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Lawrence C. Rowan

Publications and source records attributed to Lawrence C. Rowan.

At least 19 recordsLinked to original sources
USGS2011

ASTER spectral analysis and lithologic mapping of the Khanneshin carbonatite volcano, Afghanistan

Advanced Spaceborne Thermal and Reflection Radiometer (ASTER) data of the early Quaternary Khanneshin carbonatite volcano located in southern Afghanistan were used to identify carbonate rocks within the volcano and to distinguish them from Neogene ferruginous polymict sandstone and argillite. The carbonatitic rocks are characterized by diagnostic CO 3 absorption near 11.2 μm and 2.31–2.33 μm, whereas the sandstone, argillite, and adjacent alluvial deposits exhibit intense Si-O absorption near 8.7 μm caused mainly by quartz and Al-OH absorption near 2.20 μm due to muscovite and illite. Calcitic carbonatite was distinguished from ankeritic carbonatite in the short wave infrared (SWIR) region of the ASTER data due to a slight shift of the CO 3 absorption feature toward 2.26 μm (ASTER band 7) in the ankeritic carbonatite spectra. Spectral assessment using ASTER SWIR data suggests that the area is covered by extensive carbonatite flows that contain calcite, ankerite, and muscovite, though some areas mapped as ankeritic carbonatite on a preexisting geologic map were not identified in the ASTER data. A contact aureole shown on the geologic map was defined using an ASTER false color composite image (R = 6, G = 3, B = 1) and a logical operator byte image. The contact aureole rocks exhibit Fe 2+ , Al-OH, and Fe, Mg-OH spectral absorption features at 1.65, 2.2, and 2.33 μm, respectively, which suggest that the contact aureole rocks contain muscovite, epidote, and chlorite. The contact aureole rocks were mapped using an Interactive Data Language (IDL) logical operator. A visible through short wave infrared (VNIR-SWIR) mineral and rock-type map based on matched filter, band ratio, and logical operator analysis illustrates: (1) laterally extensive calcitic carbonatite that covers most of the crater and areas northeast of the crater; (2) ankeritic carbonatite located southeast and north of the crater and some small deposits located within the crater; (3) agglomerate that primarily covers the inside rim of the crater and a small area west of the crater; (4) a crater rim that consists mostly of epidote-chlorite-muscovite–rich metamorphosed argillite and sandstone; and (5) iron (Fe 3+ ) and muscovite-illite–rich rocks and iron-rich eolian sands surrounding the western part of the volcano. The thermal infrared (TIR) rock-type map illustrates laterally extensive carbonatitic and mafic rocks surrounded by quartz-rich eolian and fluvial reworked sediments. In addition, the combination of VNIR, SWIR, and TIR data complement one another in that the TIR data illustrate more laterally extensive rock types and the VNIR-SWIR data distinguish more specific varieties of rocks and mineral mixtures.

Geosphere↗
USGS2007

Mapping Phyllic and Argillic-Altered Rocks in Southeastern Afghanistan using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Data

Introduction: ASTER data and logical operators were successfully used to map phyllic and argillic-altered rocks in the southeastern part of Afghanistan. Hyperion data were used to correct ASTER band 5 and ASTER data were georegistered to orthorectified Landsat TM data. Logical operator algorithms produced argillic and phyllic byte ASTER images that were converted to vector data and overlain on ASTER and Landsat TM images. Alteration and fault patterns indicated that two areas, the Argandab igneous complex, and the Katawaz basin may contain potential polymetallic vein and porphyry copper deposits. ASTER alteration mapping in the Chagai Hills indicates less extensive phyllic and argillic-altered rocks than mapped in the Argandab igneous complex and the Katawaz basin and patterns of alteration are inconclusive to predict potential deposit types.

Open-File Report↗
USGS2007

Geologic Mapping and Mineral Resource Assessment of the Healy and Talkeetna Mountains Quadrangles, Alaska Using Minimal Cloud- and Snow-Cover ASTER Data

On July 8, 2003, ASTER acquired satellite imagery of a 60 km-wide swath of parts of two 1:250,000 Alaska quadrangles, under favorable conditions of minimal cloud- and snow-cover. Rocks from eight different lithotectonic terranes are exposed within the swath of data, several of which define permissive tracts for various mineral deposit types such as: volcanic-hosted massive sulfides (VMS) and porphyry copper and molybdenum. Representative rock samples collected from 13 different lithologic units from the Bonnifield mining district within the Yukon-Tanana terrane (YTT), plus hydrothermally altered VMS material from the Red Mountain prospect, were analyzed to produce a spectral library spanning the VNIR-SWIR (0.4 - 2.5 ?m) through the TIR (8.1 - 11.7 ?m). Comparison of the five-band ASTER TIR emissivity and decorrelation stretch data to available geologic maps indicates that rocks from the YTT display the greatest range and diversity of silica composition of the mapped terranes, ranging from mafic rocks to silicic quartzites. The nine-band ASTER VNIR-SWIR reflectance data and spectral matched-filter processing were used to map several lithologic sequences characterized by distinct suites of minerals that exhibit diagnostic spectral features (e.g. chlorite, epidote, amphibole and other ferrous-iron bearing minerals); other sequences were distinguished by their weathering characteristics and associated hydroxyl- and ferric-iron minerals, such as illite, smectite, and hematite. Smectite, kaolinite, opaline silica, jarosite and/or other ferric iron minerals defined narrow (< 250 m diameter) zonal patterns around Red Mountain and other potential VMS targets. Using ASTER we identified some of the known mineral deposits in the region, as well as mineralogically similar targets that may represent potential undiscovered deposits. Some known deposits were not identified and may have been obscured by vegetation- or snow-cover, or were too small to be resolved.

Open-File Report↗
USGS2007

Mapping known and potential mineral occurrences and host rocks in the Bonnifield Mining District using minimal cloud- and snow-cover ASTER data: Chapter E in Recent U.S. Geological Survey studies in the Tintina Gold Province, Alaska, United States, and Yukon, Canada--results of a 5-year project

On July 8, 2003, the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor acquired satellite imagery of a 60-kilometer-wide swath covering a portion of the Bonnifield mining district within the southernmost part of the Tintina Gold Province, Alaska, under unusually favorable conditions of minimal cloud and snow cover. Although rocks from more than eight different lithotectonic terranes are exposed within the extended swath of data, we focus on volcanogenic massive sulfides (VMS) and porphyry deposits within the Yukon-Tanana terrane (YTT), the largest Mesozoic accretionary terrane exposed between the Denali fault system to the south of Fairbanks and the Tintina fault system to the north of Fairbanks. Comparison of thermal-infrared region (TIR) decorrelation stretch data to available geologic maps indicates that rocks from the YTT contain a wide range of rock types ranging in composition from mafic metavolcanic rocks to felsic rock types such as metarhyolites, pelitic schists, and quartzites. The nine-band ASTER visible-near-infrared region--short-wave infrared region (VNIR-SWIR) reflectance data and spectral matched-filter processing were used to map hydrothermal alteration patterns associated with VMS and porphyry deposit types. In particular, smectite, kaolinite, opaline silica, jarosite and (or) other ferric iron minerals defined narrow (less than 250-meter diameter) zonal patterns around Red Mountain and other potential VMS targets. Using ASTER we identified some of the known mineral deposits in the region, as well as mineralogically similar targets that may represent potential undiscovered deposits. Some known deposits were not identified and may have been obscured by vegetation or snow cover or were too small to be resolved.

Alaska;Yukon↗
USGS1998

Remote sensing in the USGS Mineral Resource Surveys Program in the eastern United States

Mineral deposits commonly occur within special geologic units or structures, such as fault zones, which can be detected and mapped from aircraft and satellite images. Modern techniques analyze multispectral images that record the way solar energy is reflected or emitted by the materials exposed at the Earth's surface. In sparsely vegetated regions, including most of the Western United States, mineral composition is determined directly by analyzing the spectral properties of rock outcrops. In more densely vegetated terrain, such as the Eastern United States, rock and soil composition can be determined directly in manmade exposures, such as plowed fields and construction sites, or much more general determinations can be made indirectly by analyzing the distribution and apparent health of naturally occurring plants. The association of certain plants with particular rock or soil types has been known for decades. For example, coniferous trees grow preferentially on well-drained sandy soil, whereas deciduous trees dominate on shaly bedrock. These two forest types reflect solar radiation quite differently and, therefore, are distinguished readily in conventional aerial photographs. More subtle plant-bedrock associations require digital multispectral image analysis to infer compositional information from the spectral characteristics of the forest canopy.

Information Handout↗
USGS1987

Analysis of Shuttle Multispecral Infrared Radiometer measurements of the western Saudi Arabian shield.

During the November 12–14, 1981, mission of the space shuttle Columbia, the Shuttle Multispectral Infrared Radiometer (SMIRR) recorded radiances in ten channels along a 100 m wide groundtrack across the western Saudi Arabian shield. The ten channels are located in the 0.5 to 2.4 μm region, with five positioned between 2.0 and 2.40 μm for measuring absorption features that are diagnostic of OH‐bearing and CO 3 &#x2010; bearing "> CO 3 ‐ bearing minerals. This exceptionally well exposed area consists of late Proterozoic metamorphic, intermediate to silicic intrusive, and interlayered clastic sedimentary and intermediate silicic volcanic rocks that have not been studied previously using SMIRR data. Plots or traces of unnormalized SMIRR channel ratios were examined before field studies to locate areas with high spectral contrast, especially in the 2.0 μm to 2.40 μm channels. Reflectance spectra were measured in the laboratory for rock and soil samples collected in these areas, and the mineralogic causes of the main absorption features were determined using X‐ray diffraction. Laboratory SMIRR spectra were produced by convolving the ten SMIRR filters with the laboratory spectra. Then, normalized SMIRR reflectance spectra were generated along the groundtrack using normalization coefficients calculated for a field sample representing a uniform, low‐spectral contrast area. Field evaluation shows that unnormalized SMIRR ratio traces are useful, even without specific mineralogic information, for distinguishing rocks that are characterized by Al‐OH, Mg‐OH, and/or CO 3 "> CO 3 "> CO 3 , Fe 3 + "> Fe 3 + , and Fe2+ absorption features. Analysis of field samples permits suites of minerals causing absorption features to be identified. However, specific mineral identification cannot be achieved consistently using the SMIRR ratio traces or normalized SMIRR spectra, because the Al‐OH and Mg‐OH absorption features can be caused by more than one of the minerals commonly present. The normalized SMIRR spectra are especially useful for identifying subtle Al‐OH and Mg‐OH absorption features that are difficult to identify in the unnormalized ratio traces and for comparing the relative intensities of absorption features. Al‐OH absorption is related to muscovite, smectite, illite, and kaolinite, whereas Mg‐OH absorption is caused by chlorite, amphibole, and biotite. The principal sources of error in using SMIRR spectral measurements for identifying mineral groups along the orbit 27 groundtrack are inaccuracies in field location and lithologic heterogeneity that is not represented adequately by field samples. Calibration errors may account for systematic albedo and absorption intensity differences between calculated laboratory SMIRR spectra and normalized SMIRR spectra. SMIRR instrument noise and atmospheric factors appear to be less important sources of error. However, as higher spectral and spatial resolution systems are developed for mineral identification, radiometric precision and atmospheric factors will become more important.

Geophysics↗
USGS1980

Evaluation of new spectral bands for multi-spectral imaging: SMIRR aircraft test results

A 10-channel radiometer called the Shuttle Multispectral Infrared Radiometer (SMIRR) is scheduled to take data from orbit on the second shuttle orbital light test. As part of the instrument test sequence, a series of aircraft flights was carried out over 10 test areas in Utah and Nevada. Apart from vegetation, the materials exposed at the surface were volcanic sequences ranging from tuffs to basalts, areas of hydrothermally altered volcanic rocks, sedimentary sequences of sandstone and carbonate rocks, and alluvial cover.

Sixth Annual Pecora Symposium and Exposition↗
USGS1980

Need for new sensors to map lithologic units

One of the most important contributions that remote sensing can make to mineral energy explorations to provide data from satellites to augment regional geological mapping. Geologic maps, which show information on the subsurface, are the main basis for formulating models of resource genesis that guide exploration. However, conventional compilation procedures are time-consuming and therefore often slow the pace of exploration, especially in large, inaccessible areas. Landsat Multispectral Scanner (MSS) images have been applied to a wide variety of specific geological problems, including discrimination of lithologic and delineation of previously unrecognized tectonic features. However, these lithologic distinctions are based on brightness, spectral reflectance, and, less commonly, the morphology of the unit, which in the wavelength region of MSS images are only rarely diagnostic of specific mineralogical content. Limonite is the only lithological material that can be identified be analyzing MSS spectral radiance.

Sixth Annual Pecora Symposium and Exposition↗
USGS1979

Evaluation of Landsat multispectral scanner images for mapping altered rocks in the East Tintic Mountains, Utah

The East Tintic Mountains, Utah consist of folded and faulted Paleozoic sedimentary rocks, which are partly covered by Tertiary volcanic rocks. Clastic rocks dominate the lower one-third of the Paleozoic section, whereas carbonate rocks with subordinate amounts of shale and elastic rocks predominate in the remainder. Some of the rocks, especially the Tintic Quartzite and some shales, are commonly limonitic, an important factor in analysis of Landsat MSS images. The volcanic rocks, mainly tuffs, flows, and agglomerates of quartz latitic and latitic composition, are limonitic in a few places where hematite is present in the groundmass. Emplacement of monzonite and biotite monzonite porphyry bodies resulted in several types of altered rocks. Most widespread are argillized and silicified rocks, which are commonly bleached and limonitic. Locally, the intrusive rocks arc also altered. Hydrothermal dolomite is common in the northern part of the area, and in the East Tintic mining district, calcitic, chloritic, and weakly argillized volcanic rocks and pebble dikes are widespread. Volcanic rocks subjected to an early phase of "intravolcanic weathering" in this district are weakly altered but commonly limonitic. Barren as well as mineralized veins are present throughout the study area. In situ spectral reflectance curves representing the most abundant altered and unaltered rocks show that the argillized and silicified rocks generally have intense ferric-iron and hydroxyl absorption bands owing to the presence of iron-oxide and hydroxyl-bearing phases, respectively. These features are generally absent in the unaltered rocks, except the limonitic rocks, which have prominent iron absorption hands. Both spectral features are weakly expressed in the volcanic rocks subjected to accelerated weathering, On the other hand, hydrothermal dolomite and calcitic volcanic rocks generally lack both features, and thus are spectrally similar to the unaltered rocks. Chloritic rocks are of limited distribution and have not been measured spectrally. Most of the silicified and argillized areas are apparent in Skylab S190B, high altitude-, and low altitude color aerial photographs because of the high albedo of these rocks. However, many unaltered rocks have similar albedos and therefore are not distinguishable from the altered rocks. Moreover, very little color information is available in these photographs. These problems are further complicated by brightness variations related to topographic slope. MSS ratio images were generated to subdue the effects of topographic slope and albedo, and combined into several color composite images for displaying the spectral reflectance differences between the most widespread altered and unaltered rocks. The most effective combination proved to be MSS 4/5, MSS 4/6, and MSS 6/7 using blue, yellow and magenta diazo films, respectively, rather than the MSS 4/5, MSS 5/6, and MSS 6/7 combination used so successfully in south-central Nevada. Consideration of schematic frequency distributions of ratio values for these two areas suggests that the lack of enhancement of limonitic rocks in MSS 5/6 images of the present study area is due to the higher frequency of low ratios representing vegetation. Comparison of a limonitic bedrock map produced by scanning the optimum color-ratio composite image with a map of the silicified rocks shows good agreement, except where they are obscured by vegetation. Measurements of vegetation density indicate that shrub cover and. juniper, pinyon, and sage cover greater than 40-50 and 33-43 percent, respectively, obscure limonitic rocks in these images. Argillized rocks, the most widely distributed altered rock type, were consistently detected in exposed areas. On the other hand, hydrothermal dolomite and calcitic and chloritic volcanic rocks are not portrayed in the limonitic bedrock map because of their general lack of limonite. Some altered rocks, especially veins and pebble dikes, are too small to be detected by the MSS except where they are closely spaced and well exposed. Another important limitation is that exposures of unaltered limonitic sedimentary and volcanic rocks are included in the limonitic bedrock map. Analysis of in situ spectral reflectance measurements indicates that this limitation can be largely overcome by obtaining radiance information in the 2.2 and 1.6 μ m regions.

Utah↗
USGS1979

Geologic evaluation of major Landsat lineaments in Nevada and their relationship to ore districts

Analysis of diverse geologic, geophysical, and geochemical data shows that eight major lineament systems delineated in Landsat images of Nevada are morphological and tonal expressions of substantially broader structural zones. Southern Nevada is dominated by the 175 km-wide northwest-trending Walker Lane, a 150 km-wide zone of east-trending lineament systems consisting of the Pancake Range, Warm Springs, and Timpahute lineament systems, and a 125 km-wide belt of northeast-trending faults termed the Pahranagat lineament system. Northern Nevada is dominated by the northeast-trending 75-200km wide Midas Trench lineament system, which is marked by northeasterly-oriented faults, broad gravity anomalies, and the Battle Mountain heat flow high; this feature appears to extend into central Montana. The Midas Trench system is transected by the Northern Nevada Rift, a relatively narrow zone of north-northwest-trending basaltic dikes that give rise to a series of prominent aeromagnetic highs. The northwest-trending Rye Patch lineament system, situated at the northeast boundary of the Walker Lane, also intersects the Midas Trench system and is characterized by stratigraphic discontinuities and alignment of aeromagnetic anomalies. Field relationships indicate that all the lineament systems except for the Northern Nevada Rift are conjugate shears formed since mid-Miocene time during extension of the Great Basin. Metallization associated with volcanism was widespread along these systems during the 17-6 m.y. period. However, these zones appear to have been established prior to this period, probably as early as Precambr-an time. These lineament systems are interpreted to be old, fundamental, structural zones that have been reactivated episodically as stress conditions !changed in the western United States. Many metal districts are localized within these zones as magma rose along the pre-existing conduits.

Open-File Report↗
USGS1978

Discrimination of hydrothermally altered rocks along the Battle Mountain-Eureka, Nevada, mineral belt using Landsat images

Landsat Multispectral Scanner (MSS) images of the northwestern part of the Battle Mountain-Eureki, Nevada mineral belt were evaluated for distinguishing hydrothermally altered rocks associated with porphyry copper and disseminated gold deposits. Detection of altered rocks from Landsat is based on the distinctive spectral reflectance of limonite present at coatings on weathered surfaces Some altered rocks are visible as bleached areas in individual MSS bands; however, they cannot be consistently distinguished from unaltered rocks with high albedo nor from bright areas resulting .from topographic slope. Black-and-white ratio images were generated to subdue .topographic effects, and three ratio images were composited in color to portray spectral radiance differences, forming an image known as a color-ratio composite (CRC). The optimum CRC image for this area has MSS 4/5 as blue, MSS 4/6 as yellow, and MSS 6/7 as magenta, and differs in two respects from most CRC images of arid areas. First, as a result of the increased vegetation cover in the study area, MSS 5/6 was replaced by MSS 4/6 as the yellow layer. Second, 70 mm positive transparencies were replaced by large format images (64 cm), thereby improving the internal registration of the CRC image and the effective spatial resolution. The pattern of limonitic rocks depicted in the CRC closely agrees with the mapped pattern of the alteration zones at the Copper Canyon and Copper Basin porphyry copper deposits. Certain west-facing topographic slopes in the altered areas are depicted as unaltered in the CRC, apparently due to atmospheric scattering, and illustrate the need for atmospheric correction. The disseminated gold deposits at Gold Acres and Tenabo are poorly represented in the CRC because of the general absence of limonite on these deposits. The presence of unaltered limonitic sedimentary and volcanic rocks is the largest obstacle to discriminating altered areas within the mineral belt. Reflectance spectra, made in situ and in the laboratory indicate differences between altered and unaltered rocks in the Spectra region between 1.1 ?m and 2.5 ?m. Such differences may be detectable by a remote scanner with a longer wavelength range than current Landsat MSS (0.6 ?m-1.1 ?m).

Open-File Report↗
USGS1978

Applying satellite technology to energy and mineral exploration

IGCP Project 143 ("Remote Sensing and Mineral Exploration"), is a worldwide research project designed to make satellite data an operational geological tool along with the geologic pick, hand lens, topographic map, aerial photo and geophysical instruments and data that comprise the exploration package. While remote sensing data will not replace field exploration and mapping, careful study of such data prior to field work should make the effort more efficient.

Episodes↗
USGS1971

Relations of folded dikes and Precambrian polyphase deformation, Gardner Lake area, Beartooth Mountains, Wyoming

Two cross-cutting mafic dikes in the headwall of Gardner Lake in the eastern Beartooth Mountains, Wyoming, have structural relations with Archean migmatite and gneiss that suggest intrusion between deformational phases recognized in the eastern part of the range. Fabric data show that the older dike, an orthoamphibolite, was emplaced subsequent to the F 1 deformational event, but prior to the main episode of metamorphism, metasomatism, and folding, F 2 . The younger dike, a metanorite, was intruded after F 2 , but is folded by west-trending, open F 3 folds. The style and general trend of these F 3 folds are consistent with those observed elsewhere in the eastern part of the range. The data available and absolute age relations in other parts of the range indicate that F 2 occurred 2750 m.y. ago and F 3 1600 to 1800 m.y. ago.

Wyoming↗
USGS1969

Structural geology of the Quad-Wyoming-Line Creeks area, Beartooth Mountains, Montana

The Quad-Wyoming-Line Creeks area is in the northeastern part of the Beartooth Mountains of Montana. The rocks of the area consist mainly of banded migmatite, granitic gneisses, amphibolite, quartzite, and agmatite; small amounts of biotite schist and biotite gneiss, iron-silicate rocks, ultramafic rocks, mafic dikes, and felsic porphyries are also present. Quartzite outcrops continuously around the major folds, and agmatite is especially widespread in the axial zone of a major anti-form. Two fold sets have been revealed by detailed mapping (scale 1:7200) and statistical analysis of the penetrative subfabric elements. The large later folds, F 2 , are the south-plunging Quad Creek synform and the south-southwest-plunging Wyoming Creek antiform; they are nearly upright and non-isoclinal. A metanorite intrusion is located in the axial zone of the Quad Creek synform. Small F 2 fold style varies from similar in relatively ductile rocks to open, concentric folds in non-ductile rocks. Only one large F 1 , located immediately north of the metanorite pluton, has been mapped. F 1 folds are characteristically similar in style. Refolding of small F 1 folds by small F 2 folds has been observed throughout the map area. Statistical analysis of the compositional layering, So, and the axial planes of small F 1 folds, S 1 , shows that the πS 0 - and πS 1 -axes constructed in the Wyoming Creek antiform are nearly coincident with each other and with south-southwest-plunging maxima of small F 2 fold axes, B 2 . Both B 2 and the axial planes of F 2 folds, S 2 , are dispersed, showing that the later folds are noncylindrical and nonplane. The axes of small F 1 folds, B 1 , are generally dispersed along well defined π-girdles. B 1 and B 2 maxima are generally coincident at some stations in the Wyoming Creek antiform-axial zone, suggesting that B 1 and B 2 are, at least locally, collinear. The long axes of hornblende crystals, L 1 , are also dispersed, but are not parallel to B 1 ; the L 1 -subfabric appears to indicate that L 1 developed during the F 1 folding but plunged somewhat more steeply to the south-southwest to south than Bi and that the F 2 folds are concentric rather than similar in the amphibolite. In the Quad Creek synform (F 2 ), all of the above-mentioned fabric elements diverge around the north side of the metanorite body. The Precambrian deformational history of the present map area, and probably the remainder of the eastern Beartooth Mountains, appears to be comprised of two phases of folding. During the first phase (F 1 ) south-southwest plunging, isoclinal or nearly isoclinal folds formed by passive flow during metamorphism to at least the upper amphibolite facies; these F, folds were not upright and may have been nearly recumbent. L 1 was formed during this phase. Metamorphic differentiation of a sedimentary sequence occurred at this time, but it is not clear whether any new material was added. The second phase of deformation is characterized by upright, non-isoclinal folds which also formed about south-southwest to south plunging axes. Flexural flow was the dominant mechanism in the development of the Quad Creek synform and the Wyoming Creek antiform, but passive flow was important in layers of ductile rocks. Granitization was generally synchronous with the F 2 phase; pegmatite dikes were emplaced later in a passive manner. Quartzite and amphibolite acted as resisters to granitization; their termination in granitic gneisses and migmatites is usually most adequately explained by refolding rather than granitization of the quartzite. It is possible that partial melting occurred in some areas such as the agmatized zones. Time of emplacement of the metanorite body is still questionable, but it clearly predates the F 2 folding; intrusion prior to F 1 is probable because B 1 , B 2 , and L 1 diverge in the vicinity of the body.

Montana↗