A moraine is any glacially formed accumulation of unconsolidated glacial debris that occurs in both and glaciated regions on Earth, through geomorphological processes. Moraines are formed from debris carried along by a glacier and consisting of somewhat rounded particles ranging in size from large boulders to minute glacial flour. Lateral moraines are formed at the side of the ice flow and terminal moraines at the foot, marking the maximum advance of the glacier. Other types of moraine include ground moraines, till-covered areas with irregular topography, medial moraines which are formed where two glaciers meet. Moraines may be composed of debris ranging in size from silt-sized glacial flour to large boulders; the debris is sub-angular to rounded in shape. Moraines may be on the glacier’s surface or deposited as piles or sheets of debris where the glacier has melted. Moraines may form through a number of processes, depending on the characteristics of sediment, the dynamics on the ice, the location on the glacier in which the moraine is formed.
Moraine forming processes may be loosely divided into active. Passive processes involve the placing of chaotic supraglacial sediments onto the landscape with limited reworking forming hummocky moraines; these moraines are composed of supraglacial sediments from the ice surface. Active processes form or rework moraine sediment directly by the movement of ice, known as glaciotectonism; these form push moraines and thrust-block moraines, which are composed of till and reworked proglacial sediment. Moraine may form by the accumulation of sand and gravel deposits from glacial streams emanating from the ice margin; these fan deposits may coalesce to form a long moraine bank marking the ice margin. Several processes may combine to form and rework a single moraine, most moraines record a continuum of processes. Moraines can be classified either by origin, location with respect to a glacier or former glacier, or by shape; the first approach is suitable for moraines associated with contemporary glaciers—but more difficult to apply to old moraines, which are defined by their particular morphology, since their origin is debated.
Some moraine types are known only from ancient glaciers, while medial moraines of valley glaciers are poorly preserved and difficult to distinguish after the retreat or melting of the glacier. Lateral moraines are parallel ridges of debris deposited along the sides of a glacier; the unconsolidated debris can be deposited on top of the glacier by frost shattering of the valley walls and/or from tributary streams flowing into the valley. The till is carried along the glacial margin; because lateral moraines are deposited on top of the glacier, they do not experience the postglacial erosion of the valley floor and therefore, as the glacier melts, lateral moraines are preserved as high ridges. Lateral moraines stand high because they protect the ice under them from the elements, causing it to melt or sublime less than the uncovered parts of the glacier. Multiple lateral moraines may develop as the glacier retreats. Ground moraines are till-covered areas with irregular topography and no ridges forming rolling hills or plains.
They are accumulated at the base of the ice as lodgment till, but may be deposited as the glacier retreats. In alpine glaciers, ground moraines are found between the two lateral moraines. Ground moraines may be modified into drumlins by the overriding ice. Rogen moraines or ribbed moraines are a type of basal moraines that form a series of ribs perpendicular to the ice flow in an ice sheet; the depressions between the ribs are sometimes filled with water, making the Rogen moraines look like tigerstripes on aerial photographs. Rogen moraines are named after Lake Rogen in Härjedalen, the landform’s type locality. End moraines, or terminal moraines, are ridges of unconsolidated debris deposited at the snout or end of the glacier, they reflect the shape of the glacier's terminus. Glaciers act much like a conveyor belt, carrying debris from the top of the glacier to the bottom where it deposits it in end moraines. End moraine size and shape are determined by whether the glacier is advancing, receding or at equilibrium.
The longer the terminus of the glacier stays in one place, the more debris accumulate in the moraine. There are two types of end moraines: recessional. Terminal moraines mark the maximum advance of the glacier. Recessional moraines are small ridges left. After a glacier retreats, the end moraine may be destroyed by postglacial erosion. Recessional moraines are observed as a series of transverse ridges running across a valley behind a terminal moraine, they form perpendicular to the lateral moraines that they reside between and are composed of unconsolidated debris deposited by the glacier. They are created during temporary halts in a glacier's retreat. A medial moraine is a ridge of moraine, it forms when two glaciers meet and the debris on the edges of the adjacent valley sides join and are carried on top of the enlarged glacier. As the glacier melts or retreats, the debris is deposited and a ridge down the middle of the valley floor is created; the Kaskawulsh Glacier in the Kluane National Park, has a ridge of medial moraine 1 km wide.
Supraglacial moraines are created by debris accumulated on top of glacial ice. This debris can accumulate due to ice flow toward the surface in the ablation zone, melting of surface ice or from debris that falls onto the glacier from valley sidewalls. Washboard moraines known as minor or corrugated moraines, are low-amplitude ge
George Wheeler (explorer)
George Montague Wheeler was an American pioneering explorer and cartographer, leader of the Wheeler Survey, one of the major surveys of the western United States in the late nineteenth century. He graduated from West Point in 1866, ranked sixth in his class, he was commissioned as a lieutenant in the US Army Corps of Engineers, his first exploration was in 1869, when Wheeler was 27 years old, on the staff of the Commanding General of the Department of California of the US Army. In 1872, the US Congress authorized an ambitious plan to map the portion of the United States west of the 100th meridian at a scale of 8 miles to the inch; this plan necessitated what became known as the Wheeler Survey, lasting until 1879, when the survey, along with the King and Powell Surveys, were terminated and their work was reorganized as the United States Geological Survey. Wheeler was promoted to captain in 1879. In 1881 he represented the United States at the Third International Geographical Congress and Exhibition in Venice, Italy.
He entered semi-retirement in 1883 but continued to write scientific reports until his full retirement from the army in 1888 at the rank of major. He died in New York City in 1905. Wheeler Peak in Nevada, Wheeler Peak in New Mexico, the scenic Wheeler Geologic Area in southern Colorado are named for George Wheeler. Preliminary report concerning explorations and surveys, principally in Nevada and Arizona Address of Lieut. Geo. M. Wheeler...before the American Geographical Society Report upon the determination of the astronomical co-ordinates of the primary stations at Cheyenne, Wyoming Territory, Colorado Springs, Colorado Territory Preliminary report upon a reconnaissance through southern and southeastern Nevada, made in 1869 A Guide to the Field notebooks of the Wheeler Survey, NC319. Special Collections, University Libraries, University of Nevada, Reno. USGS history circular
The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, its base, are somewhat in flux; the period was established by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells; as a result, our understanding of the Cambrian biology surpasses that of some periods. The Cambrian marked a profound change in life on Earth. Complex, multicellular organisms became more common in the millions of years preceding the Cambrian, but it was not until this period that mineralized—hence fossilized—organisms became common; the rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia; the seas were warm, polar ice was absent for much of the period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified; the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult since the occurrence of similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia and Newfoundland, in the western USA.
The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, in Spain. The Cambrian Period was followed by the Ordovician Period; the Cambrian is divided into ten ages. Only three series and six stages are named and have a GSSP; because the international stratigraphic subdivision is not yet complete, many local subdivisions are still used. In some of these subdivisions the Cambrian is divided into three series with locally differing names – the Early Cambrian, Middle Cambrian and Furongian. Rocks of these epochs are referred to as belonging to Upper Cambrian. Trilobite zones allow biostratigraphic correlation in the Cambrian; each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance: *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.
The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian was held to represent the first appearance of complex life, represented by trilobites; the recognition of small shelly fossils before the first trilobites, Ediacara biota earlier, led to calls for a more defined base to the Cambrian period. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, to be correlated worldwide by the earliest appearance of Treptichnus pedum. Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, it is the T. pedum ichnofossil assemblage, now formally used to correlate the base of the Cambrian. This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate.
A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, to the disappearance of distinctive Ediacaran fossils. There are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which w
White Pine County, Nevada
White Pine County is a rural, mountain county along the central eastern boundary of the U. S. state of Nevada. As of the 2010 census, the population was 10,030, its county seat is Ely. The name "White Pine" is an old name for a common tree in the county's mountains; the county boasts clean air and millions of acres of unspoiled public land. It is the home of Great Basin National Park, one of America's most remote and least visited national parks, it is home to no less than 14 federally designated wilderness areas, offering an abundance of terrain available to explore for hikers, skiers and anglers. The Ely Shoshone Indian Reservation is located on the south side of the City of Ely; the reservation has a land area of 104.99 acres and a 2000 census official resident population of 133 persons. European settlement in White Pine County began with mining exploration, activity generated by the Pony Express Trail, farming; the county was established by the Nevada legislature in 1869 from Lander County and named after the heavy growth of limber pine trees in the area, which were called white pine.
Hamilton was the first county seat from 1869 to 1887. According to the U. S. Census Bureau, the county has a total area of 8,897 square miles, of which 8,876 square miles is land and 21 square miles is water. Several sections of the Humboldt-Toiyabe National Forest exist within the county, within the Snake Range, Egan Range, White Pine Range, Ruby Mountains, Schell Creek Range; the county contains Ward Charcoal Ovens State Historic Park and Cave Lake State Park. In the southeastern part of the county within Great Basin National Park lies 13,065 ft Wheeler Peak, the tallest independent mountain within Nevada and the second-highest point within the state, it is the most topographically prominent peak in the county and the second-most prominent peak in Nevada. White Pine County is home to a number of designated wilderness areas, they were created by Congress on December 20, 2006, by the "White Pine County Conservation and Development Act of 2006." About half are integral parts of Humboldt National Forest.
The rest are managed by the Bureau of Land Management. One is shared between the two agencies; some extend into neighboring counties. Bald Mountain Wilderness Becky Peak Wilderness Bristlecone Wilderness Currant Mountain Wilderness in Nye County, NV Goshute Canyon Wilderness Government Peak Wilderness High Schells Wilderness Highland Ridge Wilderness Mount Grafton Wilderness in Lincoln County, NV Mount Moriah Wilderness Red Mountain Wilderness in Nye County, NV Shellback Wilderness South Egan Range Wilderness in Lincoln County, NV. Although mining has been a major employer throughout the county's history and services related to outdoor recreation are beginning to account for a larger share of jobs in the region; as of the census of 2000, there were 9,181 people, 3,282 households, 2,159 families residing in the county. The population density was 1.0 people per square mile. There were 4,439 housing units at an average density of 0.50 per square mile. The racial makeup of the county was 86.35% White, 4.14% Black or African American, 3.29% Native American, 0.78% Asian, 0.24% Pacific Islander, 3.09% from other races, 2.10% from two or more races.
10.98% of the population were Hispanic or Latino of any race. There were 3,282 households out of which 31.20% had children under the age of 18 living with them, 51.80% were married couples living together, 9.3% had a female householder with no husband present, 34.2% were non-families. 29.6% of all households were made up of individuals and 11.5% had someone living alone, 65 years of age or older. The average household size was 2.42 and the average family size was 3.01. In the county, the population was spread out with 24.2% under the age of 18, 7.6% from 18 to 24, 29.9% from 25 to 44, 24.8% from 45 to 64, 13.5% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 128.6 males. For every 100 females age 18 and over, there were 138.5 males. The median income for a household in the county was $36,688, the median income for a family was $44,136. Males had a median income of $36,083 versus $26,425 for females; the per capita income for the county was $18,309.
About 10.3% of families and 11.0% of the population were below the poverty line, including 11.8% of those under age 18 and 7.6% of those age 65 or over. As of the 2010 United States Census, there were 10,030 people, 3,707 households, 2,344 families residing in the county; the population density was 1.1 inhabitants per square mile. There were 4,498 housing units at an average density of 0.5 per square mile. The racial makeup of the county was 85.5% white, 4.2% American Indian, 3.9% black or African American, 1
James H. Simpson
James Hervey Simpson was an officer in the U. S. Army and a member of the United States Corps of Topographical Engineers, he was born in New Jersey on March 9, 1813, the son of John Simpson and Mary Brunson. He graduated from the United States Military Academy in 1832 and was assigned to the 3rd U. S. Artillery, he served in the Second Seminole War and was promoted to first lieutenant in 1837. In 1838, a separate department known as the U. S. Army's Topographical Engineers was created. Simpson was one of the officers transferred to the newly created bureau and assigned as an assistant to Cpt. W. G. Williams, in charge of harbor construction on Lake Erie; the following year, he worked on road construction in Florida and lake surveys in Wisconsin and Ohio. From 1845 to 1847, he was in charge of the harbor of Erie. In 1849, Simpson surveyed areas in the American Southwest, between Santa Fe and the Navajo tribal lands, he had recruited wilderness artists Edward and Richard Kern to record the expedition in watercolors, oils and maps.
He surveyed a road from Fort Smith, Arkansas to Sante Fe, New Mexico and served for a year as the Chief Topographical Engineer for the Department of New Mexico. After six months sick leave, Simpson returned to duty and was transferred to St. Paul, Minnesota in 1851 where he spent the next five years overseeing the roads of the territory. During this period, he was promoted to captain. From June 1856 to February 1858, Simpson was engaged in coastal survey of Florida. In early 1858, Simpson was ordered to join the Army's reinforcements for the Utah War, he and his team resurveyed the trails from Fort Leavenworth to Utah and his photographer, Samuel C. Mills, produced the earliest surviving photographs of features along the trail. Upon his arrival at Camp Floyd, he was directed to open a new road between that post and Fort Bridger. Simpson and his team surveyed the military reservation at Fort Bridger, at Camp Floyd and in the Rush Valley. In May 1859, he headed an expedition to survey a new route from Camp Floyd across the Great Salt Lake Desert of Utah and through the Great Basin to Genoa, Nevada near California.
The Army contracted Frederick Lander to to develop the more direct route to California for use by wagons, Simpson's survey was published in 1876. Simpson's Central Route played a vital role in the transportation of mail and passengers between the established eastern states and California when hostilities of the Civil War closed the Butterfield Overland Mail stagecoach route that ran along the southern border states. George Chorpenning switched to Simpson's route to run his existing mail and stage line, the Pony Express used it as well. In 1861 the Transcontinental Telegraph was laid along the route. Afterwards, Wells Fargo & Co. hauled mail and passengers along Simpson's route until 1869, when transportation and telegraphy were switched to the newly completed Transcontinental Railroad. During the American Civil War, Simpson served as colonel of the 4th New Jersey Volunteer Infantry and was captured at the Battle of Gaines's Mill, he was paroled and returned, but resigned his commission of volunteers on August 24, 1862 to return to his rank of major in the Corps of Topographical Engineers.
He was attached to the Department of the Ohio and was in charge of the expansion and improvements of the defenses for Cincinnati, Ohio. For his services during the war, Simpson was brevetted to the rank of brigadier general. Simpson was named chief engineer of the Interior Department, he oversaw the construction of the Transcontinental Railroad, the completion of which made his Central Nevada Route obsolete. In 1880 he retired to St. Paul and died there on March 2, 1883; the Simpson Park Mountains in central Nevada, a small range in west-central Utah, Simpson Springs Pony Express Station are all named after him. Navajo Expedition: Journal of a Military Reconnaissance from Santa Fe, New Mexico to the Navajo Country, Made in 1849 The Shortest Route to California: Illustrated by a History of Explorations of the Great Basin of Utah with its Topographical and Geological Character and Some Account of the Indian Tribes Essay on Coronado's March in Search of the Seven Cities of Cibola Report of Explorations across the Great Basin in 1859 "Dictionary of American Biography", vol.
IX, p. 179. HQ. USACE. Army.mil Works by or about James H. Simpson at Internet Archive
In modern mapping, a topographic map is a type of map characterized by large-scale detail and quantitative representation of relief using contour lines, but using a variety of methods. Traditional definitions require a topographic map to show both man-made features. A topographic survey is published as a map series, made up of two or more map sheets that combine to form the whole map. A contour line is a line connecting places of equal elevation. Natural Resources Canada provides this description of topographic maps:These maps depict in detail ground relief, forest cover, administrative areas, populated areas, transportation routes and facilities, other man-made features. Other authors define topographic maps by contrasting them with another type of map. However, in the vernacular and day to day world, the representation of relief is popularly held to define the genre, such that small-scale maps showing relief are called "topographic"; the study or discipline of topography is a much broader field of study, which takes into account all natural and man-made features of terrain.
Topographic maps are based on topographical surveys. Performed at large scales, these surveys are called topographical in the old sense of topography, showing a variety of elevations and landforms; this is in contrast to older cadastral surveys, which show property and governmental boundaries. The first multi-sheet topographic map series of an entire country, the Carte géométrique de la France, was completed in 1789; the Great Trigonometric Survey of India, started by the East India Company in 1802 taken over by the British Raj after 1857 was notable as a successful effort on a larger scale and for determining heights of Himalayan peaks from viewpoints over one hundred miles distant. Topographic surveys were prepared by the military to assist in planning for battle and for defensive emplacements; as such, elevation information was of vital importance. As they evolved, topographic map series became a national resource in modern nations in planning infrastructure and resource exploitation. In the United States, the national map-making function, shared by both the Army Corps of Engineers and the Department of the Interior migrated to the newly created United States Geological Survey in 1879, where it has remained since.1913 saw the beginning of the International Map of the World initiative, which set out to map all of Earth's significant land areas at a scale of 1:1 million, on about one thousand sheets, each covering four degrees latitude by six or more degrees longitude.
Excluding borders, each sheet was up to 66 cm wide. Although the project foundered, it left an indexing system that remains in use. By the 1980s, centralized printing of standardized topographic maps began to be superseded by databases of coordinates that could be used on computers by moderately skilled end users to view or print maps with arbitrary contents and scale. For example, the Federal government of the United States' TIGER initiative compiled interlinked databases of federal and local political borders and census enumeration areas, of roadways and water features with support for locating street addresses within street segments. TIGER was used in the 1990 and subsequent decennial censuses. Digital elevation models were compiled from topographic maps and stereographic interpretation of aerial photographs and from satellite photography and radar data. Since all these were government projects funded with taxes and not classified for national security reasons, the datasets were in the public domain and usable without fees or licensing.
TIGER and DEM datasets facilitated Geographic information systems and made the Global Positioning System much more useful by providing context around locations given by the technology as coordinates. Initial applications were professionalized forms such as innovative surveying instruments and agency-level GIS systems tended by experts. By the mid-1990s user-friendly resources such as online mapping in two and three dimensions, integration of GPS with mobile phones and automotive navigation systems appeared; as of 2011, the future of standardized, centrally printed topographical maps is left somewhat in doubt. Topographic maps have multiple uses in the present day: any type of geographic planning or large-scale architecture; the various features shown on the map are represented by conventional symbols. For example, colors can be used to indicate a classification of roads; these signs are explained in the margin of the map, or on a separately published characteristic sheet. Topographic maps are commonly called contour maps or topo maps.
In the United States, where the primary national series is organized by a strict 7.5-minute grid, they are called topo quads or quadrangles. Topographic maps conventionally show land contours, by means of contour lines. Contour lines are curves. In other words, every point on the marked line of 100 m elevation is 100 m above mean sea level; these maps show
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, the natural hazards that threaten it; the organization has four major science disciplines, concerning biology, geography and hydrology. The USGS is a fact-finding research organization with no regulatory responsibility; the USGS is a bureau of the United States Department of the Interior. The USGS employs 8,670 people and is headquartered in Reston, Virginia; the USGS has major offices near Lakewood, Colorado, at the Denver Federal Center, Menlo Park, California. The current motto of the USGS, in use since August 1997, is "science for a changing world." The agency's previous slogan, adopted on the occasion of its hundredth anniversary, was "Earth Science in the Public Service." Since 2012, the USGS science focus is directed at six topical "Mission Areas", namely Climate and Land Use Change, Core Science Systems, Ecosystems and Minerals and Environmental Health, Natural Hazards, Water.
In December 2012, the USGS split the Energy and Minerals and Environmental Health Mission Area resulting in seven topical Mission Areas, with the two new areas being: Energy and Minerals and Environmental Health. Administratively, it is divided into six Regional Units. Other specific programs include: Earthquake Hazards Program monitors earthquake activity worldwide; the National Earthquake Information Center in Golden, Colorado on the campus of the Colorado School of Mines detects the location and magnitude of global earthquakes. The USGS runs or supports several regional monitoring networks in the United States under the umbrella of the Advanced National Seismic System; the USGS informs authorities, emergency responders, the media, the public, both domestic and worldwide, about significant earthquakes. It maintains long-term archives of earthquake data for scientific and engineering research, it conducts and supports research on long-term seismic hazards. USGS has released the UCERF California earthquake forecast.
As of 2005, the agency is working to create a National Volcano Early Warning System by improving the instrumentation monitoring the 169 volcanoes in U. S. territory and by establishing methods for measuring the relative threats posed at each site. The USGS National Geomagnetism Program monitors the magnetic field at magnetic observatories and distributes magnetometer data in real time; the USGS collaborates with Canadian and Mexican government scientists, along with the Commission for Environmental Cooperation, to produce the North American Environmental Atlas, used to depict and track environmental issues for a continental perspective. The USGS operates the streamgaging network for the United States, with over 7400 streamgages. Real-time streamflow data are available online. National Climate Change and Wildlife Science Center implements partner-driven science to improve understanding of past and present land use change, develops relevant climate and land use forecasts, identifies lands and communities that are most vulnerable to adverse impacts of change from the local to global scale.
Since 1962, the Astrogeology Research Program has been involved in global and planetary exploration and mapping. In collaboration with Stanford University, the USGS operates the USGS-Stanford Ion Microprobe Laboratory, a world-class analytical facility for U--Pb geochronology and trace element analyses of minerals and other earth materials. USGS operates a number of water related programs, notably the National Streamflow Information Program and National Water-Quality Assessment Program. USGS Water data is publicly available from their National Water Information System database; the USGS operates the National Wildlife Health Center, whose mission is "to serve the nation and its natural resources by providing sound science and technical support, to disseminate information to promote science-based decisions affecting wildlife and ecosystem health. The NWHC provides information, technical assistance, research and leadership on national and international wildlife health issues." It is the agency responsible for surveillance of H5N1 avian influenza outbreaks in the United States.
The USGS runs 17 biological research centers in the United States, including the Patuxent Wildlife Research Center. The USGS is investigating collaboration with the social networking site Twitter to allow for more rapid construction of ShakeMaps; the USGS produces several national series of topographic maps which vary in scale and extent, with some wide gaps in coverage, notably the complete absence of 1:50,000 scale topographic maps or their equivalent. The largest and best-known topographic series is the 7.5-minute, 1:24,000 scale, quadrangle, a non-metric scale unique to the United States. Each of these maps covers an area bounded by two lines of latitude and two lines of longitude spaced 7.5 minutes apart. Nearly 57,000 individual maps in this series cover the 48 contiguous states, Hawaii, U. S. territories, areas of Alaska near Anchorage and Prudhoe Bay. The area covered by each map varies with the latitude of its represented location due to convergence of the meridians. At lower latitudes, near 30° north, a 7.5-minute quadrangle contains an area of about 64 square miles.
At 49° north latitude, 49 square miles are contained within a quadrangle of that size. As a unique non-metric map scale, the 1:24,000 scale requires a separate and specialized romer scale for pl