and U.S. Army
Earth Science Applications, National Training Center, Fort Irwin

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Other interesting geology information

Tors and desert erosion
As rocks at and just beneath the desert surface are subjected to the physical wear of wind and rain, and to chemical breakdown from water and acids produced from plants, they weather to form interesting erosional shapes. Granite (shown in reds and greens on the geologic map) makes some of the most distinctive shapes because it has big crystals that separate from one another readily. As granite erodes, much chemical action typically is focused along planes of fractures, or joints, where water can carry on its breakdown of the minerals. Big cubes of granite result, which then undergo rounding of their corners. The final result is round, boulder-like masses of granite. These often appear in big stacks that look from a distance to be pinnacles of gigantic boulders. Geologists call these pinnacles 'tors'. Outstanding examples can be seen at Fort Irwin, as well as in many parts of the Mojave Desert; some of the best are in Joshua Tree National Park and Mojave National Preserve.

Because the minerals that erode from granite are big and resistant to being broken down to smaller sizes, they form coarse sand sheets on the granite and around tors. After millions of years of erosion, masses of granite commonly take on a gently domed shape several miles across, with tors protruding from the gentle dome surface. These features are referred to as 'desert domes'. An excellent example is traversed by the main highway into Fort Irwin along the first 5 miles or so within the boundary of the base.


"Painted Rock", a tor in Cretaceous granite at Fort Irwin used
for painting logos of units that trained at the Center.


A tor in the Granite Mountains, Mojave Desert

Balanced rocks
One interesting kind of information that can be gleaned from these unusual tor formations is the recency of major earthquakes. As tors form by disintegrating granite grain by grain, they often result in delicately balanced rocks. A large earthquake or similar shaking will dislodge these balanced rocks. If major earthquakes have occurred since the formation of balanced rocks, we should see no balanced rocks; whereas if no major earthquakes have occurred, balanced rocks should be evident. The erosion and chemical alteration that produced the balanced rocks is over 10,000 years old, it is estimated. The Fort Irwin area has no balanced rocks, whereas the Victorville area has several. From this information we can infer that Fort Irwin was shaken by a major earthquake in the recent past, geologically speaking. Jim Brune of the University of Nevada, Reno, has studied precariously balanced rocks in the Mojave Desert and used them for predicting earthquake hazards

Balanced pebbles on pedestals
In fact, Fort Irwin seems to have been shaken quite recently. At one place near the south border of the NTC, pebbles were perched on pedestals formed from the clayey material that surrounded them. The perched pebbles formed by rain washing away unprotected clay next to the pebbles. These pedestals form relatively quickly, perhaps in decades or a few centuries. However, all pebbles at this location have toppled from their perches!

Unusual rocks

• Magnetite affects your compass needle. Altered rocks in and around the edges of granite bodies in some cases formed deposits rich in magnetite, an iron-bearing mineral that is magnetic. Magnetite was used in early compasses. It will also deflect your compass needle if you are very close to the rock. It won't affect your compass if you hold it above the ground a normal distance. Magnetite rock is only observed in two places at Fort Irwin, but aeromagnetic studies suggest that much more magnetite exists beneath dirt and rock.
  
  
• Opal associated with volcanic rocks
  Opaline material has formed in several places near the base of basalt flows southwest and northwest of the Main Post on Fort Irwin (basalt is shown in orange on the geologic map). The opal has been prospected in some of these places. While eye-catching because of its many hues, and of possible decorative value to Indians, the opal is opaque and therefore not of gem grade, and probably is not of commercial value. Opal is made up of silica (like quartz, but not crystalline) and also contains water, or is hydrated. It is a low-temperature product of hydrothermal activity and is common in and near volcanic rocks.
  
  
• Marble
  Limestone (shown in blue on the geologic map) that was deposited on an ocean bottom hundreds of millions of years ago was heated and squeezed during a mountain-building event about 160 million years ago. The heating caused calcite crystals to grow to almost half an inch across, and the calcite comes in different hues: white, gray, bluish gray, yellowish and brown. Impurities in the limestone formed other metamorphic minerals, from quartz and micas to more unusual minerals listed in the section below. The color variations are swirled like marble-cake in many places. However, the marble is fractured and not very strong, and so is not suitable for building stone.

Minerals
Many interesting minerals can be found at Fort Irwin. Most are in igneous and metamorphic rocks. Igneous rocks formed from melted rock, or magma, either deep in the earth as granite, or on the surface as volcanic rocks such as rhyolite and basalt. Metamorphic rocks form at high pressure and temperature by causing new minerals to grow. Some of the interesting minerals are:

• Long needles of hornblende can be found in schist and quartzite northeast of the Tiefort Mountains.
• Small crystals of several minerals can be found in impure marble: olivine, sphene, garnet, diopside, tremolite, and wollastonite.
• Amphibolite consists almost entirely of inch-wide black hornblende crystals, with white plagioclase feldspar filling between the crystals and a few small shiny metallic crystals of chalcopyrite scattered about.
• Unusual schist south of the Tiefort Mountains in one location has unusually abundant sillimanite, and has large black crystals of magnetite.
• Tiny red garnets can be found in granite in hills near Goldstone. This granite also contains feldspars, quartz, and muscovite.

Museum photographs of minerals

Archeology
Several sites with relics of human occupation are known at Fort Irwin. Many are near springs. Excavations over the years have yielded many relics.

Archeology sites in the nearby desert


Tortoise habitat can be predicted from geology
Geologists have noted in several parts of the Mojave Desert that desert tortoises are more abundant on older alluvial fans composed of materials eroded from certain kinds of granite. At Fort Irwin, the Cretaceous granite, about 83 million years old (shown in orange on the geologic map), provides the favored materials for tortoise habitat. Recognizing the prime habitat, the Army set aside part of Fort Irwin as a tortoise reserve. Intensive studies in this reserve have been underway for the last few years.

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