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Geology of the Pacific Northwest

Basics -- Topographic & Geologic Maps

Introduction

Maps are essential tools in geology. In fact, maps are as important in geology as written texts are in the study of literature. By studying maps, a geologist can see the shape and geology of the earth's surface and deduce the geological structures that lie hidden beneath the surface.

It takes some training to read maps skillfully. Geologists are trained in map reading and map making. Many geologists have experience mapping some part of the earth's surface.

You are not expected to become a geological expert in reading maps. However, you will be expected to develop your map reading skills as you use maps to help you learn the geology of the Pacific Northwest.

What are topographic maps and why are they important?

A topographic map is one type of map used by geologists. Topographic maps are also a basic tool for hikers, planners who make decisions on zoning and construction permits, government agencies involved in land use planning and hazard assessments, and civil engineers. The topographic maps drawn and published by the U. S. Geological Survey portray the grids that are used on deeds to identify the location of real estate, so homeowners and property owners often find it useful to refer to topographic maps of their area.

Topographic maps show the three-dimensional shape of the land. Most topographic maps make use of contour lines to depict elevations above sea level. The contour lines reveal the shape of the land in the vertical direction, allowing the 3-dimensional shape of the land to be portrayed on a 2-dimensional map. When you know how to read contour lines, you can look at a topographic map and visualize the mountains, plains, ridges, or valleys that it portrays.

Topographic maps are important in geology because they portray the surface of the earth in detail. This view of the surface shows patterns that provide information about the geology beneath the surface.

The landforms of the earth result from surface processes such as erosion or sedimentation combined with internal geological processes such as magma rising to create a volcano or a ridge of bedrock being pushed up along a fault. By studying the shape of the earth's surface through topographic maps, geologists can understand the nature of surface processes in a given area, including zones of erosion, zones subjected to landslides, and zones of sediment accumulation. They can also find clues to the underlying geologic structure and geologic history of the area.

In addition to a topographic map, a complete understanding of the underlying geologic structure and history of an area requires completion of a geologic map and cross-sections. A topographic map provides the frame of reference upon which the geologic map is constructed.

Reading a Topographic Map

Reading a topographic map requires familiarity with how it portrays the three-dimensional shape of the land, so that in looking at a topographic map you can visualize the shape of the land. To read a topographic map, you need to understand the rules of contour lines.

Some Rules for Contour Lines

  1. On a topographic map, a contour line is a line of constant elevation. For example, every point on a 200-foot contour line represents a point on earth that is 200 feet above sea level.
  2. Contour lines never intersect. (A point on the surface of the earth cannot be at two different elevations).
  3. A constant specified vertical distance called the contour interval separates each contour line from adjacent contour lines. A commonly used contour interval is 40 feet. On a map with a 40-foot contour interval, the elevation difference between two contour lines that are next to each other is 40 feet, regardless of the physical distance between the two lines on the map.
  4. Points lying between contour lines must be interpolated to find the elevation. For example, a point lying midway between the contours 5440 and 5480 would be 5460 feet above sea level. (Assuming that the contour line elevations are given in feet and the contour interval is 40 feet.)
  5. Contour lines curve up stream when they cross a valley (and down hill as they cross ridges).
  6. Where contours are close together, the topography is steep; where they are far apart, the slopes are gentle.

What are geologic maps and why are they important?

A geologic map shows mappable rock units, mappable sediment units that cover up the rocks, and geologic structures such as faults and folds. A mappable unit of rock or sediment is one that a geologist can consistently recognize, trace across a landscape, and describe so that other people are able to recognize it and verify its presence and identity. Mappable units are shown as different colors or patterns on a base map of the geographic area.

Geologic maps are important for two reasons. First, as geologists make geologic maps and related explanations and cross-sections, they develop a theoretical understanding of the geology and geologic history of a given area.

Second, geologic maps are essential tools for practical applications such as zoning, civil engineering, and hazard assessment. Geologic maps are also vital in finding and developing geological resources, such as gravel to help build the road you drive on, oil to power the car you travel in, or aluminum to build the more fuel-efficient engine in your next vehicle. Another resource that is developed on the basis of geologic maps is groundwater, which many cities, farms, and factories rely on for the water they use.

Essential Components of Geologic Maps

A complete geologic map has at least two features: (1) the map itself, and (2) the map legend or key that explains all the symbols on the map. Professional geologic maps usually have two other components as well: (3) an accompanying explanation of the rock or sediment units, and (4) geologic cross-sections of the map area.

The legend or key to a geologic map is usually printed on the same page as the map and follows a customary format. The symbol for each rock or sediment unit is shown in a box next to its name and brief description. These symbols are stacked in age sequence from oldest at the bottom to youngest at the top. The geologic era, or period, or epoch--the geologic age--is listed for each rock unit in the key. By stacking the units in age sequence from youngest at the top to oldest at the bottom, and identifying which interval of geologic time each unit belongs to, the map reader can quickly see the age of each rock or sediment unit. The map key also contains a listing and explanation of the symbols shown on the map, such as the symbols for different types of faults and folds.

The explanations of rock units are often given in a separate pamphlet that accompanies the map. The explanations include descriptions with enough detail for any geologist to be able to recognize the units and learn how their ages were determined.

If included, cross-sections are usually printed on the same page as the geologic map. They are important accompaniments to geologic maps, especially if the map focuses on the geology of the bedrock underneath the soil and loose sediments.

Geologic Cross-Sections

A geologic cross-section is a sideways view of a slice of the earth. It shows how the different types of rock are layered or otherwise configured, and it portrays geologic structures beneath the earth's surface, such as faults and folds. Geologic cross-sections are constructed on the basis of the geology mapped at the surface combined with an understanding of rocks in terms of physical behavior and three-dimensional structures.

Web Links

To learn more about topographic maps visit the US Topo page http;//nationalmap.gov/ustopo/index.html.

The USGS website shows the standard US topographic map symbols. http://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf

Glossary terms that appear on this page: topographic map; contour line; contour interval geologic map; cross-section; fault; fold


Geology of the Pacific Northwest
Basics--Geologic Maps
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 9/12/13