Montana Tech THE UNIVERSITY OF MONTANA |
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Research 98 Back to Past Research Activities Page In This Issue In-Depth
Look at Young
Researchers Geologic
Maps Environmental Research
Activity Chemistry
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Geologic Maps for Montana Karen Porter, Ph.D. We pave it, plow it, build on it, and play on it. Earth’s surface is a significant element in our lives, affecting our travel, economy, safety, and recreation. But what is this surface made of? What lies beneath the vegetation, soil, and pavement, and of what use is this information to us? Over Montana’s eastern plains, “gumbo” soils dominate. In the Roundup area, black coal seams are exposed in road cuts. One rancher has well water at 40 feet while his neighbor must drill 1,400 feet. A farmer’s hay varies from one pasture to another. Lewistown residents prize the local spring water, and the Ninemile Valley is remarkably straight. Are any of these phenomena predictable? Yes, because they are controlled by the variation and distribution of rock types and geologic structures such as faults, folds, and joints—and rock types and geologic structures are mappable. Geologic maps visually depict the earth’s surface using lines, symbols, patterns, and colors to represent geologic information. Lines represent such things as the contact between two rock units or the location of a fault. Specialized symbols represent such things as the direction of rock movement along a fault, the age and formation name of a rock unit, or the tilt of rock layers. Patterns may indicate the areal extent of an ancient soil horizon or a fossil bed. Standardized colors indicate the general geologic age of the rocks in the map area. Together, these mapped visual representations of geologic information clarify the age, type, and distribution pattern of rocks, sediments, and geologic structures found on the surface in the map area. The map’s margin provides additional information such as the map’s scale and orientation relative to longitude, latitude, and true and magnetic north. The interplay of color and pattern on geologic maps appeals to geologists and non-geologists alike. By overlaying geologic map
information on a map of familiar land features Who Uses Geologic Maps? Geologic maps are used by many government agencies, groups, and citizens. These maps are needed by federal and state agencies responsible for assessment, protection, and management of the state’s surface and subsurface resources. They are needed by county and municipal agencies responsible for land-use planning decisions. However, geologic maps are often overlooked as a source of valuable information, especially by local planning agencies and by the ranching and farming communities. In addition, adequate geologic maps are not yet available for many parts of the state. Montana’s urban areas and intermountain valleys are experiencing rapid population growth with the attendant home-site platting, road construction, waste disposal, and groundwater use. In many areas, these developments are proceeding without adequate information about earthquake hazards, slope stability, flood-prone areas, swelling clays, aquifers and recharge areas, and sites of potentially economic materials. Ideally, geologic information—rock and sediment types, geologic structures, and regional geologic history—should contribute to every land-use decision. The task of the Montana Bureau of Mines and Geology’s (MBMG) Geologic Mapping Program is to provide detailed, up-to-date geologic maps for the entire state as rapidly as possible. A further objective is to educate land-use constituencies about the critical importance of geologic information to sound land-use planning. Geologic information can also benefit a much wider audience. For example, an adequate water supply for livestock and crops is vital to farmers and ranchers. A map of rock types and geologic structures at the surface in an area provides immediate useful information about the subsurface water resources (see fig. 1). Maps showing rock-type distribution are useful for optimizing agricultural land use because soil types are derived directly from parent rock types (except in areas of glacial deposits). Geologic maps also provide a basis for initial exploration for minerals, fuels, and other commodity resources. Finally, these maps record the geologic history of an area. They provide a window to the past, through which we can view the extent of great seaways that covered the land in earlier times, when a fault was last active, or where vast glacial lakes once stood. Although a geologic map of the entire state of Montana does exist, it was published more than 40 years ago based on early 1900s information and has been out of print for a number of years. Moreover, it was mapped at 1:500,000 (1 inch equals approximately 8 miles), a scale too small to show useful detail. Since 1986, MBMG has been producing detailed and accurate geologic maps in digital form at a scale of 1:100,000 (1 inch equals 1.6 miles). This scale is the smallest at which geologic information can be effectively used and integrated with other data such as mineral, fuel, or groundwater resources. Substantial new and useful information is resulting from this detailed mapping. In western Montana, major fault systems are being reevaluated relative to age of movement and potential earthquake hazard. In the Bitterroot Valley, detailed mapping of complex sedimentary units is assisting land-use decisions in the rapidly growing corridor. South of Billings, mapping of the Pryor Mountains sheds new light on the mechanics of rock deformation. In the Little Belt Mountains, previously unmapped igneous intrusions now extend the opportunity for mineral exploration. On the south flank of the Big Snowy Mountains, possible exploration targets for new oil and gas reserves are suggested by newly mapped gentle folds. Finally, ranchers and farmers in nearly every part of the state have located new water wells assisted by carefully mapped geology. How are Geologic Maps Made? Geologic data are primarily gathered by geologists working in the field (see fig. 2), and a great deal of this work is done on foot. Mapping requires a knowledge of rocks, minerals, and geologic structures; geologic processes such as weathering, erosion, and sedimentation; and volcanic and igneous processes. Simple tools are employed in this field work: rock hammer, hand lens, dilute hydrochloric acid, sturdy boots, and topographic maps. Essential skills are keen observation, attention to subtle differences, and the ability to think in three dimensions. As always, experience is the best teacher.
Figure 2. Central Montana
Country. This landscape, with the Big Snowy Mountains on the skyline, is typical of the
country mapped on the Lewiston 30 x 60-minute geologic quadrangle map. Geologic mapping of
rock units and geologic structures clarified that this area is characterized by several
dome-shaped features that strongly control groundwater recharge and offer potential for
petroleum exploration. Field data are usually plotted directly onto topographic map bases in the field and preliminary maps are drawn. Back in the office, field data are combined with study of aerial photographs, detailed examination of collected samples, and extensive search of the literature for existing geologic information for the map area. Most final map products are produced at a 1:100,000 scale and cover an area of approximately 1,500 square miles. After assembling all the data, the geologist submits the map to the MBMG geographic information systems (GIS) lab for conversion to digital form (see fig. 3). The GIS lab performs detailed data processing and verification steps, during which the geologist remains closely involved.
Figure 3. Producing digital products. View of the GIS lab at MBMG where state-of-the-art equipment and software is used to digitize, scan, process, and plot geologic maps on a daily basis. Work is conducted by GIS professionals and students from several disciplines on the Montana Tech/ College of Technology campus. Final map products are available to the public in several different forms.
Montana Bureau of Mines and
Geology’s mapping projects are team efforts, drawing on the experience and expertise
of each field geologist to complete map areas of widely varying rock types, structure, and
terrain. Two or more field seasons are required to produce each map, depending on the
complexity of the map area, the amount and quality of previous mapping, and the
availability of funding. |
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| Since 1986, geologic mapping of the state has been partially funded by the U.S. Geological Survey, under several cooperative programs, including current support under the STATEMAP Program of the National Cooperative Geologic Mapping Program. Appreciation for this support is gratefully expressed. | Additional geologic mapping is conducted by university graduate programs funded by EDMAP, the educational component of the National Cooperative Geologic Mapping Program of the U.S. Geological Survey. In Montana, two master’s level theses have been supported by this program: Montana Tech Geological Engineering student Ron Dixon, supervised by Professor Diane Wolfgram, completed a master’s thesis mapping problem in the Elkhorn Mountain Volcanics, and Indiana University Geology student William Elliott, supervised at Indiana, completed mapping of Cenozoic deposits in an area of southwest Montana. The EDMAP program requires field work to be integrated with mapping objectives of the state geological survey in the state where the work is being conducted. Availability of Geologic Maps When completed, geologic maps are released as MBMG open-file reports (see fig. 4). Since 1996, the MBMG’s geologic maps have been prepared digitally; hand-drafted maps prepared prior to 1996 are being converted to digital form as time and funding permit. Data for completed maps are also available as digital files over the Internet or on a CD ROM. |
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