Country sloping gradually from the Rocky Mountains eastward to the Prairie region, includes Nebraska, Kansas, Oklahoma, and Texas, as well as the eastern parts of Montana, Wyoming, Colorado, and New Mexico, and the western parts of North and South Dakota. The region is marked by broad, level river valleys and rolling plains, which were the habitat of great herds of bison and the Plains Indians who hunted them. Later the plains were devoted to cattle raising, but during most of the 20th century they have been planted in wheat and other crops. This change, involving the removal of grass and consequent erosion of the topsoil, resulted in dust storms and drought, especially in the 1930s, and caused the region to be called the Dust Bowl. Oklahoma and Texas in particular were disastrously affected, causing a great migration of refugees, popularly and invidiously called “Okies,” to other regions, notably California. Parkman's Oregon Trail is the most famous narrative of the Indians and white pioneers on the plains; Andy Adams has written of the cattle industry; the plays of Lynn Riggs deal with the transitional Indian Territory; Edna Ferber's Cimarron tells of the Oklahoma land rush and its aftermath; E. W. Howe and William Allen White, both Kansas editors, wrote stories of local life; Willa Cather describes the life of the region in many novels; Mari Sandoz treats Nebraska, past and present, in novels and nonfiction, as does Wright Morris; and Steinbeck's Grapes of Wrath tells of the migrants from the Dust Bowl.