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

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Geologic Information shown on geologic maps

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Generalized geologic map of southwestern part of Fort Irwin

What the map shows (click on the image to see a large map [82 kb])

The map depicts the rock formations and surfice deposits in different colors. In general, surface deposits are shown in shades of yellow, volcanic rocks such as basalt and rhyolite are in oranges, granites are in reds, pinks, and greens, and metamorphic rocks are in blues and browns. Comparing the geologic map to the landforms map made from DEMs shows that most surficial deposits (yellows) are in the basins and most other rocks form mountains.

Geologic maps not only show the rocks and unconsolidated deposits exposed at the surface of the earth, but they also give information on the deformation of those materials (such as by earthquakes), the age of the materials, and the past geography and environments of the area. With the information shown on geologic maps, it is possible to predict what rocks are beneath the surface and to describe the changes in the geology of the region for hundreds over millions of years.

Heavy lines on the map represent faults, most of which were recently active and moved during earthquakes. Two sets of faults, those oriented east and those oriented northwest, are evident. The east-oriented faults very commonly are the boundaries between uplifted hills of bedrock on their south sides and alluvial basins on their north sides. This correlation between today's landforms and the faults leads us to infer that the faults are young and that part of their movement included an up-to-the-south motion.

How the map was made

This geologic map was constructed by examining rocks and soils in the field, and by using aerial photographs and satellite images as aids to better understand the distribution of the rocks. Geophysical data also aid in interpreting the geology and locating features such as faults. This particular map is generalized from six more detailed maps, available below.

In the office, the geology was drafted onto topographic maps, then scanned into the computer. All lines and polygons were tagged in ARC/INFO and other information was added to create a geologic data base. One use of the data base is to create a full-color map, and it is a powerful tool for creating all kinds of derivative maps. Examples are flood and earthquake hazard maps, maps showing change in the environment, and maps for analyzing questions about the past history of the earth.

The base map (showing roads, streams, and elevation contours) is the Soda Mountains 1:100,000-scale topographic map. Download base map

Ages of rocks

We can tell the ages of rocks by identifying fossils whose ages are known from other places, by dating the rocks using isotopic decay 'clocks', and by identifying volcanic ashes with ages determined elsewhere. All of these methods have been used in the geologic studies at Fort Irwin.

Map of granite and metamorphic rocks at Fort Irwin, indicating isotopic ages measured by several methods and on several minerals (ages are given in millions of years; 'K' refers to Cretaceous and 'J' to Jurassic). [Click on the image; 165 kb]

Detailed geologic maps of southwestern Fort Irwin

Click on a quadrangle to download graphics files (~xx kb each) (under construction)

Digital data base for published geologic map of Fort Irwin Basin:

Preliminary Geologic Map of Fort Irwin Basin, north-central Mojave Desert, California
by James C. Yount, Elizabeth R. Schermer, Tracey J. Felger, David M. Miller, and Kirk A. Stephens: U.S. Geological Survey Open-File Report 94-173.

Download digital data (under construction)
Download text and graphic files (under construction)
Topographic base maps