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Reading a US Geological Survey Map

written by: Jayant R Row • edited by: Lamar Stonecypher • updated: 10/12/2010

We are more used to maps that indicate roads, towns and other details required for our travel or information. Geological survey maps indicate a number of details including what the engineer is likely to face below the ground.

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    The Need for Geological Survey Maps

    geology 1 

    The geology of any area can help to determine the presence of minerals, availability of groundwater, likelihood of landslides, potential for earthquake damage, and even the best plants that can grow in an area. It promotes the understanding of what lies below the ground and how it can affect life in that area. How to read a survey map that includes geological details includes the need to understand the symbols, colors, lines, and other information that are available.

    Image source: Wikimedia: Geological map of North America

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    Understanding the symbols on a geological survey map

    Survey map symbols for geological survey maps can be divided into letter, colors, and lines.

    Letter symbols indicate the age of the rocks and the type of rock. The history of the earth is divided into Eons, Eras, Periods, and Epochs, and this classification is based on the age of the fossils found in rocks. The age of these is determined by carbon dating, among other methods, which gives a fairly accurate idea of when the fossil was formed. A capital letter on a geological survey map represents the age of the particular geological unit in that survey. The smaller letters after the capital letter indicates the type of rock. Rocks are said to be from the Jurassic (P) (195 to 141 million years ago), Cretaceous (K)(141 to 65 million years ago), Tertiary (T)(65 to 2 million years ago), or Quaternary (Q)(2 million years ago until today) periods. Rocks can span two periods and would be indicated by a combination of the capital letters representing that period. Rocks that have no age determined would not have any letter in front of the geological name of the rock.

    Colors on a geological survey map indicate different geological units or rocks. The colors would indicate the rock that is seen at the surface and would disregard any soil on top of the first layer of rock. The type of rock would be indicated in the color scheme schedule and can even include rocks that have not yet been identified. Age differences in the same rock can also be indicated by different colors.

    A thin line on a geological survey map indicates a depositional contact between any two types of rocks like rock from lava formed on an existing rock layer. There are times when the position of the rocks vis-a-vis each other is changed due to movement caused by the shifting of the plates caused by earthquakes. Such positional contacts are indicated by thick lines and this line indicates a fault line along which earthquakes can occur. These fault lines would normally be in the same type of rock or geological unit. Geological units can get folded into wavelike shapes and this can cause the formation of an axis, which is represented in a geological survey map between a line that is in between that shown for depositional and fault contacts. Lines can also be dotted, dashed, or solid and indicate different things and dashes or dots can indicate the uncertainty of the information. Other symbols like triangles on a line can indicate different things like strike, thrust, dip, etc.

    A map key is essentially a part of all geological survey maps and would indicate the significance of the colors, lines and other symbols used in the map.

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    Other information on geological survey maps

    800px-Jerseyt geological map Contour lines on a geological survey map give an indication of the levels of the land being surveyed with respect to a known datum. Most maps are linked to sea level. These contour lines can never be discontinuous, and the figures generally written within the line would indicate the level of that particular contour. Such contour maps allow the geologist to know the shape of the land and easily indicate where ridges are formed or where valleys run. This can easily indicate the way water will run off from the land, and thus the route of rivers or streams.

    For civil engineers reading of a geological survey is important especially if they are concerned with projects that cover vast areas and distances. All the information on such maps will allow the engineer to understand the nature of the earth and land that he is trying to mold to suit the needs of his project whether it be a road, dam, irrigation, bridge, port, or other structures that cover large areas of land .

    Image Source: Wikimedia: Geological Map