Although agricultural activity occurs in every state in the union, it is particularly concentrated in the Great Plains, a vast expanse of flat, arable land in the center of the nation in the region west of the Great Lakes and east of the Rocky Mountains. The eastern, wetter half is a major corn and soybean producing region known as the Corn Belt, while the western, drier half is known as the Wheat Belt for its high rate of wheat production. The Central Valley of California produces fruits, vegetables, and nuts. The American South has historically been a large producer of cotton, tobacco, and rice, but it has declined in agricultural production over the past century.