Introduction to Maps and Geologic Time
Introduction to maps
Since Earth is a large, complex object, geologists must use a scaled-down replica, or model, on which to plot their data or to get around in the field. The most widely used tools used by geologists are maps. There are several types of maps, as described below.
Geologic maps show the ages, distribution, types, and structural features of rocks on the surface of the earth. A mappable unit of rock is called a formation. Geologic maps show these features at a greatly reduced scale (the relationship between a unit of distance on the map and a corresponding distance on the ground. Three types of scale are used. Verbal scale is a statement of the relationship between map and true distances (one inch on the map equals one mile on the ground; 1”=1 mile). Graphic scale is the use of a “bar scale” calibrated in miles or kilometers. Fractional scale is the use of a mathematical fraction relating one unit of map-distance to a corresponding number of similar units of ground-distance. For example, 1/62,500 means that one inch of map distance equals 62,500 inches (1 mile) of ground-distance.
We use geologic maps to tell us where oil and other natural resources might be, for land use planning, and a variety of other things. Digital Relief maps show the location of mountain ranges, valleys, and flat surfaces on the earth in two dimensions. Raised relief maps show the same features in three dimensions along with some hydrogeologic and man-made features. Raised relief maps also have contour lines (lines connecting points of equal elevation) on them to show the land elevation at a given area. The contour interval of the map is difference in elevation (vertical difference) between two adjacent contour lines. The total relief of a map can be calculated by subtracting the lowest elevation on a map from the highest elevation.
Purpose. To introduce different types of maps and to learn why maps are so important in the study of Geology.
Since Earth is a large and complex sphere, geologists must use a scaled-down replica, or model, on which to plot and portray information, etc. The most widely used tools to do so are maps, of which are there many. Maps are meant to be a 2-D visual representations of given areas that can be easily read and studied. They are so useful because on one colorful sheet of paper they provide people with an abundant amount of information that would be very tedious to obtain in any other way. Many maps have legends which in a succinct manner provide the necessary information to read the map. Geologic maps show the age, distribution, rock type, and structural features of the bedrock on the surface of the Earth. Bedrock consists of rock that occurs below the loose soil and young glacially deposited material. A mappable unit of rock is called a formation.
Geologic maps show features at a greatly reduced scale (the relationship between a unit of distance on the map and a corresponding distance on the ground). Three types of scale are used:
1) Verbal scale is a statement of the relationship between map and true distance (one inch on the map equals one mile on the ground; 1" = 1 mile).
2) Graphic scale is the use of a bar scale calibrated in miles or kilometers.
3) Fractional scale is the use of a mathematical fraction relating one unit of map distance to a corresponding number of similar units of ground distance. For example, 1/62,500 means that one inch of map distance equals 62,500 inches (1 mile) of ground distance.
We use geologic maps to tell us where oil and other natural resources might be found, land use planning, and for a variety of other purposes. Digital relief and physiographic maps show the location of mountain ranges, valleys, and other physical features on the Earth's surface in two dimensions. Raised relief maps show the same features in three dimensions, along with some hydrogeologic and man-made features. Raised relief maps also have contour lines (lines of equal elevation) to show the land elevation at a given area. The contour interval of the map is the difference in elevation (vertical difference) between two adjacent contour lines. The total relief of a map can be calculated by subtracting the lowest elevation on a map from the highest elevation. In a few weeks we will learn more about 2-D topographic maps (which also portray topography using contour lines) and later in the semester we will spend a lab making a simple geologic map.
Finally, some maps also show a cross-section view of the area. A cross-section depicts what features look like on and below the Earth's surface. It is comparable to cutting a loaf of bread and looking at it from the side (cross-section view) instead of looking only at its top surface (which would be the map view).
The Geologic Time Scale
Geology (the study of Earth) deals with a vast amount of time (4.6 billion years). Geologists have constructed a time scale which gives names to different periods of time in millions of years (see geologic timescale). Giving a name to a certain period of time is convenient – note how we refer to people born between 1945 and 1964 as ‘baby boomers’. Similarly, formations of sedimentary rocks that are deposited between 540 and 490 million years are referred to as Cambrian in age, whereas rocks deposited between 65 and 2 million years are called Tertiary in age.
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See examples of Bedrock and Surficial Geology of Ohio at Ohio Dept. of Natural Resources Geologic Survey site