There are several distinct regions of prairies around our valley. Farther south, near Coalinga and Bakersfield, and adjacent to the Coast Ranges, the prairies are drier and more alkaline, more nearly a desert landscape. A portion of this region has been preserved as the Carrizo Plains National Monument, which I may describe in a separate post. It is also a great place to see the San Andreas fault!
The region in which I live is a transition zone between the rich agricultural soils of the Central Valley around Fresno, Merced and Modesto, and the rocky ridges of the Sierra Nevada foothills. The underlying rocks include alluvial fans that developed as muddy rivers flowed off the Sierra Nevada block during the ice ages, and some of the volcanic lahar deposits and ash flows that preceded the glacial times. Rivers have not flowed across these surfaces for tens of thousands of years since the ice last invaded the high country. This long period of chemical weathering has resulted in the development of a clay-filled soil rich in iron oxide and hardpan (caliche deposits). The occasional winter rains that we get cannot sink far into the surface, so the primary vegetation is grass and wildflowers. A few oaks dot the higher hills.
In historic time, before the ranches, these prairies were grazed by tule elk and pronghorn antelope. The pronghorns were hunted to extinction by 1920, and the elk that roam a few isolated refuges are descended from single pair found in Kern County in the 1920's. California grizzly bears also lived here. In much earlier times, prior to about 12,000 years ago, the fauna included horses, camels, bison, mastodons, and mammoths. Giant condors flew overhead. Sabertooth cats, dire wolves and American lions hunted the grazers.
Some surfaces (as in the photo above) are covered by a regular pattern of small hummocks called mima mounds. Their origin is enigmatic, but is probably related to thousands of generations of ground squirrels occupying the same individual hills year after year. Other more exotic explanations involve Native American burials, odd earthquake waves, and periglacial soil activity.
The bottom photo shows the geological relationships in the prairie outside my town. In the foreground, the protruding rocks are part of the metamorphic bedrock of the Sierra Nevada, dating to Mesozoic time. These rocks were highly deformed and deeply eroded during the late Cretaceous and early Cenozoic, between 80 and 40 million years ago (approximate dates, and mildly controversial). The erosional unconformity is covered by ash flow deposits from about 22-28 million years ago that originated near the present-day Sierra Crest around 40 miles to the east (the Valley Springs Formation). The ash is in turn covered by voluminous lahar deposits (volcanic mudflows) of the Mehrten Formation, dating to around 9-11 million years. The lahars are probably responsible for the prominent ledges on the hillsides.
When I first came to live in the Central Valley, I didn't always appreciate the serene beauty of these wide-open landscapes. The geologist in me always wanted to see high mountains and steep-sided plateaus. The landscape has grown on me...the prairies are a special place.
And just wait til you see them in springtime. No black and white photos then! Have a good Thanksgiving holiday, if you live where it is celebrated. If not, give yourself a moment to think of the things you can be thankful for! And be safe in your travels...