The older and wealthier part of the city (''ville basse'') is located on a flat area of alluvial sand and clay near the river, while many newer areas are found on the eroding red soil of surrounding hills. Older parts of the city were laid out on a geometric pattern, with ''de facto'' racial segregation becoming ''de jure'' in 1929 as the European and African neighborhoods grew closer together. City plans of the 1920s–1950s featured a ''cordon sanitaire'' or buffer between the white and black neighborhoods, which included the central market as well as parks and gardens for Europeans.