What is a DESERT? = area where the rate of evaporation is high relative to the rate of precipitation (less than 25 cm or 10 in. or rain per year).

For example, Death valley receives 1" of precipitation a year, but the evaporation rate is 10 feet per year.

Most of California � especially in southern California � evaporation exceeds precipitation. So most of California is semi-arid.

Figure 6-1, in California the principal deserts are located in the southern Basin and Range, Mojave, and Colorado deserts.

Point out the National and largest State Parks:

Death Valley National Monument

Mojave National Preserve

Joshua Tree National Monument

Anza-Borrego Desert State Park

What causes the Deserts in California? Topography.

"Orographic Effect" = [oro = Greek for mountain] Relatively moist air blowing in from the Pacific rises over the mountains and cools.

When air rises it cools. Cool air cannot hold as much moisture as warm air, thus clouds form and it eventually rains.

Rain Shadow = as air descends it compresses and warms. Thus, when air descends on the east side, it is relatively dry creating a situation of high evaporation rates on the east side.

(Inside back Cover of Text)

Which mountains cause the orographic effect for the California deserts?

Southern Basin and Range = Sierra Nevada

Mojave Desert = Transverse Ranges (San Gabriel and San Bernardino Mtns.)

Colorado Desert = Peninsular Ranges

Western Great Valley = Coast Ranges (low effect) (Bakersfield receives about 1 to 2 inches of rain per year!! They get their water as snow melt from the adjacent Sierra Nevada.)

Desert Varnish = a thin, very dark coating of Fe-Mg oxides on rocks (Mn = manganese). This results from a CHEMICAL WEATHERING that is restricted to deserts.

Petroglyph "rock drawing" made by the Native Americans. Chipped off the desert varnish.

Two common features in CA deserts produced by wind.

Desert Pavement = a thin armor of gravel, usually coated with desert varnish, that mantles the desert surface. The material underlying the desert pavement is usually very fine, light colored, silt.

A. Deflation (Harden, p. 94) may explain desert pavement. That is, the fine material is carried away by the wind, leaving the desert pavement as a residual gravel.

B. BEETLES may explain desert pavement by somehow tucking the fine material beneath the rocks. Not only beetles but also desert rodents and reptiles.

C. Swelling Soils may explain desert pavement. During cycles of wetting, soils expand and bring up gravel size material to the surface. During cycles of drying, the soil cracks and sand plus mud size material falls into the cracks below the gravel armor.

Desert Pavement surfaces are very prominent in the desert (especially on alluvial fans) and they may be thousands of years old.

Intaglios = large scale markings made by Native Americans by scraping aside the desert pavement to expose the lighter soil beneath.

(Overhead, Figs. 6-4 and 6-5)

When most people think about the desert, they picture a hot dry area full of Sand Dunes. However, sand dunes only make up a small percentage of the desert environment.

For example, only 2% of Death Valley is covered by sand dunes.

Sand Dunes = mounds of loose sand grains heaped up by wind action.

(Overhead, Fig. 6-7 and 6-8, p. 95)

Again, Sand Dunes are formed by the action of wind currents moving sand grains from one area to another.

Three conditions must be met for a sand dune to accumulate:

1. Wind speed in and area must be adequate to lift and transport sand grains.
2. The area must have little or no vegetation; plant roots are effective anchors that stabilize sand.
3. Sand-sized sediment (1/16 mm to 2 mm) must be available for transport.

The sandy alluvium of alluvial fans and dry washes are ideal sources for sand.

Wind can pick up speed through mountain valleys. Sand is picked up by prevailing winds (usually west to east) and deposited on the eastern sides of valleys.

For example (Fig. 6-9), sand is channeled through Emigrant Pass in the Panamint Range where it picks up sand from the Cottonwood alluvial fan. Sand Dunes are deposited at Stovepipe Wells in Death Valley.

Ventifacts = rocks with flat, wind-blasted surfaces.

Wind seldom moves particles larger than sand grains, but wind-blown sculpture isolated pebbles, cobbles, or boulders.

Even though deserts are usually very dry, the effects of water action are far more evident in deserts than the work of wind action.

(Overhead, Fig. 6-10, Flash Floods) The Desert geomorphology is sculptured mainly by water.

Flash Flood = high run-off created by sudden local floods of high discharge and short duration.

The limited rainfall that does occur in the deserts often comes from violent thunderstorms.

Because such a large amount of rainfall (average of 13 cm or 5 inches) cannot soak into the ground as fast, this creates high run-off and flooding.

The thunderstorms may occur miles away in the mountain ranges, but the flash floods will occur in the narrow gullies and channels in the valleys.

These Flash Floods are effective agents of erosion in desert regions.

Alluvial Fans = large, fan-shaped pile of sediment that forms at the mouths of narrow canyons where they open up to valleys or basins at the foot of a mountain range.

Bajadas = broad, gently sloping depositional surface formed by the coalescing of individual alluvial fans.

Playas = dry lake beds. Spanish word for "beach" or "shore"

Pediment = gently inclined erosion surface above the bajada and before the mountain front. Carved into bedrock and has thin veneer of alluvium.

Mountain Front = the sharp angled intersection where the gentle pediment meets the steep lower slope of the mountain range.

Inselberg = erosional remnant of mountain range now isolated hill on the pediment.

Bolson = a basin into which water drains from surrounding mountains to a central playa � no outlet to the sea.

The Playas are either covered by fine mud size sediment or evaporite minerals.

When precipitation is high enough to form temporary small, shallow lakes in the Playa, fine sediment is deposited which eventually forms a clay, hard packed bottom that becomes riddled with mud cracks.

When precipitation is so low that water seldom reaches the central playa, the playas are covered by evaporite salt deposits: HALITE; GYPSUM; BORATES

Evaporite minerals are chemical precipitates from the evaporation of water. They are usually white in color. Consequently, the dry playas are light colored and clearly distinguished by aerial photographs.

The source of the evaporating water is the leaching of ground water. As the ground water is leached to the surface and evaporated, the ions brought to the surface precipitate in the form of evaporite minerals.

(Overhead, Vignette 5, Sharp and Glazner)

Because of Pleistocene and Holocene volcanism, much Boron gas from the magmas as moved up into the desert valleys. This Boron volcanic gas contributes to the wealth of Borate minerals found in the California Desert. Ulexite is a common Borate mineral found in Death Valley. Boron is used as a meat preserver, ice cream hardener, jet fuel additive, detergent and a host of other uses.

Playas are very abundant in the desert, and if you stop and think about it, that implies that LAKES must have been abundant at one time in the present deserts.

They were!! During the Pleistocene.

About 11,000 years ago, during the final stages of the "Great Ice Age" of the Pleistocene Epoch, many lakes filled the enclosed basins of the California deserts.

We had a COOLER Climate and Glaciers were abundant in the Sierra Nevada. When this Glacial Ice melted, much of the water flowed into the deserts.

Even DEATH VALLEY was filled by a lake estimated to be between 500 and 600 feet deep and close to 100 miles long.

This lake is often referred to as Pleistocene Lake Manly.

The only lake remaining today is MONO LAKE.

OWENS LAKE in the Owens Valley had water in it until 1913, when the Owens river was diverted into the Los Angeles Aqueduct.

On Fig. 6-17, note inset showing the connecting system between Pleistocene lakes.

Strandlines = eroded shorelines or wave-cut benches of these Pleistocene lakes. They are conspicuous around the playas and are useful for reconstructing the sizes of the former lakes.

(Overhead, Vignette 5, Sharp and Glazner) Death Valley�s Shoreline Butte

Tufa Towers = CaCO3 (calcite) chemical precipitates formed where springs of fresh water enter saline lakes. They form underwater on the lake bottom and are only exposed as the lake level drops.

Tufa towers are common in many of the playas, and the most famous and most photographed ones are those of Mono Lake.

Other famous ones are the ones at Pleistocene Searles Lake.

(Overhead, Fig. 6-18 and 6-19, Harden)

Final subject Harden covers in the Deserts Chapter is "Riches in the Desert Lake Beds"

This has to do with the Borate minerals that are found in the playa lake beds.

Borate Minerals = evaporates containing the element Boron (B) � there is even a town named Boron, California.

These borate minerals were and are still used in an enormous variety of products, including ceramics, soaps, water softeners, and as meat preservatives.

One of these minerals is "Borax" � Na2B4O7 . 10H20 sodium borate

You may have heard of the "20 Mule Team Boraxo"

The first deposits of Borate minerals were mined from Death Valley between 1882 and 1928. The greatest problem with mining these deposits was transporting them over 275 km of dry, steep mountains and salt marshes between Death Valley and the nearest rail yard in Mojave.

As the urban areas in southern California become more and more populated, the open spaces of the nearby California desert become more and more increasingly used.