Laboratory #3 Topography and Landscapes. Today's lab will be an introduction
to topographic maps and the shape of the land surface. Topographic maps are ...
Laboratory #3 Topography and Landscapes
Today's lab will be an introduction to topographic maps and the shape of the land surface. Topographic maps are maps that show the configuration of the Earth's surface by means of lines of equal elevation. They are constructed to scale, so that distance, directions, areas, elevations, slope angles and volumes can be measured with great accuracy. As scale models, they contain a vast store of information that may not be readily apparent to the casual observer. Topographic map - a two dimensional representation of a three dimensional land surface. The third dimension (height, or relief) is shown through the use of contour lines that represent areas of the same elevation. This vertical information is the distinguishing feature of a topographic map, making these maps invaluable in geologic and engineering studies. Topographic Quadrangles: Topographic maps published by the US Geological Survey depict rectangular sections of the Earth's surface called quadrangles; an area bounded by lines of latitude at the top (north) and bottom (south) and by lines of longitude on the left (west) and right (east), measured in degrees, minutes and seconds.
Latitude lines are measured from the equator to the poles. 90o S to 90o N. Longitude lines are measured east and west from the prime meridian (an imaginary line that runs through Greenwich, England). Think of them as the intersection lines of a glass plate that is pushed through a sphere to exactly split it in two, only there are several of these glass plates and they all meet at the north and south poles. 0o to 360o. For finer measurements, each degree can be broken down into subdivisions (60) of minutes; each minute can be broken down into 60 subdivisions called seconds. While the boundaries of a quadrangle map can vary, two sizes are most common: 15minute quadrangle maps and 7 1/2-minute quadrangle maps. A 15-minute topographic map represents an area that measures 15 minutes of latitude by 15 minutes of longitude. But what does this mean in terms of distance?
Map scales Maps are scale models, therefore are a miniature version of real world features Ratio scale - 1:24000 one inch on the map equals 24000 inches on the world Fractional scale - 1/24000 - same thing Verbal scale - saying the words "1 inch on the map equals 24000 inches on the World" Graphic scale bar scale shown on the bottom that graphically lays out what A mile is (a one inch bar on the bottom of the map will say it is equal to One mile) Contour lines connect all points on the map that have the same elevation above sea level. Contour lines cannot cross each other, or else you have two elevations in the same exact place… unless of course you are standing on a cliff. Where contour lines are spread out, you have a gentle slope. Where contour lines are close together, you have a steep slope. Individual contour lines are usually brown in color for land masses (blue for water, or bathymetry measurements). Heavy brown lines are referred to as index contours because they often have elevations printed on them. The distance (or elevation) between contours is referred to as the contour interval and is often specified on most topographic maps in feet or meters.
Elevations of specific points on topographic maps (tops of peaks, bridges, survey points) sometimes are indicated directly on the maps beside the symbols indicated for that purpose. The notation "BM" denotes a benchmark, a permanent marker placed by the USGS at the point indicated.
Relief is the difference in elevation between two points on a map. Local relief refers to adjacent hills and valleys; it can be determined from contours. Total relief is the difference in the elevation between the highest and lowest points on the map. Gradient is a measure of the steepness of a slope and is usually expressed in feet per mile or meters per kilometer. Gradient is determined by dividing the relief between two points by the distance between those two points.
Topographic profiles and vertical exaggeration: A topographic map provides an overhead view of an area, depicting features and relief by means of its symbols and contour lines. Occasionally a cross section of the topography is useful. A topographic profile is a cross section that shows the elevation and slopes along a given line.
How to construct a topographic profile: Step 1: On the map, draw a line of section along which the profile is to be constructed. Label the section line A-A'. Make sure the line intersects all of the features that you wish the profile to show. Step 2: On a strip of paper placed along the section line A-A', make tick marks at each place where a contour line intersects the section line, and note the elevation at the tick marks. Step 3: Draw the profile. On a separate sheet of paper, draw a series of equally spaced horizontal and parallel lines that are the same length as the line of section. Each horizontal line represents a constant elevation and therefore corresponds to a contour line. In the end, you will have a grid, with the horizontal axis representing the length of your profile, and the vertical axis the elevation of your profile. Take your strip of paper, line it up with the base of your horizontal lines. Mark a dot on the grid above it the strip for each elevation. Smoothly connect the dots to complete the topographic profile. Step 4: The vertical scale of your profile will vary greatly, depending on how you draw your grid. This difference causes an exaggeration of the vertical dimension. To calculate the vertical exaggeration, divide the vertical fractional scale (1 inch on your scale equals how many feet of elevation) by the horizontal fractional scale of your map.
Some examples:
LANDFORMS Landforms, river systems and coasts commonly have many distinctive characteristics that reflect certain facts about their origin and history.
Some of the geologic features we will be studying in class and lab, as shown above, can be studied via topographic maps, aerial photographs and relief maps. Next week, we will be focusing on river and glacial systems as it applies to the Genesee River. This week, we will be focusing on desert and coastline processes.
DESERTS AND DRYLANDS Drylands are lands in arid, semi-arid and dry sub-humid climates. These climatic zones are driven by global wind patterns, pressure systems, and the absence of appreciable rainfall. Many drylands have specific landforms that result primarily from processes associated with degradation (a state of declining agricultural productivity due to natural and/or human causes), erosion by streams and flash floods (fluvial processes), or erosion and deposition associated with the wind (eolian processes).
Two characteristics of dryland precipitation combine to create some of the landforms commonly associated with dessert morphology. First, rainfall is minimal. Second, when rainfall does occur, it is usually in the form of violent thunderstorms. The high volume of water falling in such storms causes flash floods over dry ground. These floods develop suddenly, have a high discharge, and last for a short period of time. They carve steep-walled canyons, often floored with gravel that is deposited as water flow decreases and ends. These steepwalled canyons are called arroyos (wadis, dry washes).
Flash flooding in arid regions also erodes vertical cliffs along the edges of hills. When bedrock lies roughly horizontal (remember Law of Superposition?), such erosion creates broad, flattopped mesas bounded by cliffs. In time, the mesas can erode to small, stout, barrel-like rock columns called buttes. In regions where the Earth's crust has been lengthened by tensional forces (pulled apart, like a mid-oceanic ridge), mountain ranges and basins develop by block faulting. The higher blocks (mountains, or ranges) are called horsts and the lower blocks (basins) are called grabens.
Some dryland landforms: • Alluvial fan - a fan-shaped, delta-like deposit of alluvium made at the mouth of a stream or arroyo, where it enters a graben, level-plain, or basin. • Bajada - a continuous apron of coalescing alluvial fans below the mountain front • Pediment - a gently inclined erosion surface in the upper part of the piedmont slope. It is carved into bedrock and generally has a thin veneer of alluvium • Playa - the shallow, almost flat, central part of a desert in which water gathers after a rain and evaporates to leave behind silt, clay and evaporites (salts). • Sand dune - a small hill, mound or ridge (linear or sinuous) of windblown sand
Unconsolidated sediment can be redistributed by vigorous wind action. This may happen wherever particle movement is not retarded by vegetation, moisture, and/or cementing materials. When wind transports and deposits sand, it creates sand dunes, and silty deposits called loess.
Types of sand dunes The size and shape of a sand dune is controlled by: 1. The strength and directional stability of prevailing winds 2. The rate and amount of sediment supplied 3. The type and amount of vegetation 4. The nature of the surface over which sand is transported
Barchan dunes are crescent-shaped and occur where sand supply is limited and wind direction is fairly constant. Barchans generally form around shrubs and large rocks, which serve as minor barriers to sand transportation. The horns, or tips, of the barchans point downwind. Transverse dunes occur where sand supply is greater. They form as ridges perpendicular to the prevailing wind direction. The crests of transverse dunes generally are sinuous. Parabolic dunes somewhat resemble barchan dunes, however their horns point in the upwind direction. They always form adjacent to blowouts, oval depressions from which the sandy sediments form the dunes. Longitudinal (linear) dunes occur where sand is abundant and cross winds merge to form these high, elongated dunes. They can be quite large (up to 200 km long and up to 100 m high). The crests of longitudinal dunes generally are straight to slightly sinuous.
SHORELINE PROCESSES The shorelines of lakes and oceans are among the most rapidly changing parts of the Earth's surface. All coastlines are subject to erosion by waves; depending on the nature of the coastline (soft sediments versus solid bedrock) the rate of erosion will vary. Several factors determine the characteristic landforms of shorelines. They include the shape of the shoreline, the materials that comprise the shoreline (rock, mud, loose sediment, concrete), the source and supply of sediments, the direction in which currents move along the shoreline and the effects of major storms. Most coastlines are also affected by changes in mean (average) sea level: •
A rising sea level creates a submergent coastline - one that is flooding and receding. Sealevel rise is caused either by water level rising (called a transgression) or by the land getting lower (subsidence).
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A falling sea level creates an emergent coastline - one that is being elevated above sea level and building out into the water. Sea level fall is caused either by the water level actually falling (called regression) or by rising of the land (uplift).
Some features of coastline landforms: • Barrier island - a long narrow island that parallels the mainland coastline and is separated from the mainland by a lagoon, tidal flat or salt marsh • Beach - a gently sloping deposit of sand or gravel along the edge of a shoreline • Berm crest - the highest part of a beach; it separates the foreshore (seaward part of the shoreline) from the backshore (landward part of a shoreline) • Washover fan - a fan-shaped deposit of sand or gravel deposited landward of the beach during a storm or high tide. • Estuary - a valley flooded by a rise in the level of an ocean or a lake • Longshore current - a water current in the surf zone (zone where waves break). It flows slowly parallel to the shoreline, driven by waves that were caused by the wind • Delta - a sediment deposit at the mouth of a river where it enters an ocean or a lake • Headland - projection of land that extends into an ocean or lake and generally has cliffs along its water boundary • Spit - a sand bar extending from the end of a beach in to the mouth of an adjacent bay • Tidal flat - marshy or sandy area that is covered with water at high tide and exposed at low tide • Wave-cut cliff - seaward facing cliff along a steep shore and formed by wave erosion • Marine terrace - an elevated platform that is bounded on its seaward side by a cliff or a steep slope (and formed when a wave-cut platform is elevated by uplift or regression)
