The fertile land along both banks of the Red River of the North was once the bed of Glacial Lake Agassiz, and when the last glacier retreated and the lake slowly drained some 9,000 years ago, it left one of North America's richest farming regions. The flat valley lands were well suited to the new farm machinery of the 1870's and 1880's, and settlers followed the railroads west and shipped huge wheat crops back to the growing flour mills in the Twin Cities. Many bonanza farms in the valley exceeded 2,000 acres, and the largest was more than 30,000 acres. Farm protest movements followed the wheat belt because single-cash-crop farmers with heavy machinery investments were especially vulnerable to price fixing by railroads, grain buyers, and milling monopolies, and the Farmers' Alliance, the cooperative marketing movement, the Nonpartisan League, and the Farmer - Labor Party all found strength in the Red River Valley. Although wheat is still grown, sugar beets, sunflowers, and potatoes are now the staples of valley agriculture, which remains characterized by large-scale operations, high capital investment, and the employment of seasonal workers.