Prehistoric SD County, Part 4

By Tom Oberbauer, President CNPS-San Diego


During the Pleistocene, the San Diego County deserts would have looked very different. The coniferous forest would have grown far down slope onto the leeward side of the mountains into the upper deserts. Acer macrophyllum (Big leaf maple) grew in stands on the upper slopes. Lower down, Pinus monophylla (Single leaf pinyon) grew down to the desert floor near Blair Valley and Oriflamme Canyon.

Granite Mtn and the Pinyon Mtns not far from Earthquake Valley (aka Shelter Valley) as well as the slopes of the San Ysidro Mountains, and the slopes of Rabbit Peak northeast of Borrego near the San Diego/Riverside County Line were covered with coniferous forest of Pinus jeffreyi (Jeffrey pines), maybe some Pinus coulteri (Coulter pines), Quercus chrysolepis (Canyon live oak) and Juniperus occidentalis (Western junipers). Pinus flexilis (Limber pine) grew on Rabbit Peak. They were predominantly coniferous forest but the deciduous tree Quercus kelloggii (California black oak) would have also been found on the western parts.

Juniper woodland grew in Borrego Palm Canyon with 14 inches of precipitation per season. The Junipers occurred in groves around the perimeter of Borrego Valley except for the very sandy areas and the bottom of the Borrego Sink, the low point of the basin in the valley, that held water during winter and spring. Clark Lake was also full of water most of the year and was surrounded by Juniper and Pinyons. South toward Ocotillo and east toward Ocotillo Wells, the vegetation gradually dried and thinned. The Junipers dropped out to the east of Borrego Valley and near what is now the east County Line, the vegetation was semi-desert scrub with Larrea tridentata (Creosote bush) and Fouqueria splendens (Ocotillo). This vegetation would have existed on the eastern and southern edges of the County where rainfall was roughly 5-6 inches a season. The snow would have occasionally reached the desert floor. However, during summer, monsoonal moisture still occurred on the floor of the desert, augmenting the winter/spring rainfall.

California Fan Palm (Washingtonia filifera) anonymous, Washingtonia filifera, CC BY-SA 1.0

The Mesquite Bosque near Borrego Sink was large, covering thousands of acres of the floor of the valley and creating a forest of thorny small trees. Washingtonia filifera (Desert fan palm) would have extended down farther into the floor of the desert, displaced from the higher elevations by the colder temperatures. They were able to extend farther down due to the increased precipitation and runoff into the valley floors because of increased levels of runoff in the canyon stream courses. However, at the bottom of the valley, cold winter night temperatures would have also limited their growth since cold air flows downhill. Herds of American camels and Zebras moved across the valley in scattered locations, following the open areas among the Junipers. Three of the big cats, the Saber-toothed cat, American lion and Jaguar were also present. Mammoths fed on a variety of the vegetation but favored the riparian woodlands along the drainages much as the one that exists in Coyote Canyon today. Dire wolves were also present, moving over from the outer edges of the 2,200 square mile Lake Cahuilla just east of what is now the County line. The lake just touched the eastern boundary of what is now San Diego County at Ocotillo Wells. In terms of precipitation, the lake existed in the portion of the region that received the least actual rainfall, however, it existed due to the runoff from mountainous areas to the north and east with the exception of the drainages that ended in the Borrego Sink mentioned above.

Joshua Tree (Yucca brevifolia) Yucca brevifolia inflorescence, CC BY-SA 3.0

Desert vegetation surrounded the lake on the west but grassy meadows grew on the edges as well and wetlands with some riparian forests existed in the drainages that approached the lake from the west and east. However, the lake’s basin was large enough so that now it holds the Salton Sea, and it would have enhanced vicinity’s moisture. Its shores would have generated moist vegetation that served as grazing and browsing food for herbivores, which in turn served as the basis for carnivores. Yucca brevifolia (Joshua tree) grew on both sides of the lake especially on the lower hills (Rinehard and McFarlane 1995).

A trip down what is now County Highway S-2 from Borrego Springs over Sweeny Pass and all the way down to Ocotillo would have passed from Junipers to Pinyons over Sweeny Pass to Junipers again in the shadow of forested Laguna Mountains, often white capped from periodic snows. San Felipe Valley, which currently supports a large riparian area, would have been even more forested, including the terrain surrounding the existing marsh at the intersection of S-2 and State Route 78 and the marsh at the beginning of Sentenac Canyon at Sentenac Cienega. Earthquake, or Shelter, Valley would have been a moist meadow of grasses and browsable shrubs with bands of Antilocapra americana (Pronghorn) in various parts. As mentioned, the upper portions of the Pinyon Mountains would have been clothed with forest. Most of the uplands were vegetated with Junipers but the Pinyons were present in the low ridge through which Box Canyon passes, connecting Granite Mountain and Whale Peak. Farther south, the entire Vallecito Valley would have been a marshy, mesquite and riparian woodland area with seeps and standing water created by runoff from the Laguna Mountains to the west. Continuing to the south, the vegetation began to dry more significantly past Agua Caliente Park and Canebrake. Approaching the area of Mountain Palm Springs, the Washingtonia filifera grew down past the modern location of the road and crossed it. They had thickly thatched trunks and branches with leaves that glistened and moved in the afternoon breeze. The entire low area affected by Carrizo Creek was vegetated with a Mesquite Bosque and the existing Carrizo Creek marsh near the County Line with Imperial County was vast and held standing water. This far south, the surroundings were no longer Junipers but low desert of Fouqueria splendens (Ocotillo) and Larrea tridentata (Creosote bush) and it continued down to the town of modern day Ocotillo. Traveling up the Mountain Springs Grade into the area of Jacumba, one would have passed through more Washingtonia filifera groves and back into Junipers and Pinyon pines until at the top of the grade around Jacumba, Jeffrey pines would have been mixed in with the Pinyons, Chaparral and Junipers.

The vegetation during the Pleistocene would have been very interesting with the displacement of the various vegetation communities to lower elevations and farther south than they currently grow. The level of vegetative growth must have been phenomenal to support the mass numbers of large mammalian wildlife, particularly the major grazers, browsers and predators. It would have been a fascinating place to visit but the safety of an individual human in the midst of all of these predators would have been easily compromised if one was left alone. The native people would have worked in harmony to keep from having continual losses from predators, though the amount of grazing and browsing animals generated a good food supply for humans as well as for the carnivorous predators.


References that support the imagined habitat conditions:

Axelrod, D.I. 1980. History of the maritime closed-cone pines, Alta and Baja California. University of California Publications in Geological Sciences v. 120.

Axelrod, D.I.; Govean, F., 1996: An early Pleistocene closed-cone pine forest at Costa Mesa, southern California. International Journal of Plant Sciences 157(3): 323-329

Close, D. H., Gilbert, D. E. and G. D. Peterson eds. 1970. Climates of San Diego County, Agricultural Relationships. Univ. Calif. Agri. Extension Service. 127p.

Jefferson, G. T. and L. Lindsay. 2006. Fossil Treasures of the Anza-Borrego Desert: The Last Seven Million Years. Sunbelt Publications 394p.

Minnich, R.A. 2007. California climate and paleovegetation. In, Terrestrial Vegetation of California, 3 rd edition, (M.G. Barbour, T. Keeler-Wolf, and A.A. Schoenherr, eds.), pp. 43-70. University of California press.

Oberbauer, Thomas A. 1986. The Tree that Skipped San Diego. Environment Southwest 512:12-13.

Oberbauer, T. A. 2013a. Searching for Representatives of a Previous Era in San Diego County. San Diego Newsletter California Native Plant Society.

Oberbauer, T. A. 2013b. Land of Perpetual Shade. San Diego Chapter Newsletter California Native Plant Society. March

Oberbauer, T. A. 2013c. Searching for Representatives of Previous Eras; A Mysterious Canyon. San Diego Chapter Newsletter September California Native Plant Society. September

Oberbauer, T. A. 2013c. Searching for Representatives of Previous Eras; A Mysterious Canyon Part 2 San Diego Chapter Newsletter September California Native Plant Society. November.

Oberbauer, T. A. 2013c. Searching for Representatives of Previous Eras; A Mysterious Canyon Part 3 San Diego Chapter Newsletter September California Native Plant Society. December.

Rhode, D. 2002. Early Holocene Juniper Woodland and Chaparral Taxa in the Central Baja California Peninsula, Mexico. Quaternary Research, Volume 57, Issue 1, January 2002, Pages 102-108.

Rinehart, R. B. and D. A. McFarlane. 1995. Early Holocene Vegetation Record from the Salton Basin, California. Quaternary Research 43:259-262.

Stock, C. 1992. Rancho La Brea: A record of Pleistocene life in California. 7th ed. Revised by J.M. Harris. Science Series no.37. Los Angeles: Natural History.

Websites with information on birds and animals, extinct and living: z/camel/ z/dire-wolf/ z/giant-sloth/