Chapter 2 – Physical Geography of Simi Valley

Simi Valley is located within a mountainous region known as the Transverse Ranges province which extends from Point Arguello in Santa Barbara County to central Riverside County. The province contains chains of east/west-running mountain ranges separated by valleys. Simi Valley is one of these valleys.

Simi Valley is surrounded by the Big Mountain area to the north, the Santa Susana Mountains to the north and east, and the Simi Hills to the south. Rock formations date from 75 million years to as recently as 100,000 years ago, and consist of sandstone, marine deposits, and sedimentary rock. Igneous rocks can be found that are associated with volcanic activity to the southwest 14 million years ago.

This area is quite seismic. The Simi Fault, which is part of the Simi-Santa Rosa fault system, runs for 30 miles along the base of the foothills to the north of the valley.

The valley floor contains a rich deposit of alluvium which contributed to the valley’s historic agricultural success. The valley was once a major source of citrus fruit, walnuts, and other agricultural products.

Despite its almost desert-like appearance, the area has diverse natural vegetation ranging from chaparral, to coastal scrub, and even a few patches of oak woodland in the deeper, moister canyons where oak trees thrive.

Historically, the alluvial geology of the valley floor contained within the mountainous surroundings supported the formation of artesian wells (confined aquifers that create pressure that allows water to seep to the surface). In a few areas, year-around water sources can still be found. Just a few hundred yards from where the picture to the left was taken, a rich riparian habitat of sycamores, oaks, and a wide variety of shrubs surround a small marsh where deer, coyotes, and even a mountain lion occasionally leave their tracks while getting a drink. It is easy to see how game, acorns, and other edible vegetation may have supported the original Chumash residents long before European immigration.

The Mediterranean climate of the region produces a mild rainy winter season and a hot, dry summer. Sunshine is predominant throughout the year (from roughly 70% in the winter to nearly 100% in the summer), with most rain occurring between the end of October and the beginning of March. Winter temperatures rarely fall below freezing, but summer highs are commonly well above 100 degrees Fahrenheit.

The most significant weather pattern, and the one which Simi Valley is named for (from the Chumash word for clouds associated with wind which they called shimiyi), is the Santa Ana wind. Created by high pressure built up over the Great Basin (Nevada and Utah) and heated during their passage through the Mojave and Sonoran deserts, the Santa Ana winds always seem to blow about 10 mph harder in Simi Valley than anywhere else. The Santa Anas are most noticeable in the fall when they fan the flames of destructive and deadly brush fires, but they seem to blow hardest in the winter when they are colder, sometimes reaching hurricane force. It feels like they get a little worse each year and they seem to last longer and longer. I have always associated them with temporary insanity, something Raymond Chandler noted long ago in a short story called “Red Wind” where he wrote:

“Those hot dry winds that come down through the mountain passes and curl your hair and make your nerves jump and your skin itch. On nights like that every booze party ends in a fight. Meek little wives feel the edge of the carving knife and study their husbands' necks. Anything can happen.”

Really, the wind is the only thing besides the traffic that I do not like about Simi Valley.

Sources:

Havens, Patricia and Bill Appleton. Simi Valley: A Journey Through Time. Simi Valley: Simi Valley Historical Society and Museum, 1997. pp 293-301.