The term Cross Timbers, also known as Ecoregion 29, Central Oklahoma/Texas Plains, is used to describe a strip of land in the United States that runs from southeastern Kansas across Central Oklahoma to Central Texas. Made up of a mix of prairie, savanna, and woodland, it forms part of the boundary between the more heavily forested eastern country and the almost treeless Great Plains, and also marks the western habitat limit of many mammals and insects. No major metropolitan areas lie wholly within the Cross Timbers, although roughly the western half of the Dallas-Fort Worth Metroplex does, including the cities of Fort Worth, Denton, Arlington, and Weatherford. The western suburbs of the Tulsa metropolitan area and the northeastern suburbs of the Oklahoma City metropolitan area also lie within this area. The main highways that cross the region are I-35 and I-35W going north to south (although they tend to skirt the Cross Timbers' eastern fringe south of Fort Worth) and I-40 going east to west. Numerous U.S. Highways also cross the area. As an ecoregion The Cross Timbers are defined by the United States Environmental Protection Agency as Ecoregion 29, a Level III ecoregion. Some organizations and maps refer to the Cross Timbers ecoregion as the Central Oklahoma/Texas Plains. The Cross Timbers are contained within the WWF central forest-grasslands transition ecoregion. The woodland and savanna portions of the Cross Timbers are mainly post oak and blackjack oak on coarse, sandy soils; fire suppression in recent years has increased forest density and allowed eastern redcedar to invade as well. The short, stout oaks that grow in the Cross Timbers were not usable as timber, and those that were not cleared for farmland constitute one of the least disturbed forest types in the eastern United States (MADISON), with some of old-growth forest scattered throughout the region. These old-growth forests contain millions of post oak from 200 to 400 years old and red cedar over 500 years old. The prairie portions are chiefly tallgrass on finer, dry soils. Overall, the Cross Timbers are not as arable as the surrounding ecoregions. Today, land use is a mixture of rangeland, pastures, and farmland. The area has also been an important site of oil extraction for over 80 years. Geologically speaking, the Cross Timbers are underlain by Pennsylvanian and Cretaceous-era sandstone and limestone that has been moderately dissected, giving the region a gently to moderately rolling