Contour Lines

Topographic maps are essential tools in geologic studies because tehey show the configuration of the earth's surface in remarkable detail and permit one to measure horizonial distances and vertical elevations.This is accomplished by means of carefully surveyed contour lines.A contour line is a line connecting points of equal elexation.Natural expressions of contour lines are illustrated by elevated shorelines,cultivated terraces or pallerns developed from contour plowing.
The concept of contours may be clearer to you if you consider an island in a lake and the patterns made by receding water level.The shoreline represents the same elevation all around the island and is thus a contour line(figure 115).Suppose that the water level of the lake should contour lines (the lake level and the old stranded beach)would now be indicated, and each woluld accurately depict the shape of the island at these two elevations.If the water level should continue to drop in increments of two feet,additional contours would be formed each acurately expressing the island's configuration at different elevations.
The relation between a contour map and the features it expresses is illustrated in Figure 116.The conical hills are represented by a series of roughly circular contour lines.As the elevation icreases to the peak of the hills,the circles become smaller.A careful study of this diagram reveals the following characteristics of contour lines:
1-Contour lines do not cross or divide
2-Closely spaced contour lines represent steep slopes,contours spaced far apart represented gentle slopes.
3-Contour lines trend up valleys,cross the stream and return down the valley on the opposite side.They thus form a V which points upstream.

On standard topographic maps,index contours(generally every fifth contour line) are numbered and printed darker for ease of identification.Lighter contours are not numbered,but their elevation can easily be determined.Count up or down from the nearest index contour and multiply by the contour interval.The contour interval is indicated on the margin of the map.The most frequent interval used on 7.1/2 minute and 15 minute quadrangles is 20 feet,although 5,10,40,50,100, and 200 feet ae used if needed to express the topography being mapped.

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