English: The mountain barrier between southern and northern California, historically important because it greatly limited human movement, and still important because it affects weather, plants and animals.

Environmental biologists Peter Berg and Ray Dasmann wrote about the Northern California bioregion in "Reinhabiting California", saying the following:

The Northern California bioregion is ringed by mountains on the north, east and south and extends some distance into the Pacific Ocean on the west. Since the boundaries depend in part on human attitudes they cannot be clearly mapped. These attitudes, however, have been persistent since prehistoric times. The region is separated from Southern California by the barrier of the Tehachapi Mountains and their extension through the Transverse Ranges to Point Conception on the seaward side. Flora and fauna change to some extent on either side of this boundary, but human attitudes are more important in the separation. Eastward, the region is enclosed by the Sierra Nevada which stops the rain and defines the dry Nevadan bioregion.

• Peter Berg (2014) The Biosphere and the Bioregion: Essential Writings of Peter Berg, Routledge, p. 37 ISBN: 9781134504091.