion and would soon after carry large amounts of trade across the desert. A series of states developed in the Sahel on the southern edge of the Sahara which made immense profits from trading across the Sahara. The first of these was the Kingdom of Ghana, reaching it peak in the 12th century. Soon, others such as the Mali Empire and Kanem-Bornu, also arose in the region. The main trade of these states was gold, which was plentiful in Guinea. Also important was the trans-Saharan slave trade that shipped large numbers of slaves to North Africa. 600–1600 AD Many wealthy empires grew around coastal areas or large rivers that served as part of important trade routes. The kingdoms of Mali and Songhai Empire grew along the Niger River between 1200 and 1590. Berber traders from the Sahel—a region south of the Sahara Desert—traded dates, copper, horses, weapons and cloth that they brought from north Africa in Camel trains. Trade with the Berber people, and other groups, drove the growth of the Ghana empire, which traded its gold, kola nuts, and slaves. West Africans created a demand for salt, which was collected at desert oases, and which they used to preserve food as well as for seasoning it. In 1324, Mansa Musa, the king of Mali, made a historically famous Hajj (pilgrimage) to Mecca. There was an enormous group organized to undertake the Hajj with the king. It included "60,000 men, including 1200 servants" and records show that Mansa Musa gave