Scientific Tools

List tools and concepts commonly used by geologists.

This section discusses some of the common tools and concepts geologists employ.

What You’ll Learn to Do

  • Discuss several tools and their use in geology.
  • Understand how to read various maps.
  • Find a location using longitude and latitude.
  • Identify commonly used geological models.

Geologic Tools

Geologists use a lot of tools to aid their studies. Some of the most common tools used are compasses, rock hammers, hand lenses, and field books.


There are a number of different (specialised) magnetic compasses used by geologists to measure orientation of geological structures, as they map in the field, to analyse (and document) the geometry of bedding planes, joints, and/or metamorphic foliations and lineations. In this aspect the most common device used to date is the analogue compass.

Classic geological compasses

Classic geological compasses that are of practical use combine two functions, direction finding and navigation (especially in remote areas), and the ability to measure strike and dip of bedding surfaces and/or metamorphic foliation planes. Structural geologists (i.e. those concerned with geometry and the pattern of relative movement) also have a need to measure the plunge and plunge direction of lineations.

Compasses in common use include the Brunton compass and the Silva compass.

Modern Geological Compasses

The concept of modern geological compass was developed by Eberhard Clar of the University of Vienna during his work as structural geologist. He published it in 1954.[1] An advantage of his concept is that strike and dip is measured in one step, using the vertical circle for dip angle and the compass for the strike direction. The first implementation was done by the VEB Freiberger Präzisionsmechanik in Freiberg, Germany. The details of the design were made in a close cooperation with the Freiberg University of Mining and Technology.[2]

Setup of a modern geological compass after Prof. Clar (Freiberger), total view

Setup of a modern geological compass after Prof. Clar (Freiberger), total view

top view of compass

top view

bottom side of the compass

bottom side

Strike line and dip of a plane describing attitude relative to a horizontal plane and a vertical plane perpendicular to the strike line

Strike line and dip of a plane describing attitude relative to a horizontal plane and a vertical plane perpendicular to the strike line

At first sight it appears confusing to the novice user, for the numbers on the compass dial ascend in an anticlockwise direction. This is because the compass is used to determine dip and dip-direction of surfaces (foliations), and plunge and plunge-direction of lines (lineations). To use the compass one aligns the lid of the compass with the orientation of the surface to be measured (to obtain dip and dip direction), or the edge of the lid of the compass with the orientation of the line (to obtain plunge and plunge direction). The compass must be twisted so that the base of the compass becomes horizontal, as accomplished using the spirit level incorporated in it. The needle of the compass is then freed by using the side button, and allowed to spin until the damping action slows its movement, and then stabilises. The side button is released and the needle is then firmly held in place, allowing the user thereafter to conveniently read the orientation measured. One first reads the scale that shows the angle subtended by the lid of the compass, and then depending on the colour shown (red or black) the end of the compass needle with the corresponding colour. Data are then recorded as (for example) 25°->333° (dip and dip-direction) or (plunge and plunge-direction).

This compass has the most use by structural geologists, measuring foliation and lineation in metamorphic rocks, or faults and joints in mining areas.

Digital Compasses

With the advent of the smartphone, geological compass programs based on the 3-axis teslameter and the 3-axis accelerometer have also begun to appear. These compass programs use vector algebra to compute plane and lineation orientations from the accelerometer and magnetometer data, and permit rapid collection of many measurements. However, some problems are potentially present. Smartphones produce a strong magnetic field of their own which must be compensated by software; as well, because the Earth’s magnetic field fluctuates rapidly, measurements made by smartphone geological compasses can potentially be susceptible to considerable noise. Users of a smartphone compass should carefully calibrate their devices and run several tests against traditional magnetic compasses in order to understand the limitations of their chosen program.

Rock Hammers

A geologist's hammer used to break up rocks, as well as a scale in the photograph

A geologist’s hammer used to break up rocks, as well as a scale in the photograph

A geologist’s hammer, rock hammer, rock pick, or geological pick is a hammer used for splitting and breaking rocks. In field geology, they are used to obtain a fresh surface of a rock to determine its composition, nature, mineralogy, history, and field estimate of rock strength. In fossil and mineral collecting, they are employed to break rocks with the aim of revealing fossils inside. Geologist’s hammers are also sometimes used for scale in a photograph.


Geologist’s hammers, as with most hammers, have two heads, one on either side. Most commonly, the tool consists of a flat head on one end, with either a chisel or a pick head at the other end.

  • A chisel head (pictured), which is shaped like a chisel, is useful for clearing covering vegetation from exposures and is sometimes (though inadvisedly) used to pry open fissures. Some rocks can be easily split, like slate or shale, to reveal any fossils.
  • A pick head, which terminates in a sharp point to deliver maximum pressure, is often preferred for harder rocks. A geologist’s hammer bearing a pick end is often referred to as a rock pick or geological pick instead of a geologist’s hammer.
  • A flat head is used to deliver a blow to a rock with the intention of splitting it. Specimens or samples can be trimmed to remove sharp corners or reduce them in size.


A geologist's hammer with tubular shaft and chisel head

A geologist’s hammer with tubular shaft and chisel head

The effective power of a geologist’s hammer is mainly considered to be a reflection of its head weight and handle length. Head weight may range from 8 oz (225 g) or less on a small hammer—such as would generally be used for casual use or by children—to 24 oz (680 g) and greater. A hammer of 16 oz (450 g) is often quoted as sufficient for all rock types, although metamorphic or igneous rocks often require heavier hammers for a more powerful blow.

The best geologist’s hammers are forged from one piece of hardened steel, which renders them sturdy and long-lasting. Alternatives such as tubular- and wooden-shafted hammers are more commonly used, in part due to their lower cost. Such alternative handles sacrifice strength and make the hammer unsuitable for high-strain activities such as prying.

The form and weighting of the shaft defines the balance, which itself defines the ease, efficiency, and comfort of use of the geologist’s hammer.

Hand Lenses

Loupe used by a geologist

Loupe used by a geologist

The hand lens is a vital geological field tool used to identify small mineral crystals and structures in rocks. It is a simple, small magnification device used to see small details more closely. Unlike a magnifying glass, a loupe does not have an attached handle, and its focusing lens(es) are contained in an opaque cylinder or cone or fold into an enclosing housing that protects the lenses when not in use.

Three basic types of hand lenses exist:

  • Simple lenses, which result in the highest degree of optical aberration and are generally lower magnification.
  • Multiple lenses, generally higher magnification because of the reduced optical aberration.
  • Prismatic, Multiple lenses with prisms used to change the perspective.

Jewelers typically use a monocular, handheld loupe in order to magnify gemstones and other jewelry that they wish to inspect. A 10x magnification is good to use for inspecting jewelry and hallmarks and is the Gemological Institute of America’s standard for grading diamond clarity. Stones will sometimes be inspected at higher magnifications than 10x, although the depth of field, which is the area in focus, becomes too small to be instructive. The accepted standard for grading diamonds is therefore that inclusions and blemishes visible at 10x impact the clarity grade.

Field Books

Field books are used to take fieldnotes; they can be anything from a composition type notebook to a spiral, but most use an actual “field book” like those available for purchase hereFieldnotes refer to qualitative notes recorded by scientists during or after their observation of a specific phenomenon they are studying. They are intended to be read as evidence that gives meaning and aids in the understanding of the phenomenon. Fieldnotes allow the researcher to access the subject and record what they observe in an unobtrusive manner.

One major disadvantage of taking fieldnotes is that they are recorded by an observer and are thus subject to (a) memory and (b) possibly, the conscious or unconscious bias of the observer. It is best to record fieldnotes immediately after leaving the site to avoid forgetting important details.

Fieldnotes are particularly valued in  geology and other descriptive sciences such as ethnography, biology, and archaeology.


There are two components of fieldnotes: descriptive information and reflective information.

  • Descriptive information is factual data that is being recorded. Factual data includes time and date, the state of the physical setting, social environment, descriptions of the subjects being studied and their roles in the setting, and the impact that the observer may have had on the environment.
  • Reflective information is the observer’s reflections about the observation being conducted. These reflections are ideas, questions, concerns, and other related thoughts.

Fieldnotes can also include sketches, diagrams, and other drawings. Visually capturing a phenomenon requires the observer to pay more attention to every detail as to not overlook anything.


Maps are essential tools in geology. 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. Geologists are trained in map reading and map making. Many geologists have experience mapping some part of the earth’s surface.

It takes some training to read maps skillfully. 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 geology.

Topographic Maps

A complex map of Yellowstone. There are several natural features on the map, including springs, geysers, and plains.

Figure 1. Map of Yellowstone.

A topographic map (like the one in figure 1) is one type of map used by geologists. Topographic maps show the three-dimensional shape of the land and features on the surface of the earth. Topographic maps are also used by 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 sometimes find it useful to refer to topographic maps of their area.

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 sheet of paper or computer screen. When you know how to read contour lines, you can look at them on a topographic map and visualize the mountains, plains, ridges, or valleys that they 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 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 subjected to landsliding, places undergoing erosion and places where sediment is accumulating. 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 most geologic maps are 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.

Rules for Contour Lines
  • A contour line connects all the points on a map area that are at a specific elevation. For example, every point on a 600-foot contour line represents a point on earth that is 600 feet above sea level. You can visualize a contour line as the shoreline that would exist if the ocean were to cover the land to that elevation.
  • The contour interval is the vertical distance, also known as the elevation difference, between adjacent contour lines. On a map with a 40-foot contour interval, the vertical distance between two contour lines that are next to each other, is 40 feet, regardless of the horizontal distance between the two lines on the map.
  • Contour lines do not intersect each other, because a point on the surface of the earth cannot be at two different elevations. (However, in the rare case of a vertical cliff showing up on a topographic map, contour lines along the cliff may appear to join together into a single line.)
  • Circles that are closed contours generally signify hills.
  • Depressions that have no outlet are signified by closed contours with short lines that stick out of them and point toward the center. (The short lines sticking out of the contour lines are called hachures, hatch marks, or tick marks.)
  • Contour lines on standard US Geological Survey topographic maps are brown — except on the surfaces of glaciers, where the contour lines are blue.
  • The elevation of a point on the map that is not on a contour line must be estimated as greather than the elevation of the nearest contour line below it, and less than elevation of the nearest contour line above it. For example, a point lying midway between the contours 5440 ft and 5480 ft would be at approximately 5460 ft elevation.
  • Contour lines curve upstream when they cross a valley. This leads to the “Rule of Vs”: Where they cross streams, contour lines make Vs that point upstream.
  • Where contours are close together, the topography is steep; where contour lines are far apart, the slope is gentle or flat.
  • The relief on a landscape is the elevation difference between two given points. The maximum relief on a topographic map is the elevation difference between the highest and lowest points on the map.
Map Quadrangles, Latitudes, and Longitudes

Standard United States Geological Survey topographic maps cover a quadrangle. A map quadrangle spans a fraction of a degree of longitude east-to-west and the same fraction of a degree of latitude north-to-south. Because lines of longitude degrees (also called meridians) in the Northern Hemisphere come closer and closer together the nearer they get to the North Pole, whereas lines of latitude degrees remain the same distance apart as they circle the earth, quadrangle maps span less distance east-to-west than they do north-to-south.

Latitude is how far north or south of the equator a point is on earth, measured in degrees, from 0° at the equator to 90° at the poles. When specifying a latitude, always state whether it is in the Northern Hemisphere (N) or Southern Hemisphere (S).

Longitude is how far east or west, up to a maximum of 180°, a point on earth is from the Prime Meridian. The Prime Meridian, 0° longitude, is a north-south line that runs through Greenwhich, England. When specifying a longitude, state whether it is in the Western Hemisphere (W) or Eastern Hemisphere (E).

Meridians, lines of longitude, run from the South Pole to the North Pole, converging (coming together) at the poles. Because the meridians converge at the poles, a degree of longitude gets smaller and smaller near each pole. In contrast, a degree of latitude remains approximately 69 miles across, no matter how near or far from the poles or equator it is.

Degrees of latitude and longitude are divided into arc minutes and arc seconds. In this context, they are usually just called minutes and seconds, but it must be kept in mind that these minutes and seconds are units of angles, not units of time. These units, which divide angles into smaller parts, work as follows:

  1. There are 60 arc minutes in 1 degree.
  2. The symbol for minutes is a single apostrophe: ‘.
  3. In symbols, 60′ = 1° means there are 60 minutes in 1 degree.
  4. There are 60 arc seconds in 1 arc minute.
  5. To convert arc minutes into a decimal fraction of a degree, multiply the number of arc minutes by 1°/60′. For example, to convert 15′ into a decimal fraction of a degree, 15′ x 1°/60′ = 0.25°. In simpler terms, just divide the number of arc minutes by 60 to convert to decimal degrees.
  6. The symbol for arc seconds is a double apostrophe or quotation mark: “.
  7. In symbols, 60″ = 1′ means there are 60 seconds in 1 minute.

Two common quadrangle sizes are 7.5 minutes (1/8 of a degree), and 15 minutes (1/4 of a degree).

Name, Size, and Latitude-Longitude of a Topographic Map Quadrangle

A map portion. It reads Juniper Quadrangle, Oregon–Washington. 7.5 minute series (topographic). Below this is the lines of the top right corner of the map. There are markings indicating the east at 119 degrees 00' and the north ad 46 degrees 00'. The corner is labelled Wallula 1:125,000).

The image above shows the northeastern corner portion of the topographic map of the Juniper quadrangle, which spans the border of the states of Oregon and Washington. The name of the quadrangle comes from the name of a place on the map. Find the following information using this corner of the map:

  1. The name of the quadrangle
  2. The state(s) in which the quadrangle is located
  3. The size of the quadrangle
  4. The name and fractional scale of the quadrangle map that is located adjacent to the northeast of the corner corner
  5. The longitude of the eastern boundary of the map
  6. The latitude of the northern boundary of the map

Map Scale, Contour Interval, and Magnetic Declination

The scale at the bottom of a map. The bottom edge of the map can be seen. 5’ and 2’30’’ are noted on the edge of the map. On the left side of this clip, you can read 360,000 Feet (Oreg.). In the center of the bottom edge it reads Umatilla 1:125,000. The scale is stated to be 1:24,000. There are three scale bars indicating the length on the map that equals a mile, 7000 feet, and 1 kilometer. The contours interval is 20 feet. Datum is mean sea level. Depth curves and sounds in feet; datum is normal pool elevation 340 feet. The approximate man declination, 1962 measures a 20 and a half degree difference between true north and magnetic north. Below this is a short passage that reads as follows: This map complies with National map accuracy standards. For sale by the U.S. Geological Survey, Denver 25, Colorado or Washington 25, D.C. A folder describing topographic maps and symbols is available on request.

Important information is shown at the bottom of a USGS quadrangle map, including the map scale, the contour interval, and the magnetic declination. The image above is from the bottom of the Juniper 7.5 minute quadrangle. It tells you, among other things:

  1. The map scale. The map scale is listed in terms of the fractional scale as 1:24,000. This means that 1 inch on the map corresponds to 24,000 inches in the real world represented by the map, or 1 cm equals 24,000 cm; in other words, distances on the map have been shrunk by a factor of 24,000 from their real-world size. Beneath the fractional scale, the map scale is also depicted a different way, in bar scales using three different units. One of the bar scales is in miles, one is in units of thousands of feet, and one is in kilometers.
  2. The contour interval, the difference in elevation between adjacent contour lines on the map, is listed below the map scale as 20 feet.
  3. There is also a reminder that elevations shown on the map are elevations above mean (average) sea level on earth.
  4. (You might note that this map does something unusual for a topographic map. It shows depths in the Columbia River in feet below the surface of the river when the river is backed up in its reservoir behind a dam to a normal pool surface elevation of 340 feet above sea level.)
  5. To the left of the bar scales, the magnetic declination is shown as an arrow diverging from a line oriented toward true north. True north is the direction toward the geographic North Pole. The geographic North Pole is where the northern end of earth’s axis of rotation is located. The magnetic North Pole is in northeastern Canada. In 1962, the magnetic North Pole, as measured from the Juniper quadrangle, was located 20.5° east of true north. If you took a magnetic compass to the Juniper quadrangle in 1962, its north arrow would point 20.5° east of true north, so you would have had to set your magnetic compass to compensate for the declination. The magnetic North Pole wanders a few miles every year and the magnetic declination of 20.5° east of true north was determined in the year 1962; it may be slightly different now.

Constructing a Topographic Profile

One of the important tools you can use to extract the vertical information from a topographic map, and see more clearly the shape of the earth’s surface that it represents is a topographic profile.

Construction of a topographic profile allows you to visualize the vertical component of a landscape. A topographic profile is similar to the view you have of a landscape while standing on earth, looking at hills and valleys from the side rather than from above.

Given a topographic map such as the one below, here’s how to construct a topographic profile.

A topographical map: A series of ovals expanding in size representing different levels of grade ranging between 500 and 400 feet. The left edges of the ovals are much closer to each other than the right sides. The top of the map is labelled as North.
Step 1

Determine the line of profile, the line across that part of the map that you want to see in profile or cross-section view. Depending on which part of the map you want to see in profile, you can draw your line of profile in any direction you choose, across any part of the map you choose. For the map used in this example, we choose to draw the profile from A to A’ as shown in the diagram below, to see the entire length of the hill in profile.

Step 2

Draw a grid that will contain the profile. The width of the grid should be the same as the length of the line of profile. To draw the profile, the grid must be crossed by evenly-spaced horizontal lines that represent the contour elevations. The grid must extend high enough to span the elevation range of the contour lines spanned by the line of profile. You can see that the grid, shown below, includes the range of elevations that the line of profile crosses on the map. In addition, the grid must have an extra horizontal line at the bottom and top to accommodate the parts of the profile that go above the highest contour elevation and below the lowest contour elevation. That is why the grid in the example below goes below 400 feet and above 500 feet in elevation.

The same map; this time with a line drawn through the ovals’ centers east to west. The west side of the line is labelled A, the east is labelled A’. Above the map is a grid without vertical lines.
Step 3

Transfer the contour elevations from the topographic map to the profile grid. The point where each contour line crosses the line of profile on the topographic map determines the horizontal coordinate of each corresponding point on the grid of the topographic profile. The elevation of each contour line corresponds to the vertical coordinate of each corresponding point on the profile grid, as shown on the diagram below.

The same map and grid. The intersection points of each oval with the line have been pulled up to the grid to their appropriate heights.
Step 4

Now that you have marked the elevation points on the profile grid, draw a smooth line connecting the data points as shown below. Note that the ends of this profile go below the 400 foot contour elevation but they do not extend to the 380 foot elevation because on the map the line of profile did not reach the 380 foot contour line. Also note that the top of the profile reaches a peak above 520 feet but less than 540 feet because the line of profile does not cross the 540 foot contour line.

The same map and grid. The lines in the grid have been connected, to show a slope, which resembles the side view of a mountain.
Step 5

The completed topographic profile and the map it was drawn from are shown below. Topographic profiles are usually constructed without drawing any lines on the map. Instead, the edge of a piece of paper is laid along the line of profile and the contour line data is transferred to the edge of the piece of paper. From the edge of the piece of paper, the data are transfered to the profile grid, which is on a separate piece of paper.

The same map and grid. The guide lines have been removed, so it is just the map and a representation of the slope.

Notice on the topographic profile constructed above that the peak of the hill is above 520 ft, but below 540 ft. Similarly, the ends of the profile are below 400 ft but above 380. This is consistent with the elevations of those parts of the line of profile on the map.

Note that the vertical scale on the profile is very different from the horizontal scale on the map. In this example, the map covers 0.25 miles horizontally in less distance than the profile covers 100 feet vertically. As a result, the topographic profile is greatly exaggerated vertically. In an actual view of the hill, looking at it from the side, it would not look nearly as steep as it does in the topographic profile that we have constructed.

If the vertical scale on a topographic profile is different from the map scale, as it is in this case, then the profile will exhibit a vertical exaggeration. The vertical exaggeration of a topographic profile can be calculated. It is the fractional scale of the topographic profile’s vertical axis, divided by the fractional scale of the map. For example, if the vertical scale on the profile is 1:200 and the map scale is 1:24,000, the vertical exaggeration is [latex]\displaystyle\frac{\left(\frac{1}{200}\right)}{\left(\frac{1}{24,000}\right)}[/latex]. To divide by a fraction, you can invert and multiply, so this becomes [latex]\displaystyle\left(\frac{1}{200}\right)\times\left(\frac{24,000}{1}\right)=\frac{24,000}{200}=120[/latex]. A topographic profile with a VE of 120 would be a very exaggerated topographic profile. It would be as if a rubber model of the landscape has been pulled in the vertical direction, until it is 120 times taller than it really is.

If the vertical scale of a topographic profile is different from the map scale, the vertical exaggeration should be listed next to the profile, such as VE = 10 or VE 10x if the vertical exaggeration is 10.

Compare the profile to the topographic map. You will see that the hill is steeper on the west (left) side than on the east (right) side. This is consistent with the contour lines being spaced more closely on the west side of the hill and farther apart on the east side of the hill. This accords with the rules of contour lines, which state that slopes are steeper where contour lines are more closely spaced, and slopes are less steep where contour lines are more widely spaced.

If you drew a profile from north to south across the peak of the hill, do you think the profile would be symmetric or asymmetric?

Checklist for a Complete Topographic Profile

A properly drawn topographic profile will have the following attributes:

  • The topographic profile is drawn on a rectilinear graph with evenly spaced grid lines. (Vertical grid lines are not required.)
  • Elevation lines are labeled along the left-hand vertical axis.
  • The profile is a smooth curve where its gradient changes, rather than straight-line segments connecting the dots and only bending at the dots.
  • If the vertical scale on the profile is different from the map scale, the resulting amount of vertical exaggeration is listed.
  • The ends and any high points or low points of the topographic profile should be above or below elevation lines, not on them, except in cases where an end, high point, or low point of a line of profile happens to fall right on a contour line.

Explore More: Topographic Maps

Use this resource to answer the questions that follow. You may stop watching at the 4:04 mark.

  1. What do topographic maps do and how do they do it?
  2. What are the meanings of the terms topographic map, contour line, contour interval and index contour?
  3. If you were to walk along a contour line, what would happen to your elevation?
  4. If you walk perpendicular to contour lines what are you doing?
  5. What do close contour lines indicate?

Bathymetric Maps

Figure 3. Loihi volcano growing on the flank of Kilauea volcano in Hawaii. Black lines in the inset show the land surface above sea level and blue lines show the topography below sea level. Click on the image to view a larger version.

Figure 3. Loihi volcano growing on the flank of Kilauea volcano in Hawaii. Black lines in the inset show the land surface above sea level and blue lines show the topography below sea level. Click on the image to view a larger version.

A bathymetric map is like a topographic map with the contour lines representing depth below sea level, rather than height above. Numbers are low near sea level and become higher with depth.

Kilauea is the youngest volcano found above sea level in Hawaii. On the flank of Kilauea is an even younger volcano called Loihi. The bathymetric map pictured in figure 3 shows the form of Loihi.

Geologic Maps

A geologic map shows the geological features of a region (see figure 4 for an example). Rock units are color-coded and identified in a key. Faults and folds are also shown on geologic maps. The geology is superimposed on a topographic map to give a more complete view of the geology of the region.

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 map of the region around Old Faithful, Yellowstone National Park

Figure 4. A geologic map of the region around Old Faithful, Yellowstone National Park.

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:

  • the map itself
  • the map legend or key that explains all the symbols on the map.

Professional geologic maps usually have two other components as well:

  • an accompanying explanation of the rock or sediment units
  • 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. See the Table of Geologic Map Symbols for pictures and an overview of the map symbols, including strikes and dips, faults, folds, and an overview.

Table of Geologic Map Symbols

Strike and Dip Symbols
Strike and dip are a way of representing the three-dimensional orientation of a planar surface on a two-dimensional map. The strike is the compass direction of a horizontal line on the plane. All the horizontal lines on a plane are parallel, so they all have the same characteristic compass direction. The dip is the angle at which the plane slopes downhill from the horizontal, at its maximum slope, which is at right angles (90º) from strike.
Map Symbol Definition Explanation of symbol
vertical line with a small horizontal line on its right side labeled 38 strike and dip of beds other than horizontal or vertical
  • strike (longer line) is horizontal line on bedding plane
  • strike parallels nearby contacts between stratified rocks
  • dip shows which way beds run downhill
  • dip angle, number at end of dip symbol, is how much beds tilt down from horizontal
circle with a t cross inside horizontal beds
  • because the bed is horizontal it strikes in all directions
  • because the bed is horizontal, the dip is 0%
vertical line with a small horizontal line crossing the middle strike and dip of vertical beds
  • strike (longer line) is horizontal line on bedding plane
  • because the bed dips vertically (has a 90% dip), it dips equally in either direction at right angles to strike, so the dip line is shown extending in both directions
Geologic Fault Symbols
Type of Fault Map Symbol Definition Type of Regional Stress Geologic Associations
normal horizontal line with a U above and D below. U on uplifted side, D on down-dropped side. hanging wall down, footwall up tension
  • zones of crustal extension
  • divergent plate boundaries
  • edges of horsts and grabens
  • Basin and Range region
detachment  rectangles on horizontal line (rectangles on upper plate) low-angle normal fault, footwall—gneiss, hanging wall—shallow-crust rocks tension
  • boundaries of metamorphic core complexes
thrust triangles on horizontal line (triangles on upper plate) hanging wall up, footwall down compression
  • zones of crustal compression
  • convergent plate boundaries
reverse  triangles on horizontal line (triangles on upper plate) high-angle (45° or more dip) thrust fault compression
  • zones of crustal compression
  • convergent plate boundaries
strike-slip two half-arrows pointing in opposite directions rocks on either side move horizontally in opposite directions shear
  • continental margins undergoing oblique (not straight on) plate convergence
  • transform plate boundaries
oblique-slip two half-arrows pointing in opposite directions. U on uplifted side, D on down-dropped side combines horizontal and vertical motion combination
  • orogenic mountain belts
  • continental margins undergoing oblique (not straight on) plate convergence
Geologic Fold Symbols
Type of Fold Map Symbol Definition Appearance of Beds in Map View
anticline horizontal line with a vertical line crossing it. There are arrows on both ends of the vertical line pointing away from the horizontal line. up fold
  • roughly parallel stripes
  • dip away from center (away from axis)
  • oldest at center (along axis)
  • youngest farthest from center
plunging anticline horizontal arrow pointing to the right with a vertical line crossing it. There are arrows on both ends of the vertical line pointing away from the horizontal line. up fold with tilted axis
  • roughly a U-shaped pattern
  • plunges in direction U points
  • oldest at center (along axis)
  • youngest farthest from center
syncline Horizontal line with a vertical line crossing it. There are arrows on the internal ends of the vertical line pointing at the horizontal line. down fold
  • roughly parallel stripes
  • dip toward center (toward axis)
  • oldest farthest from center
  • youngest at center (along axis)
plunging syncline Horizontal arrow with a vertical line crossing it. There are arrows on the internal ends of the vertical line pointing at the horizontal line. down fold with tilted axis
  • roughly a U-shaped pattern
  • plunges in direction U opens
  • oldest farthest from center
  • youngest at center (along axis)
monocline Horizontal line with a vertical line crossing it. There is an arrow on the top end of the vertical line. strata tilted in one direction
  • all dip in same direction
structural dome Horizontal arrow pointing to the right and the left with a vertical line crossing it. There are arrows on both ends of the vertical line pointing away from the horizontal line. upward bulge in layered rocks
  • roughly a bull’s eye pattern
  • dip away from center
  • oldest in center
  • youngest farthest from center
structural basin Horizontal line with a vertical line crossing it. There are arrows on the internal ends of the vertical line pointing at the horizontal line. There are also arrows on the horizontal line pointing inward at the vertical line. downward bulge in layered rocks
  • roughly a bull’s eye pattern
  • dip toward center
  • youngest in center
  • oldest farthest from center

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.


  • Earth scientists regularly use topographic, bathymetric, and geologic maps.
  • Topographic maps reveal the shape of a landscape. Elevations indicate height above sea level.
  • Bathymetric maps are like topographic maps of features found below the water. Elevations indicate depth below sea level.
  • Geologic maps show rock units and geologic features like faults and folds.

Exploring Topographic Maps

Check out this augmented reality map originally developed by The University of California–Davis. It was created to help students understand topographic maps.

Check out UC Davis’s website to learn more about the project.

Location and Direction

If you found this feature while out in the field, could you find it again?

The geyser Old Faithful erupting water over 100 feet in the air

Figure 5. Old Faithful

If you’re going to make observations of geological features, you’re going to need to know the location where you are so you can mark it on a map. If you find a rock formation filled with gold, you’ll want to be able to find the location again!

You may need to tell someone when your truck gets stuck when you’re in the field so you’ll need a direction to give them.

The photo in figure 5 is of Old Faithful Geyser in Yellowstone National Park. Let’s explore just a few of the ways we can pinpoint the location of this famous geological icon.


How would you find Old Faithful? One way is by using latitude and longitude. Any location on Earth’s surface — or on a map — can be described using these coordinates. Latitude and longitude are expressed as degrees that are divided into 60 minutes. Each minute is divided into 60 seconds.


A look on a reliable website shows us that Old Faithful Geyser is located at N44o27’ 43’’. What does this mean?

Latitude tells the distance north or south of the Equator. Latitude lines start at the Equator and circle around the planet. The North Pole is 90oN, with 90 degree lines in the Northern Hemisphere. Old Faithful is at 44 degrees, 27 minutes and 43 seconds north of the Equator. That’s just about exactly half way between the Equator and the North Pole!


The latitude mentioned above does not locate Old Faithful exactly, since a circle could be drawn that latitude north of the Equator. To locate Old Faithful we need another point – longitude. At Old Faithful the longitude is W110o49’57’’.

Longitude lines are circles that go around the Earth from north to south, like the sections of an orange. Longitude is measured perpendicular to the Equator. The Prime Meridian is 0o longitude and passes through Greenwich, England. The International Date Line is the 180o meridian. Old Faithful is in the Western Hemisphere, between the Prime Meridian in the east and the International Date Line in the west.


An accurate location must take into account the third dimension. Elevation is the height above or below sea level. Sea level is the average height of the ocean’s surface or the midpoint between high and low tide. Sea level is the same all around Earth.

Old Faithful is higher above sea level than most locations at 7,349 ft (2240 m). Of course, the highest point on Earth, Mount Everest, is much higher at 29,029 ft (8848 m).

Global Positioning System

Satellites continually orbit Earth and can be used to indicate location. A global positioning system receiver detects radio signals from at least four nearby GPS satellites. The receiver measures the time it takes for radio signals to travel from a satellite and then calculates its distance from the satellite using the speed of radio signals. By calculating distances from each of the four satellites the receiver can triangulate to determine its location. You can use a GPS meter to tell you how to get to Old Faithful.


Direction is important if you want to go between two places. Directions are expressed as north (N), east (E), south (S), and west (W), with gradations in between. The most common way to describe direction in relation to the Earth’s surface is with a compass, a device with a floating needle that is actually a small magnet. The compass needle aligns itself with the Earth’s magnetic north pole. Since the magnetic north pole is 11.5 degrees offset from its geographic north pole on the axis of rotation, you must correct for this discrepancy.

Without using a compass, we can say that to get to Old Faithful, you enter Yellowstone National Park at the South Entrance, drive north-northeast to West Thumb, and then drive west-northwest to Old Faithful.


  • Latitude is the distance north or south of the Equator and is expressed as a number between 0 and 90 degrees north or south.
  • Longitude is the distance east or west of the Prime Meridian and is expressed as a number between 0 and 180 degrees east or west.
  • Elevation is the height above sea level.
  • Direction is expressed as north, south, east, or west, or some gradation between them.

Explore More

Use this resource to answer the questions that follow.

  1. What are lines of latitude?
  2. How far apart are the lines of latitude, in degrees, in miles?
  3. What are the latitudes of the Equator, the Tropic of Cancer, and the Tropic of Capricorn? What are the characteristics of the regions found between the Tropic of Cancer and Tropic of Capricorn?
  4. Where are the Arctic and Antarctic circle? What are the characteristics of the regions that are found poleward of these circles?
  5. What are lines of longitude?
  6. Where do the meridians meet?
  7. What is the Prime Meridian? Where is it located?
  8. How are longitude and latitude measured?

Scientific Models

Scientists use models to help them understand and explain ideas. Models explain objects or systems in a more simple way. Models often only show only a part of a system. The real situation is more complicated. Models help scientists to make predictions about complex systems. Some models are something that you can see or touch. Other types of models use an idea or numbers. Each type is useful in certain ways.

Scientists create models with computers. Computers can handle enormous amounts of data. This can more accurately represent the real situation. For example, Earth’s climate depends on an enormous number of factors. Climate models can predict how climate will change as certain gases are added to the atmosphere. To test how good a model is, scientists might start a test run at a time in the past. If the model can predict the present it is probably a good model. It is more likely to be accurate when predicting the future.

Physical Models

A physical model is a representation of something using objects. It can be three-dimensional, like a globe. It can also be a two-dimensional drawing or diagram. Models are usually smaller and simpler than the real object. They most likely leave out some parts, but contain the important parts. In a good model the parts are made or drawn to scale. Physical models allow us to see, feel and move their parts. This allows us to better understand the real system.

An example of a physical model is a drawing of the layers of Earth (figure 6). A drawing helps us to understand the structure of the planet. Yet there are many differences between a drawing and the real thing. The size of a model is much smaller, for example. A drawing also doesn’t give good idea of how substances move. Arrows showing the direction the material moves can help. A physical model is very useful but it can’t explain the real Earth perfectly.

Diagram showing the different layers of the earth. From the outside to the inside they are the crust, moho, upper mantle, lower mantle, D(double prime)-layer, outer core, liquid-solid boundary, and inner core.

Figure 6. Earth’s Center.

Ideas as Models

An illustration of a meteor a third of the size of the earth colliding with the planet.

Figure 7. A collision showing a meteor striking Earth.

Some models are based on an idea that helps scientists explain something. A good idea explains all the known facts. An example is how Earth got its Moon. A Mars-sized planet hit Earth and rocky material broke off of both bodies (figure 7). This material orbited Earth and then came together to form the Moon. This is a model of something that happened billions of years ago. It brings together many facts known from our studies of the Moon’s surface. It accounts for the chemical makeup of rocks from the Moon, Earth, and meteorites. The physical properties of Earth and Moon figure in as well. Not all known data fits this model, but much does. There is also more information that we simply don’t yet know.

Models that Use Numbers

Models may use formulas or equations to describe something. Sometimes math may be the only way to describe it. For example, equations help scientists to explain what happened in the early days of the universe. The universe formed so long ago that math is the only way to describe it. A climate model includes lots of numbers, including temperature readings, ice density, snowfall levels, and humidity. These numbers are put into equations to make a model. The results are used to predict future climate. For example, if there are more clouds, does global temperature go up or down? Models are not perfect because they are simple versions of the real situation. Even so, these models are very useful to scientists. These days, models of complex things are made on computers.

Geologic Modelling

Screenshot of a structure map generated by Contour map software for an 8500ft deep gas & Oil reservoir in the Erath field, Vermilion Parish, Erath, Louisiana. The left-to-right gap, near the top of the contour map indicates a Fault line. This fault line is between the blue/green contour lines and the purple/red/yellow contour lines. The thin red circular contour line in the middle of the map indicates the top of the oil reservoir. Because gas floats above oil, the thin red contour line marks the gas/oil contact zone.

Figure 8. Geological mapping software displaying a screenshot of a structure map generated for an 8500ft deep gas & Oil reservoir in the Erath field, Vermilion Parish, Erath, Louisiana. The left-to-right gap, near the top of the contour map indicates a Fault line. This fault line is between the blue/green contour lines and the purple/red/yellow contour lines. The thin red circular contour line in the middle of the map indicates the top of the oil reservoir. Because gas floats above oil, the thin red contour line marks the gas/oil contact zone.

Geologic modelling, or Geomodelling, is the applied science of creating computerized representations of portions of the Earth’s crust based on geophysical and geological observations made on and below the Earth surface. A Geomodel is the numerical equivalent of a three-dimensional geological map complemented by a description of physical quantities in the domain of interest. Geomodelling is related to the concept of Shared Earth Model; which is a multidisciplinary, interoperable and updatable knowledge base about the subsurface.

Geomodelling is commonly used for managing natural resources, identifying natural hazards, and quantifying geological processes, with main applications to oil and gas fields, groundwater aquifers and ore deposits. For example, in the oil and gas industry, realistic geologic models are required as input to reservoir simulator programs, which predict the behavior of the rocks under various hydrocarbon recovery scenarios. A reservoir can only be developed and produced once; therefore, making a mistake by selecting a site with poor conditions for development is tragic and wasteful. Using geological models and reservoir simulation allows reservoir engineers to identify which recovery options offer the safest and most economic, efficient, and effective development plan for a particular reservoir.

Geologic modelling is a relatively recent subdiscipline of geology which integrates structural geology, sedimentology, stratigraphy, paleoclimatology, and diagenesis;

In 2-dimensions (2D), a geologic formation or unit is represented by a polygon, which can be bounded by faults, unconformities or by its lateral extent, or crop. In geological models a geological unit is bounded by 3-dimensional (3D) triangulated or gridded surfaces. The equivalent to the mapped polygon is the fully enclosed geological unit, using a triangulated mesh. For the purpose of property or fluid modelling these volumes can be separated further into an array of cells, often referred to as voxels (volumetric elements). These 3D grids are the equivalent to 2D grids used to express properties of single surfaces.

Geomodelling generally involves the following steps:

  1. Preliminary analysis of geological context of the domain of study.
  2. Interpretation of available data and observations as point sets or polygonal lines (e.g. “fault sticks” corresponding to faults on a vertical seismic section).
  3. Construction of a structural model describing the main rock boundaries (horizons, unconformities, intrusions, faults)
  4. Definition of a three-dimensional mesh honoring the structural model to support volumetric representation of heterogeneity (see Geostatistics) and solving the Partial Differential Equations which govern physical processes in the subsurface (e.g. seismic wave propagation, fluid transport in porous media).

Geologic modelling components

Structural framework

Incorporating the spatial positions of the major formation boundaries, including the effects of faulting, folding, and erosion (unconformities). The major stratigraphic divisions are further subdivided into layers of cells with differing geometries with relation to the bounding surfaces (parallel to top, parallel to base, proportional). Maximum cell dimensions are dictated by the minimum sizes of the features to be resolved (everyday example: On a digital map of a city, the location of a city park might be adequately resolved by one big green pixel, but to define the locations of the basketball court, the baseball field, and the pool, much smaller pixels – higher resolution – need to be used).

Rock type

Each cell in the model is assigned a rock type. In a coastal clastic environment, these might be beach sand, high water energy marine upper shoreface sand, intermediate water energy marine lower shoreface sand, and deeper low energy marine silt and shale. The distribution of these rock types within the model is controlled by several methods, including map boundary polygons, rock type probability maps, or statistically emplaced based on sufficiently closely spaced well data.

Reservoir quality

Reservoir quality parameters almost always include porosity and permeability, but may include measures of clay content, cementation factors, and other factors that affect the storage and deliverability of fluids contained in the pores of those rocks. Geostatistical techniques are most often used to populate the cells with porosity and permeability values that are appropriate for the rock type of each cell.

Fluid saturation

Three-dimensional finite difference grid used in MODFLOW.

Figure 9. A 3D finite difference grid used in MODFLOW for simulating groundwater flow in an aquifer.

Most rock is completely saturated with groundwater. Sometimes, under the right conditions, some of the pore space in the rock is occupied by other liquids or gases. In the energy industry, oil and natural gas are the fluids most commonly being modelled. The preferred methods for calculating hydrocarbon saturations in a geologic model incorporate an estimate of pore throat size, the densities of the fluids, and the height of the cell above the water contact, since these factors exert the strongest influence on capillary action, which ultimately controls fluid saturations.


An important part of geologic modelling is related to geostatistics. In order to represent the observed data, often not on regular grids, we have to use certain interpolation techniques. The most widely used technique is kriging which uses the spatial correlation among data and intends to construct the interpolation via semi-variograms. To reproduce more realistic spatial variability and help assess spatial uncertainty between data, geostatistical simulation based on variograms, training images, or parametric geological objects is often used.

Mineral Deposits

Geologists involved in mining and mineral exploration use geologic modelling to determine the geometry and placement of mineral deposits in the subsurface of the earth. Geologic models help define the volume and concentration of minerals, to which economic constraints are applied to determine the economic value of the mineralization. Mineral deposits that are deemed to be economic may be developed into a mine.


Geomodelling and CAD share a lot of common technologies. Software is usually implemented using object-oriented programming technologies in C++, Java or C# on one or multiple computer platforms. The graphical user interface generally consists of one or several 3D and 2D graphics windows to visualize spatial data, interpretations and modelling output. Such visualization is generally achieved by exploiting graphics hardware. User interaction is mostly performed through mouse and keyboard, although 3D pointing devices and immersive environments may be used in some specific cases. GIS (Geographic Information System) is also a widely used tool to manipulate geological data.

Geometric objects are represented with parametric curves and surfaces or discrete models such as polygonal meshes.

Gravity Highs over the Mardin Uplift

Figure 10. Gravity Highs

Research in Geomodelling

Problems pertainting to Geomodelling cover

  • Defining an appropriate Ontology to describe geological objects at various scales of interest
  • Integrating diverse types of observations into 3D geomodels: geological mapping data, borehole data and interpretations, seismic images and interpretations, potential field data, well test data, etc.
  • Better accounting for geological processes during model building
  • Characterizing uncertainty about the geomodels to help assess risk. Therefore, Geomodelling has a close connection to Geostatistics and Inverse problem theory
  • Applying of the recent developed Multiple Point Geostatistical Simulations (MPS) for integrating different data sources
  • Automated geometry optimization and topology conservation


In the 1970s, geomodelling mainly consisted of automatic 2D cartographic techniques such as contouring, implemented as FORTRAN routines communicating directly with plotting hardware. The advent of workstations with 3D graphics capabilities during the 1980s gave birth to a new generation of geomodelling software with graphical user interface which became mature during the 1990s.

Since its inception, geomodelling has been mainly motivated and supported by oil and gas industry.

Geologic modelling software

Software developers have built several packages for geologic modelling purposes. Such software can display, edit, digitise and automatically calculate the parameters required by engineers, geologists and surveyors. Current software is mainly developed and commercialized by oil and gas or mining industry software vendors:

Geologic modelling and visualisation

  • SGS Genesis
  • IRAP RMS Suite
  • Geomodeller3D
  • Geosoft provides GM-SYS and VOXI 3D modelling software
  • GSI3D
  • Petrel
  • Rockworks
  • Move

Groundwater modelling

  • FEHM
    • GMS
    • Visual MODFLOW

Moreover, industry Consortia or companies are specifically working at improving standardization and interoperability of earth science databases and geomodelling software:

  • Standardization: GeoSciML by the Commission for the Management and Application of Geoscience Information, of the International Union of Geological Sciences.
  • Standardization: RESQML(tm) by Energistics
  • Interoperability: OpenSpirit, by TIBCO(r)

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.

  1. Clar, E.: A dual-circle geologist’s and miner’s compass for the measurement of areal and linear geological elements. Separate print from the negotiations of the Federal Institute of Geology Vienna, 1954, vol. 4
  2. "Geologist's Compass: Operating Manual," Freiberger Präzisionsmechanik.