Inland Empire

The most accepted physical boundaries between Los Angeles and the Inland Empire from west to east are the San Jose Hills splitting the San Gabriel Valley from the Pomona Valley, leading to the urban populations centered in the San Bernardino Valley. Cities in southeastern Los Angeles County lying east of the hills, such as Pomona, Diamond Bar, La Verne, and Claremont, are often included because of their location in the Pomona Valley and proximity to the Riverside/San Bernardino county line. From the south to north, the Santa Ana Mountains physically divide Orange from San Bernardino and Riverside Counties. The Santa Rosa Mountains, as well as the Southern California portion of the Sonoran Desert, physically divides Riverside from San Diego county. Interconnectivity provided by one of the most comprehensive freeway systems in the United States has eroded any sense of physical boundaries between the Inland Empire and the Greater Los Angeles area. http://en.wikipedia.org/wiki/Riverside-San_Bernardino-Ontario_Metropolitan_Area