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At least 73 records · Page 4Linked to original sources
USGS1949

Interpreting ground conditions from geologic maps

Intelligent planning for heavy construction, water supply, or other land utilization requires advance knowledge of ground conditions in the area. It is essential to know: the topography, that is, the configuration of the land surface; the geology and soils, that is, the deposits that compose the land and its weathered surface; and the hydrology, that is, the occurrence of water whether under or on the ground. These elements usually are considered in planning land developments that involve much investment; detailed surveys generally are made of the topography, geology, soils, and hydrology at the site selected for development. Such detailed surveys are essential, but equally essential and often overlooked is the need for general surveys prior to site selection. Only if the general surveys have been made is it possible to know that a particular site is most suitable for the purpose and that no situations in the tributary areas that might affect the project have been overlooked. Moreover, the general regional relations must be known in order to properly interpret the geology, soils, and hydrology at a particular locality. In brief, both the general and the specific are needed in order to avoid costly mistakes either during or after development. The accompanying maps illustrate how a general geologic map can be used for interpreting grc .d conditions during a planning stage prior to site selection. The topographic and geologic maps, which provide the basic data, have been simplified from some existing ones. The interpretive sheets are intended to provide some examples of the kinds of information that trained persons can read from such basic maps.

Circular↗
USGS1999

An interpretation of the aeromagnetic and gravity data and derivative maps of the Craig and Dixon Entrance 1°x 3° quadrangles and the western edges of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska

The U.S. Geological Survey (USGS) is required by the Alaska National Interest Lands Conservation Act (ANILCA, Public Law 96-487) to survey certain Federal lands to determine their mineral resource potential. As part of continuing studies designed to fulfill this responsibility, geochemical, geological and geophysical surveys of the Craig and Dixon Entrance (CDE) 1°x3° quadrangles in southeastern Alaska were undertaken by the USGS during the summers of 1983-85, 1989, and 1991. The geochemical data have been reported by Cathrall and others (1993) and Cathrall (1994). A simplified geologic map has been compiled especially for this report based largely on the work of Eberlein and others (1983) and Gehrels (1991, 1992). Brew (1996) also produced a compilation. This report presents available magnetic and gravity data and an interpretation thereof. In this report (released on CD-ROM in 1999) we are releasing USGS-acquired geophysical data for CDE 1:250,000-scale quadrangles of southeastern Alaska. Much of the data have never appeared before in the public domain, including a simplified geologic map, the raw gravity and magnetic data, and derivative products including depth-to-source images, various grids of the data, imaging representations, rock densities, and overlays of the data. In addition, we have added a limited interpretation of what the gravity and magnetic data indicate about the underlying geology, structures, and their relationship to known and potential hidden mineral resources. This interpretation is necessarily limited in the case of the gravity data by the general sparsity of stations. For convenience, we have also included here various representations of the digital topography.

Alaska↗
USGS2001

Interpretive geologic cross sections for the Death Valley regional flow system and surrounding areas, Nevada and California

This report presents a network of 28 geologic cross sections that portray subsurface geologic relations within the Death Valley regional ground-water system, a ground-water basin that encompasses a 3? x 3? area (approximately 70,000 km2) in southern Nevada and eastern California. The cross sections transect that part of the southern Great Basin that includes Death Valley, the Nevada Test Site, and the potential high-level nuclear waste underground repository at Yucca Mountain. The specific geometric relationships portrayed on the cross sections are discussed in the context of four general sub-regions that have stratigraphic similarities and general consistency of structural style: (1) the Nevada Test Site vicinity; (2) the Spring Mountains, Pahrump Valley and Amargosa Desert region; (3) the Death Valley region; and (4) the area east of the Nevada Test Site. The subsurface geologic interpretations portrayed on the cross sections are based on an integration of existing geologic maps, measured stratigraphic sections, published cross sections, well data, and geophysical data and interpretations. The estimated top of pre-Cenozoic rocks in the cross sections is based on inversion of gravity data, but the deeper parts of the sections are based on geologic conceptual models and are more speculative. The region transected by the cross sections includes part of the southern Basin and Range Province, the northwest-trending Walker Lane belt, the Death Valley region, and the northern Mojave Desert. The region is structurally complex, where a locally thick Tertiary volcanic and sedimentary section unconformably overlies previously deformed Proterozoic through Paleozoic rocks. All of these rocks have been deformed by complex Neogene ex-tensional normal and strike-slip faults. These cross sections form a three-dimensional network that portrays the interpreted stratigraphic and structural relations in the region; the sections form part of the geologic framework that will be incorporated in a complex numerical model of ground-water flow in the Death Valley region.

California, Nevada↗
USGS2004

Sidescan sonar imagery and surficial geologic interpretation of the sea floor off Branford, Connecticut

The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Connecticut Department of Environmental Protection (CT DEP), Figure 1 - Map of Study Areahas produced detailed geologic maps of the sea floor in Long Island Sound, a major East Coast estuary surrounded by the most densely populated region of the United States. These studies have built upon cooperative research between the USGS and the State of Connecticut that was initiated in 1982. The current phase of this research program is directed toward studies of sea-floor sediment distribution, processes that control sediment distribution, nearshore environmental concerns, and the relation of benthic community structures to the sea-floor geology. Anthropogenic wastes, toxic chemicals, and changes in land-use patterns resulting from residential, commercial, and recreational development have stressed the environment of the Sound, causing degradation and potential loss of benthic habitats (Koppelman and others, 1976; Long Island Sound Study, 1994). Detailed maps of the sea floor are needed to help evaluate the extent of adverse impacts and to help wisely manage resources in the future. Therefore, in a continuing effort to better understand Long Island Sound, we are constructing and interpreting sidescan sonar mosaics (complete-coverage acoustic images of the sea floor) within specific areas of special interest (Poppe and Polloni, 1998). The mosaic presented herein, which was produced during survey H11043 by NOAA 's Atlantic Hydrographic Branch, covers approximately 41.1 km 2 of the sea floor in north-central Long Island Sound off Branford, Connecticut. Shell bed provides shelter for juvenille skate.The mosaic and its interpretation serve many purposes, including: (1) defining the geological variability of the sea floor, which is one of the primary controls of benthic habitat diversity; (2) improving our understanding of the processes that control the distribution and transport of bottom sediments and the distribution of benthic habitats and associated infaunal community structures; and (3) providing a detailed framework for future research, monitoring, and management activities. The sidescan sonar mosaic also serves as a base map for subsequent sedimentological, geochemical, and biological observations, because precise information on environmental setting is important for selection of sampling sites and for appropriate interpretation of point measurements.

Connecticut↗
USGS2004

Converting analog interpretive data to digital formats for use in database and GIS applications

There is a growing need by researchers and managers for comprehensive and unified nationwide datasets of scientific data. These datasets must be in a digital format that is easily accessible using database and GIS applications, providing the user with access to a wide variety of current and historical information. Although most data currently being collected by scientists are already in a digital format, there is still a large repository of information in the literature and paper archive. Converting this information into a format accessible by computer applications is typically very difficult and can result in loss of data. However, since scientific data are commonly collected in a repetitious, concise matter (i.e., forms, tables, graphs, etc.), these data can be recovered digitally by using a conversion process that relates the position of an attribute in two-dimensional space to the information that the attribute signifies. For example, if a table contains a certain piece of information in a specific row and column, then the space that the row and column occupies becomes an index of that information. An index key is used to identify the relation between the physical location of the attribute and the information the attribute contains. The conversion process can be achieved rapidly, easily and inexpensively using widely available digitizing and spreadsheet software, and simple programming code. In the geological sciences, sedimentary character is commonly interpreted from geophysical profiles and descriptions of sediment cores. In the field and laboratory, these interpretations were typically transcribed to paper. The information from these paper archives is still relevant and increasingly important to scientists, engineers and managers to understand geologic processes affecting our environment. Direct scanning of this information produces a raster facsimile of the data, which allows it to be linked to the electronic world. But true integration of the content with database and GIS software as point, vector or text information is commonly lost. Sediment core descriptions and interpretation of geophysical profiles are usually portrayed as lines, curves, symbols and text information. They have vertical and horizontal dimensions associated with depth, category, time, or geographic position. These dimensions are displayed in consistent positions, which can be digitized and converted to a digital format, such as a spreadsheet. Once this data is in a digital, tabulated form it can easily be made available to a wide variety of imaging and data manipulation software for compilation and world-wide dissemination.

Open-File Report↗
USGS2005

Surficial geologic interpretation and sidescan sonar imagery of the sea floor in west-central Long Island Sound

The United States Geological Survey (USGS) is working cooperatively with the National Oceanic and Atmospheric Administration (NOAA) and Connecticut Department of Environmental Protection (CT-DEP) to conduct detailed studies of the surficial geology in Long Island Sound (LIS). The study goals are to interpret sedimentary environments within the Sound, to further understand processes controlling sediment distribution, and to provide a framework for future studies. Sidescan-sonar mosaics produced by USGS and NOAA show detailed acoustic images of the sea floor with 1- m resolution. These images, along with data obtained from sediment samples, seismic-reflection profiles, and seafloor video, are used to interpret the surficial geology. As part of this cooperative program, since 1995, 12 sidescan-sonar surveys of the LIS sea floor have been completed (Poppe and others, 1997; Twichell and others, 1997; Poppe and others, 1998a; Poppe and others, 1998b; Twichell and others, 1998; Poppe and others, 1999a; Poppe and others, 1999b; Poppe and others, 2001; Poppe and others, 2004; Zajac and others, 2003). The purpose of this report is to release digital versions of the imagery and interpretations from NOAA survey H11044 originally published in McMullen and others (2005). Survey H11044, which covers an area of 293 km 2 in west-central LIS , includes the area of the previously published Milford Survey (Twichell and others, 1998) and the westernmost part of the New Haven Harbor Survey (Poppe and others, 2001). These two surveys detailed surficial geology and mapped sediment distributions. In this study, we map the sediment distribution across a broader, previously unstudied area and the sedimentary environments.

Connecticut↗
USGS2006

Geological interpretation of bathymetric and backscatter imagery of the sea floor off eastern Cape Cod, Massachusetts

The imagery, interpretive data layers, and data presented herein were derived from multibeam echo-sounder data collected off Eastern Cape Cod, Massachusetts, and from the stations occupied to verify these acoustic data. The basic data layers show sea-floor topography, sun-illuminated shaded relief, and backscatter intensity; interpretive layers show the distributions of surficial sediment and sedimentary environments. Presented verification data include new and historical sediment grain-size analyses and a gallery of still photographs of the seabed. The multibeam data, which cover a narrow band of the sea floor extending from Provincetown around the northern tip of Cape Cod and south southeastward to off Monomoy Island, were collected during transits between concurrent mapping projects in the Stellwagen Bank National Marine Sanctuary (Valentine and others, 2001; Butman and others, 2004; and Valentine, 2005) and Great South Channel (Valentine and others, 2003a, b, c, d). Although originally collected to maximize the use of time aboard ship, these data provide a fundamental framework for research and management activities in this part of the Gulf of Maine (Noji and others, 2004), show the composition and terrain of the seabed, and provide information on sediment transport and benthic habitat. These data and interpretations also support ongoing modeling studies of the lower Cape’s aquifer system (Masterson, 2004) and of erosional hotspots along the Cape Cod National Seashore (List and others, 2006).

Massachusetts↗
USGS2006

Geologic interpretation and multibeam bathymetry of the sea floor in southeastern Long Island Sound

Digital terrain models (DTMs) produced from multibeam echosounder (MBES) bathymetric data provide valuable base maps for marine geological interpretations (e.g. Todd and others, 1999; Mosher and Thomson, 2002; ten Brink and others, 2004; Poppe and others, 2006a,b). These maps help define the geological variability of the sea floor (one of the primary controls of benthic habitat diversity); improve our understanding of the processes that control the distribution and transport of bottom sediments, the distribution of benthic habitats and associated infaunal community structures; and provide a detailed framework for future research, monitoring, and management activities. The bathymetric survey interpreted herein (National Oceanic and Atmospheric Administration (NOAA) survey H11255) covers roughly 95 km? of sea floor in southeastern Long Island Sound (fig. 1). This bathymetry has been examined in relation to seismic reflection data collected concurrently, as well as archived seismic profiles acquired as part of a long-standing geologic mapping partnership between the State of Connecticut and the U.S. Geological Survey (USGS). The objective of this work was to use these geophysical data sets to interpret geomorphological attributes of the sea floor in terms of the Quaternary geologic history and modern sedimentary processes within Long Island Sound.

Open-File Report↗
USGS2011

Notes on interpretation of geophysical data over areas of mineralization in Afghanistan

Afghanistan has the potential to contain substantial metallic mineral resources. Although valuable mineral deposits have been identified, much of the country’s potential remains unknown. Geophysical surveys, particularly those conducted from airborne platforms, are a well-accepted and cost-effective method for obtaining information on the geological setting of a given area. This report summarizes interpretive findings from various geophysical surveys over selected mineral targets in Afghanistan, highlighting what existing data tell us. These interpretations are mainly qualitative in nature, because of the low resolution of available geophysical data. Geophysical data and simple interpretations are included for these six areas and deposit types: (1) Aynak: Sedimentary-hosted copper; (2) Zarkashan: Porphyry copper; (3) Kundalan: Porphyry copper; (4) Dusar Shaida: Volcanic-hosted massive sulphide; (5) Khanneshin: Carbonatite-hosted rare earth element; and (6) Chagai Hills: Porphyry copper.

Open-File Report↗
USGS1963

Preliminary interpretation of an aeromagnetic survey in north-central Iowa

Publication of Lyons' gravity map of the United States in 1950 focused attention on a spectacular geophysical feature now generally referred to as the "midcontinent gravity high." This feature, the largest gravity maximum in North America, was discovered and later detailed by geophysicists at the University of Wisconsin (Woollard, 1943, 1951) and Thiel (1956). Their work reveals an essentially continuous large positive gravity anomaly extending southwestward for 800 miles from Lake Superior to the Salina Basin in Kansas. The midcontinent gravity high reaches its maximum intensity in central Iowa where the Bouguer anomaly ranges from -100 to 60 milligals. In September-October 1961, the U. S. Geological Survey made a 5,000 square mile aeromagnetic survey in north-central Iowa in cooperation with the Iowa Geological Survey. Objectives of the investigation were to prepare and interpret a detailed aeromagnetic contour map for part of the midcontinent gravity high in Iowa. Analysis of the magnetic data could be expected to yield estimates of local thickness of the Paleozoic-Mesozoic sedimentary section and provide a basis for informed speculation about the character, configuration, and distribution of the Precambrian rocks. Such information would have obvious scientific and possible economic value in an area covered by glacial till and for which little deep subsurface knowledge was available. Interpretation of the Indiana aeromagnetic survey (Henderson and Zietz, 1958) had previously demonstrated that aeromagnetic data from the central stable region of North America would provide a significant amount of geologic information. The 5,000 square mile area surveyed in 1961 has been combined with a previously unpublished 600 square mile aeromagnetic survey made in 1953, centered on Manson, Pocahontas Co., Iowa. The combined area is shown as the patterned area in figure 1. The unpatterned area outlined on this same figure is the additional 10,000 square mile cooperative aeromagnetic survey made in the summer of 1962 to complete coverage of the midcontinent gravity high in Iowa. The primary purpose of this report is to make the 1953 and 1961 aeromagnetic data available to the public in advance of more formal publication. Only salient magnetic features are discussed in this preliminary presentation and in a second report on results obtained during the 1962 flying. A more complete interpretation of the entire aeromagnetic survey will be prepared.

Iowa↗
USGS1999

New reporting procedures based on long-term method detection levels and some considerations for interpretations of water-quality data provided by the U.S. Geological Survey National Water Quality Laboratory

This report describes the U.S. Geological Survey National Water Quality Laboratory's approach for determining long-term method detection levels and establishing reporting levels, details relevant new reporting conventions, and provides preliminary guidance on interpreting data reported with the new conventions. At the long-term method detection level concentration, the risk of a false positive detection (analyte reported present at the long-term method detection level when not in sample) is no more than 1 percent. However, at the long-term method detection level, the risk of a false negative occurrence (analyte reported not present when present at the long-term method detection level concentration) is up to 50 percent. Because this false negative rate is too high for use as a default 'less than' reporting level, a more reliable laboratory reporting level is set at twice the determined long-term method detection level. For all methods, concentrations measured between the laboratory reporting level and the long-term method detection level will be reported as estimated concentrations. Non-detections will be censored to the laboratory reporting level. Adoption of the new reporting conventions requires a full understanding of how low-concentration data can be used and interpreted and places responsibility for using and presenting final data with the user rather than with the laboratory. Users must consider that (1) new laboratory reporting levels may differ from previously established minimum reporting levels, (2) long-term method detection levels and laboratory reporting levels may change over time, and (3) estimated concentrations are less certain than concentrations reported above the laboratory reporting level. The availability of uncensored but qualified low-concentration data for interpretation and statistical analysis is a substantial benefit to the user. A decision to censor data after they are reported from the laboratory may still be made by the user, if merited, on the basis of the intended use of the data.

Open-File Report↗
USGS2009

Geophysical Interpretations of the Southern Espanola Basin, New Mexico, That Contribute to Understanding Its Hydrogeologic Framework

The southern Espanola basin consists of a westward- and northward-thickening wedge of rift fill, composed primarily of Santa Fe Group sediments, that serves as an important aquifer for the city of Santa Fe and surrounding areas. Detailed aeromagnetic surveys were flown to better understand ground-water resources in this aquifer. This report presents a synthesis of these data with gravity data and other constraints. The interpretations were accomplished using qualitative interpretation, state-of-art data analysis techniques, and two- and three-dimensional modeling. The results depict the presence of and depth to many geologic features that have hydrogeologic significance, including shallow faults, different types of igneous units, and basement rocks. The results are presented as map interpretations, geophysical profile models, and a digital surface that represents the base and thickness of Santa Fe Group sediments, as well as vector files of some volcanic features and faults.

Professional Paper↗
USGS1975

Paleotectonic investigations of the Pennsylvanian system in the United States, Part II: Interpretive summary and special features of the Pennsylvanian system

Interpretation of the paleotectonic history of the Pennsylvanian System is represented on two sets of maps, one referred to as interpretive isopach maps and the other as interpretive paleotectonic maps (pi. 15A-C). Each set includes five maps, one for each interval of the Pennsylvanian. All maps of both sets were prepared at the same scale (1:5,000,000) and are published at a scale of 1:10,000,000. Therefore direct comparisons can readily be made.

Professional Paper↗
USGS2012

TracerLPM (Version 1): An Excel® workbook for interpreting groundwater age distributions from environmental tracer data

TracerLPM is an interactive Excel® (2007 or later) workbook program for evaluating groundwater age distributions from environmental tracer data by using lumped parameter models (LPMs). Lumped parameter models are mathematical models of transport based on simplified aquifer geometry and flow configurations that account for effects of hydrodynamic dispersion or mixing within the aquifer, well bore, or discharge area. Five primary LPMs are included in the workbook: piston-flow model (PFM), exponential mixing model (EMM), exponential piston-flow model (EPM), partial exponential model (PEM), and dispersion model (DM). Binary mixing models (BMM) can be created by combining primary LPMs in various combinations. Travel time through the unsaturated zone can be included as an additional parameter. TracerLPM also allows users to enter age distributions determined from other methods, such as particle tracking results from numerical groundwater-flow models or from other LPMs not included in this program. Tracers of both young groundwater (anthropogenic atmospheric gases and isotopic substances indicating post-1940s recharge) and much older groundwater (carbon-14 and helium-4) can be interpreted simultaneously so that estimates of the groundwater age distribution for samples with a wide range of ages can be constrained. TracerLPM is organized to permit a comprehensive interpretive approach consisting of hydrogeologic conceptualization, visual examination of data and models, and best-fit parameter estimation. Groundwater age distributions can be evaluated by comparing measured and modeled tracer concentrations in two ways: (1) multiple tracers analyzed simultaneously can be evaluated against each other for concordance with modeled concentrations (tracer-tracer application) or (2) tracer time-series data can be evaluated for concordance with modeled trends (tracer-time application). Groundwater-age estimates can also be obtained for samples with a single tracer measurement at one point in time; however, prior knowledge of an appropriate LPM is required because the mean age is often non-unique. LPM output concentrations depend on model parameters and sample date. All of the LPMs have a parameter for mean age. The EPM, PEM, and DM have an additional parameter that characterizes the degree of age mixing in the sample. BMMs have a parameter for the fraction of the first component in the mixture. An LPM, together with its parameter values, provides a description of the age distribution or the fractional contribution of water for every age of recharge contained within a sample. For the PFM, the age distribution is a unit pulse at one distinct age. For the other LPMs, the age distribution can be much broader and span decades, centuries, millennia, or more. For a sample with a mixture of groundwater ages, the reported interpretation of tracer data includes the LPM name, the mean age, and the values of any other independent model parameters. TracerLPM also can be used for simulating the responses of wells, springs, streams, or other groundwater discharge receptors to nonpoint-source contaminants that are introduced in recharge, such as nitrate. This is done by combining an LPM or user-defined age distribution with information on contaminant loading at the water table. Information on historic contaminant loading can be used to help evaluate a model's ability to match real world conditions and understand observed contaminant trends, while information on future contaminant loading scenarios can be used to forecast potential contaminant trends.

Techniques and Methods↗
USGS1986

A data-management system for detailed areal interpretive data

A data storage and retrieval system has been developed to organize and preserve areal interpretive data. This system can be used by any study where there is a need to store areal interpretive data that generally is presented in map form. This system provides the capability to grid areal interpretive data for input to groundwater flow models at any spacing and orientation. The data storage and retrieval system is designed to be used for studies that cover small areas such as counties. The system is built around a hierarchically structured data base consisting of related latitude-longitude blocks. The information in the data base can be stored at different levels of detail, with the finest detail being a block of 6 sec of latitude by 6 sec of longitude (approximately 0.01 sq mi). This system was implemented on a mainframe computer using a hierarchical data base management system. The computer programs are written in Fortran IV and PL/1. The design and capabilities of the data storage and retrieval system, and the computer programs that are used to implement the system are described. Supplemental sections contain the data dictionary, user documentation of the data-system software, changes that would need to be made to use this system for other studies, and information on the computer software tape. (Lantz-PTT)

Water-Resources Investigations Report↗
USGS2024

Visual interpretation of high-resolution aerial imagery: A tool for land managers

Remotely sensed imagery from various collection platforms (e.g., satellites, crewed and uncrewed aircraft) are used by biologists and other conservation personnel to support management activities ranging from monitoring invasive species to assessing land cover and vegetation characteristics. Although remote sensing–based vegetation indices and models have been developed and used for some management applications, straightforward visual interpretation of imagery by on-the-ground personnel may be a pragmatic approach for obtaining time-sensitive and spatially relevant information to support and guide local management activities. Our primary objective was to qualitatively assess our ability to identify patches of target invasive plant species based on simple visual interpretation of high-resolution aerial imagery. We also sought to compare the high-resolution imagery to widely available imagery (e.g., National Agriculture Imagery Program) to determine the efficacy of each for assessing vegetation communities and land-cover features in support of management activities. To accomplish these objectives, we obtained high-resolution imagery and visually scanned and assessed the imagery by using standard geographic information system software. We were able to differentiate patches of crownvetch Securigera varia (L.) Lassen and wild parsnip Pastinaca sativa L., but not spotted knapweed Centaurea stoebe L. or leafy spurge Euphorbia esula L. The relative success in identifying these species had a relationship to plant characteristics (e.g., flower color and morphology, height), time of year (phenology), patch size and density, and potentially site characteristics such density of the underlying vegetation (e.g., grasses), substrate color characteristics (i.e., color contrast with flowers), and physical disturbance. Our straightforward, qualitative assessment suggests that visual interpretation of high-resolution imagery, but not some lower-resolution imagery, may be an efficient and effective tool for supporting local invasive species management through activities such as monitoring known patches, identifying undetected infestations, assessing management actions, guiding field work, or prioritizing on-the-ground monitoring activities.

Journal of Fish and Wildlife Management↗
USGS1986

Interpretation and compendium of historical fire accounts in the Northern Great Plains

This interpretation and compendium of historical fire accounts in the northern Great Plains provides resource managers with background information to justify the study or use of fire in management and provides a reference of historic fire accounts for those without ready access to major library collections. Historical accounts of fire are critiqued to aid interpreting the compendium accounts. An interpretation is included by the author.Lightning-set fires were recorded in the literature far less frequently than were Indian-set fires. The kinds of fire most frequently reported were scattered, single events of short duration and small extent. Although fires occurred in wetlands, wetlands as well as sandy soil sites usually were good areas for escape from the effects of fire. Both Indians and wild animals were reportedly injured or killed during prairie fires. The frequency of historic fires was less evident in the literature than the descriptions of fire distribution in time and space. Indian-set fires were reported in every month except January. Fires occurred mainly in two periods, March through May with a peak in April, and July to early November with a peak in October. Grassland fuels burned readily within a few hours or days after rain and even during light snowfall.I agree with arguments that support the concept that Indians of the northern Great Plains generally did not subscribe to annual wholesale or promiscuous burning practices, but that they did purposely use fire as a tool to aid hunting and gathering of food and materials. Apparently, the northern plains Indians did not pattern their use of fire with the seasonal patterns of lightning fires. More likely they developed seasonal patterns of burning the prairies in harmony with bison (Bison bison) herd movements because the hunter-gatherer economy of these nomadic tribes was centrally focused and largely dependent on bison and bison ecology.

Resource Publication↗
USGS1983

An interpretation of gravity and aeromagnetic surveys of the Greater Jiddah area, Kingdom of Saudi Arabia

A gravity survey of the Jiddah area between lat 21°23' and 21°48' N., long 39°03' and 39°20' E., carried out in early 1977, covers an area of 744 km 2 and includes 215 stations, at an average spacing of 1.9 km. The resulting simple Bouguer gravity anomaly map shows a series of four large (7-15 mgal) gravity anomaly highs along a north-trending gravity gradient located immediately west of outcrops of Precambrian rocks. The regional Bouguer gravity anomaly field of the map area decreases eastward at an approximately constant rate of about 1.1 mgal-km-1. Several anomalies suggest northeast-trending, right-lateral strike-slip faulting throughout the area. Interpretation of the Bouguer gravity anomaly map, combined with geologic, aeromagnetic, and paleomagnetic data, leads to an interpretive model in which the gravity gradient marks the boundary zone between Precambrian continental crust to the east and oceanic sea-floor crust overlain by marine sedimentary rocks to the west. The strike-slip faults in the area are probably the landward extension of offshore Red Sea transform faults, and, because they penetrate and offset continental crust, their study may yield useful information about the mechanism of transform faulting. The gravity anomaly highs are interpreted to result from shallow intrusive bodies, probably gabbroic in composition, that are buried a few hundred meters below the surface and that are approximately 2 km thick.

Report↗