On the plateau cattle graze, finding shelter in the many ravines, and large ranges are an economic necessity. In the northwestern area of the state oil was discovered in 1951, and petroleum is now North Dakota's leading mineral product, ahead of sand and gravel, lime, and salt. There are also natural-gas fields. Underlying the western counties are lignite reserves; close to the lignite beds are deposits of clay of such varied types that they serve as both construction and pottery materials.
Despite mineral production and some manufacturing, agriculture continues to be North Dakota's principal pursuit, and the processing of grain, meat, and dairy products is vital to such cities as Fargo, Grand Forks, Minot, and Bismarck. The Missouri and Red rivers, once the major transportation routes, are more important now for their irrigation potential. Several dams have been built, notably Garrison Dam, and a number of federal reclamation projects have been completed as part of the Missouri River basin project. There has also been reforestation. With such attractions as the Badlands, the International Peace Garden on the Canadian border, and recreational facilities provided by reservoirs (resulting from dam building in the 1950s), tourism has become North Dakota's third-ranking source of income, behind agriculture and mineral production.