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general, the level of detail and amount of time spent per unit area is greater for the wilderness studies than for CUSMAP or AMRAP.

Mineral Deposit Research

Most mineral deposits that are exposed at the surface have been recognized by prospectors, but a large fraction of deposits are either concealed or have only an uneconomic fringe visible. Thus, a key element in the transition from the physical description of the ground, in the form of a geologic map, to the mineral assessment is the ability to recognize the possibility or

probability for a given type of mineralization to occur in a given geologic setting. The basic tool for this is the comparison of the tract with similar tracts known to be mineralized so that the geologic clues to mineralization can be recognized and interpreted. Studies of known mineral deposits are key factors in support of the assessment activity. These studies must include detailed field and laboratory investigations of mines and prospects and an interpretive synthesis leading to predictive models for mineral deposits. Use of these models can lead to discovery of new resources in previously unexplored parts of the United States as well as expanding the identified reserves in recognized mining districts.

Figure 2. Status of the Conterminous United States Mineral Assessment Program.

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Geologic Availability of Minerals

To keep track of the amount and distribution of mineral resources within the United States and to promote the adequacy and certainty of access to resources from foreign sources, the Geological Survey carries out several mutually complementary activities. Each of the major mineral commodities is assigned to an experienced earth scientist whose duty it is to acquire and maintain a thorough understanding of the national and international resource information for that specific mineral or material. These mineral commodity experts constitute a valuable source of resource information to answer direct inquiries from the private and public sectors. In addition, the amount and distribution of resources by geographic region is compiled and used through a recently revised computerized mineral and geologic data system which possesses over 60,000 entries. To improve our knowledge of foreign supplies and potential supplies, the Geological Survey is cooperating with the State Department and many agencies of foreign governments to develop improved resource information and evaluate worldwide nonfuel mineral resource potential and to keep track of international exploration activities. Survey

foreign activites are directed especially toward identification of alternate and more secure sources of the strategic minerals needed by the United States.

In summary, the Geological Survey is attacking the increasingly difficult problem of United States critical minerals import dependency by systematic mineral surveys to identify domestic mineral resource potential, mineral deposit research that can lead to the discovery of new types of deposits in hitherto unexplored parts of the United States and expanding known resources in recognized mining districts, and resource data services in the form of mineral commodity experts and an international computerized mineral resources data base. These activities are continuing programs that constitute a significant part of the Geological Survey's responsibilities as mandated by the Organic Act of 1879. Because of long experience in these activities, the Geological Survey has been able to quickly focus on strategic and critical mineral problems to support the President's National Materials and Minerals Program Plan that was released in April 1982. By these activities, the Geological Survey can help identify new domestic mineral resources and more secure foreign sources, as well as provide resource information to support land and mineral policy decisionmaking.

Climate Change

Most people are very much aware of the unusual weather that occurred during the last year. For example, record snows in the Midwest followed by early spring thaws and heavy spring rains caused flooding in a number of places such as Salt Lake City, Utah, and parts of the Southeastern United States. However, different types of geologic studies demonstrate that climate varies over a wide range of time scales from decades to millenia and that the climate changes experienced by modern man represent relatively minor wiggles superimposed on larger scale climatic fluctuations. Geologists are actively involved in the study of ancient climates (or paleoclimates) because much geologic data such as associations of fossils, structure and composition of sediments, and the occurrence of features directly related to glacial processes can provide information on past climates that extends beyond the historical records of man. Such information is essential for understanding current climate and for predicting future climate change.

The water-level fluctuations of ancient Searles Lake, California, shown in the figure, represent one kind of paleoclimate record. Although now dry, Searles Lake was one of about 100 lakes in the Great Basin that extend from the Sierra Nevada of California to the Wasatch Mountains of Utah. These lakes, which were developed in closed basins, are called pluvial lakes (rain lakes) because their water-level changes dramatically with changing climate. During times of cool and wet climate, the pluvial lakes expand, whereas during times of warm and dry climate, they

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shrink. Through studies of the geology of Searles Valley and cores from ancient Searles Lake, U.S. Geological Survey scientists have been able to reconstruct the history of Searles Lake for the last 30,000 years.

The figure shows that Searles Lake has remained dry or at a relatively low level for the last 10,000 years. In contrast, from 10,000 to 30,000 years before present, the level of ancient Searles Lake was generally much higher, and extreme rapid fluctuations in the level of the lake occurred frequently. Between 1 5,000 and 20,000 years before present, the lake level rose high enough to flow over the valley rim into the next lower basin. Because changes in the lake level reflect changes in climate, we infer that warm, dry, and relatively stable climates of the past 10,000 years were preceded by a period of cold and wet and more variable climates.

The modern and ancient record of Searles Lake generally matches the history of lake levels determined from many other pluvial lakes in the Great Basin. For example, when Searles Lake was overflowing its basin between 1 5,000 and 20,000 years before present, Great Salt Lake was greatly expanded and formed part of a huge lake called Lake Bonneville which covered an area of about 19,300 square miles, 1 7,000 square miles larger than the present Great Salt Lake.

A wide variety of paleoclimate studies indicate that climate has fluctuated regularly in the past; these studies also show that climates as warm as the last

Water-level fluctuations of ancient Searles Lake, California.

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THOUSANDS OF YEARS BEFORE PRESENT

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