Geology Cafe

Introduction to Geology

Chapter 4 - Understanding Maps

Geologic maps and topographic maps are perhaps the most important tools for evaluating landscapes for a host of issues involving land use and natural resources,above and below the surface. Maps have been used by back into prehistoric times. However, the evolution of maps in the modern digital world has changed map making—enhancing their use in nearly all aspects of modern science, technology, and culture. Modern maps are created with "geographic information systems (GIS)"—computer-based map-generating programs can combine geographic (spatial) information with many kinds of databases (medical, commercial, civic infrastructure, biological, satellite data, etc.). Whether printed on paper or in a digital format, geologic maps are essential to understanding local and regional geology, including natural resources (minerals, water, soils) and natural hazards (earthquake faults, volcanic hazards, flooding and landslide hazards).

Standard map resources:

Google Maps
provides world-wide map information of basic geography (roads, towns, parks, water, landscape features) or on satellite imagery.
Google Earth is an example interactive geographic information system (requires software download). Many kinds of information linked to a database containing geographic reference data can be imported into Google Earth.
ESRI is a corporation supporting the most widely used map editing (GIS) software in education, industry, and government.

1. What is a map?
2. Essential concepts associated with common topographic maps (topography, bathymetry, relief)
3. What are Latitude and Longitude?
4. What is the Public Land Survey System?
5. Standard Maps and Digital Map Data produced by the US Geological Survey.
6. Define geomorphology and and the concept of physiographic provinces.
7. What is a Geologic Map?
8. Finding Geologic Map Information
9. Explain the uses of maps and geologic illustrations in understanding geology.

Click on images for a larger view throughout this website.
Blank line map of the United States of America
Line map of United States
Keywords and Essential Concepts

1. What is a map?

map—a diagrammatic representation of the earth's surface or part of it, showing the geographical distributions, positions, etc., of natural or artificial features such as roads, towns, relief, rainfall, etc. Maps are a scaled, 2 dimensional representation of the surface of an area or region. Some maps attempt to portray 3rd dimensional landscape features, such as mountains or canyons. Maps may represent the surface of the land or regions in and around lakes and oceans, the seafloor, or features known or inferred to occur underground. Mapping is also used in astronomy (planet and moons, regions of space, etc.)

Types of Maps (American Geological Institute, Earth Comm website provides links to many map sources)

All "good" maps include:

title A title should include concise information related to geographic information and theme (focus) of the map's content.
scale Map scale refers to the relationship (or ratio) between distance on a map and the corresponding distance on the ground.(relative to both miles and kilometers, example: 1 inch = 2000 feet)
orientation Maps should include a north arrow and corner coordinates information (longitude & latitude)
legend explanation of symbols used on map, including colors, lines, icons, and symbols.
selected reference locations or features for orientation (such as cities, towns, highways, state boundaries, coastlines, mountain peaks, rivers, etc.)
source information authors, publisher, associated publications (text or guidebook), complete bibliographic information.
base map information (What was the base source of geographic information of a map, such as a USGS topographic map or satellite image? What is the map projection?
date published What year was the map released (or revised). Is the data new or old data?
written text Is there a publication (pamphlet or guide book) associated with this map?

2. Essential concepts associated with common topographic maps (topography, bathymetry, relief)

—the arrangement of the natural and artificial physical features of an area. A topographic map is a graphical representation of a landscape showing selected natural and artificial landscape feature including topographic relief.

relief—the variation in elevation on the surface of the Earth (topography). Areas of high relief have much elevation changes over distance, such as mountainous areas and canyons. Low relief occurs where elevation changes are minimal, such as on coastal plains.

bathymetry—the measurement or mapping of water depth (sea bed) beneath oceans, lakes, or other bodies of water.

Topography (relief) and bathymetry are graphically illustrated with contours or shaded relief.

topographic map—map showing relief and man-made features of a portion of a land surface distinguished by portrayal of position, relation, size, shape, and elevation of the features. Topographic maps have contours, which are lines that represent the location of equal elevations, typically measured in feet or meters above standard mean sea level. Standard USGS topographic maps are 7.5 minute quadrangles with scale of 1:24,000 (see more discussion below).
Chittenden 7.5 minute quadrangle
Example of a USGS topographic map: Chittenden, CA 7.5 minute quadrangle

3. What are Latitude and Longitude?

Locations on the Earth's surface are defined using latitude and longitude coordinate system.

latitude—The angular distance of a place north or south of the earth's equator, usually expressed in degrees and minutes. Lines of latitude are called parallels. Latitude lines parallel the Equator. Each degree of latitude is approximately 69 miles (111 kilometers) apart.

—the angular distance of a place east or west of the Prime Meridian usually expressed in degrees and minutes. In order to make an accurate map of the stars for use in ship navigation, in 1884, a location indicating the precise location of 0° East-West was designated in the cross hairs of a telescope in the Royal Observatory (now located on the grounds of the National Maritime Museum) in Greenwich England. This line marks the reference location of the Prime Meridian now used in all global mapping (including GPS location systems).

A meridian is a circle of constant longitude passing through a given place on the earth's surface and the terrestrial poles. Longitude lines (of equal spacing measured in degrees) are widely spaced at the equator but converge at point at the North and South Poles. The Prime Meridian is designated 0° (zero degrees). Meridian lines east of the Prime Meridian are designated positive values (0° to 180° east); whereas meridian lines west of the Prime Meridian are designated negative values (-0° to -180°). At 180° east or west is the International Date Line. A degree of longitude is widest at the equator at 69.172 miles (111.321) and gradually shrinks to zero at the poles. At 40° north or south the distance between a degree of longitude is 53 miles (85 km).

Defining locations with a latitude-longitude coordinate system
—any location on the planet surface can be defined by a number in degrees, minutes, and seconds north or south of the Equator and east or west of the Prime Meridian. (Compare to hours, minutes, seconds on a clock!)

Example: Location of the Statue of Liberty in New York Harbor

The standard coordinates of the are:
Latitude: 40°68′92"N
Longitude: 74°04′ 45"W.

Described in decimal degrees the coordinates of the Statue of Liberty are: Latitude:40.689758°

Find the latitude and longitude of any named location or landscape feature on the GeoNames website.

Global Positioning System (GPS)—a space-based global navigation satellite system that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and where there is an unobstructed line of sight to four or more GPS satellites.

quadrangle—a standardized area used in mapping to designated an area on the Earth's surface. In the United States, the area shown on one of the standard 7.5 minute quadrangle map sheets (published by the U.S. Geological Survey): approximately 17 miles (27 km) north to south and from 11 to 15 miles (17 to 24 km) east to west. In a 1° by 1° quadrangle area, there are sixty-four 7.5 minute quadrangles.

The earth is round (a sphere like a globe) but maps are flat. As a result, maps that show large regions are distorted. Map projections are attempts to portray a portion of the earth on a flat surface. The flattening of a map always causes some distortions of distance, direction, scale, and area. Large scale maps (such as a map of a continent or a world show much distortion, however, maps on small scales (such as a map of a town or neighborhood) have relatively little distortion. Learn more about map projections at the U.S. Geological Survey Map Projections website: Projections/projections.html

Map of world, Mercator Projection Map of North America Lambert Projection Globe view of Earth from space
Map of world showing with Mercator Projection - notice distortion in high latitudes because longitude lines are not converging Map of North America with Lambert Conic Projection - on this scale distortion of America is minimal, but look at South America. A global view is the only way to have perfect map projection!
Global projection Mercator Map
Longitude and Latitude projected on a globe Map of the world showing latitude and longitude in a Mercator (flat) projection
California Topographic Map Index California 7.5 minute quadrangle index
California Map Index for USGS 30 x 60 minute quadrangle designations. (Equals 0.5º latitude and 1.0º longitude on each side of quadrangle map.) Portion of the California USGS Topographic Map Index showing map 7.5 minute quadrangle designations.
Devils Tower satellite image from Google Devils Tower Quadrangle
Google Maps satellite view of Devils Tower, Wyoming Part of Devils Tower 7.5 Minute Quadrangle Map, Wyoming showing contour lines, green for wooded areas, elevation markers, etc.

Topographic Map scale 1:24,000
Scale bar on a typical 1:24,000 topographic map Gavilan Colege Quadrangle 2012
Gavilan Topographic Quadrangle
Portion of the Chittenden Quadrangle topographic map
(black and white topo map with contours for elevation)
Portion of the Chittenden Quadrangle topographic map (2012 version) showing mapping on a satellite image.
San Luis Rey, California 7.5 minute quadrangle Topographic map of a portion of Washington DC
Southwest corner of the Washington DC 7.5 minute topographic quadrangle map showing locations where most of the government building and monuments are located.
San Luis Rey, CA 7.5 minute quadrangle (large .pdf file)  

5. What is the Public Land Survey System?

Historically, as the United States expanded its territories in the 18th, 19th, and early 20th centuries, land was surveyed (mapped) and subdivided using the Public Land Survey System. This method of surveying involved measuring a grid of lines east-to-west and north-to-south starting with a designated initial point (such as a mountain peak like Mt. Diablo in central California). From an established initial point, a north-to-south was mapped (called the Principal Meridian). From the initial point, a baseline was mapped east-to-west following a line of latitude.

North-south lines are called township lines and east-west lines are called range lines. These lines of a grid are parallel mapped at intervals of at 1 mile apart. Flying over the country today it is easy to see the historical significance of the township and range lines because they are the locations of roads and property boundaries throughout the landscape we see today.

On a limited regional scale, township and range lines work fairly well, but with increasing distances the rectangular pattern begins to fail due to distortion caused by the curvature of the earth. To solve the distortion problem, new grids of township and range lines were set up throughout the country over time. The Public Land Survey System is used for describing locations in where the land grid has been establish. Throughout much of the United States, roads (commonly dirt tracks) follow section lines east and west, and north and south. These lines are considered public access routes in most states, and are a means of finding and locating specific pieces of property or locations on public lands.
BLM Map showing Public Land Survey grids BLM Map showing Public Land Survey System grid in parts of the American West
Principal Meridians and Base Lines of region that used the Public Land Survey System (map provided from US BLM) Detail view of a public land survey of Nebraska Territory showing the initial point, principal meridian, and baseline.
Initial Point marker for the 1855 land survey of Nebraska Territory Public Land Survey System
Google image of the initial point for the Public Land Survey of Nebraska Territory started in 1855. Locations in areas mapped with the Public Land Survey System are designated by township, range, and section.

5. Standard Maps and Digital Map Data produced by the US Geological Survey.

USGS Topographic Maps and information about maps can be gotten from:

USGS Map Locator & Downloader
(a source for "free" digital topographic maps on several scales for most of the United States)

USGS National Map Viewer
(free online access to digital topographic maps)

US Geological Survey Topographic Map Symbols
is at:

Selected sources of map information

See the USGS Maps, Imagery, and Publications (a website to download free digital maps and imagery)

Learn about all kinds of maps for California (Humboldt State University Library website)

See Historic USGS topographic quadrangle maps of the Monterey Bay region (UC Berkeley)

Other common types of maps and digital map data that show topography and integrated into geographic information systems.

digital elevation model (DEM)—A digital elevation model (DEM) is a digital representation of ground surface topography or terrain. It is also widely known as a digital terrain model (DTM) commonly used as a base map in a geographic information system (GIS). A DEM can be represented as a raster (a grid of squares) or as a triangular irregular network. A DEM is used for the generation of contours, shaded relief, 3-D terrain models and elevation profiles. DEMs are used to make shaded relief maps using GIS software.

shaded relief map—A map of an area whose relief is made to appear three-dimensions using gray-scale shading based on a hypothetical sun angle, typical of late afternoon (north and east facing slopes appear darker than south and west facing slopes).

satellite image map
—a map generated from raw satellite imagery data that has been rectified to match a grid associated with a standardize map grid, such as a USGS 7.5 minute quadrangle, a "digital orthoquad", or DOQ. provides satellite image maps along with standardized road maps on their maps search website.

Mount St. Helens Austin Texas DEM Mount St. Helens DEM shaded relief Devils Tower satellite image
Photograph of Mount St. Helens, Washington. Standard photographs are useful in describing landscapes, but they are less important for analyzing data compared with maps. Example of "raw" DEM data: Digital elevation model of Mount St. Helens, Washington. Note that a DEM shows relief only in a vertical orientation in 256 shades of gray. It is only raster elevation data and does not show geographic names, roads, etc. Shaded relief model made with DEM data for Mount St. Helens, Washington. This image was created with a geographic information system by reprojecting the raw DEM data from the previous image. Satellite imagery data (digital othoquad [DOQ]) of Mount St. Helens, Washington. DOQs are photographs (images) taken from airplanes or satellites. DOQs are commonly used as base maps in creating other kinds of maps or map layers in a GIS database.

5. Define geomorphology and and the concept of physiographic provinces.

The links between the traditional studies of Geography and Geology intersect in the science of Geomorphology.

geomorphology—the study of the earth's surface including classification, description, nature, origin, and development of landforms and their relationships to underlying structures and the history of geologic changes as recorded by these surface features.

Primary studies of landscapes involve generating map of many kinds, including natural landscape features (such as rivers, mountains, shorelines), man-made features (cities, roads, dams, etc.), ecology and land use (natural and agricultural) and much more. Regions of the North American landscape have been subdivided in to areas sharing similar physical characteristics, such as topography (relief), geologic history, ecology, and climate. These subdivisions on a map are called Physiographic Provinces.

physiographic province—a geographic region with a specific geomorphology and often specific subsurface rock type, age, or structural elements. See more information on the Physiographic Provinces of the United States web page.

Shaded Relief Map of the United States Physiographic provinces of the United States Physiographic Provinces Map (blank) Physiographic provinces of California
Physiographic Provinces on a geologic map
Shaded Relief Map of the United States
(USGS Map i-2206)
Physiographic province of the United States Blank physiographic Provinces Map
Physiographic Provinces
Tapestry of Time and Terrain (a USGS map website) compares landscapes with underlying geology with physiographic provinces of the United States.

Gail P. Thelin and Richard J. Pike, 1991, Landforms of the Conterminous United States - A Digital Shaded-Relief Portrayal: U.S. Geological Survey
Miscellaneous Investigations Series Map I-2206,
Physiographic Provinces on a geologic map  

6. What is a Geologic Map?

geologic map
—a special-purpose map made to show geological features. Types and ages of rock units are shown by color or symbols to indicate where they are exposed at or near the surface. A geologic map records the distribution, nature, and age relationships of rock units and the occurrence of structural features (such as the location of faults). Geologic features are illustrated as colors, lines and symbols. They depict the land as if all soil and vegetation were stripped away. Geologic maps show "bedrock" geologic materials and features, and may display shallow surface sediments (alluvium, landslide deposits, floodplain deposits, sand dunes, etc.)
Geologic map of the Cincinnati Ohio region
Geologic maps are used to interpret the geologic history of a region. Geologic maps are used by paleontologists to find areas that are likely to contain fossils, and by geologists and engineers to define the location of faults, economic mineral resources, to find potential underground water resources, used in civic planning, and more. General geologic map of the Cincinnati Arch region,
Ohio, Kentucky, Indiana

Essential concepts to understand geologic maps
(common geologic information concepts needed to interpret geologic maps)

geologic time scale
rock types (see rock cycle)
Law of Superposition
Law of Original Horizontality
Law of Cross-Cutting Relationships
relative and absolute dating
strata (beds, rock formations)
surficial deposits (stream deposits, alluvium, landslides, etc.)
strike & dip
Common geologic map symbols Basic Geologic Principles Unconformities
Common symbols representing geologic features on geologic maps. Basic geologic principles: laws of original horizontality, superposition, and cross-cutting relationships Unconformities: types include nonconformities, disconformities, and angular unconformities.

Comparison of a topographic map and a geologic map
San Juan Bautista 15 minute topographic map Geology on topography Geologic map
Topographic Map
San Juan Bautista, CA
15 minute quadrangle
Topographic map with geology mapping Geologic Map
San Juan Bautista, CA
15 minute quadrangle
Dibblee, Thomas, W, 1979, Preliminary Geologic Map of the San Juan Bautista Quadrangle: U.S. Geological Survey Open-File Report 79-375, scale 1:24,000.
(revised version at

7. Explain the uses of maps and geologic illustrations in understanding geology.

Interpreting the Earth structure with maps and illustrations

Geologic Maps, Cross Sections, Block Diagrams
are used to show the structure of the Earth in 3 dimensions. Geologic maps show the arrangement of geologic features and materials exposed on or near the surface. Cross sections show interpretations of geologic features, structures, and materials in vertical profiles, usually perpendicular to the surface.

Examples below are from the Grand Canyon, Arizona area. (Map modified from Billingsley, G. H., 2000, Geologic map of the Grand Canyon 30' x 60' quadrangle, Coconino and Mohave Counties, northwestern Arizona: U.S. Geological Survey Geologic Investigations Series I-2688, map scale 1:100,000.
Grand Canyon photo
Grand Canyon (photo view is looking north from the South Rim)
Grand Canyon cross section
Grand Canyon geologic map Geologic Map Legend
Grand Canyon Geologic Map. Line D - D' is the location of the cross section shown below.

Grand Canyon cross section
Block Diagram of the Grand Canyon Geologic Cross Section of the Grand Canyon Geologic map legend for the Grand Canyon

8. Finding Geologic Map Information

To find a geologic map of portions of the United State it is usually essential to find the name of the topographic map name of a particular region. The USGS typically names small scale geologic maps with the name of the 7.5 minute topographic map quadrangle. Larger scale (regional maps) are named after the 1:100,000 maps (1° x 2° quadrangle) or larger 1:250,000 scale. State geologic maps are typically 1:1,000,000 scale or larger.

Geologic maps can be located using the National Geologic Map Database:

FGDC Digital Cartographic Standard for Geologic Map Symbolization

Symbols Used On Geological Maps

Example of a geologic map showing northern San Diego County region:

Geologic map of the Oceanside 30'x60' Quadrangle, California
California Geological Survey, Regional Geologic Map No. 2, 1:100,000 scale
Compiled by: Michael P. Kennedy, Siang S. Tan; Kelly R. Bovard, Rachel M. Alvarez, Michael J. Watson, and Carlos I. Gutierrez, release date 2002:
Website provides options to download Adobe .pdf files of the geologic map, and map explanation and pamphlet documents.

Map Assignment - Physiographic Provinces (Due day of first exam)

Quiz 4 questions