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Start with the map’s explanation, not its colors. The explanation identifies what each unit label and symbol means on that particular map. Then trace boundaries between units, distinguish contacts from faults, and use orientation marks and any cross section to understand the mapped geology. Colors and symbol conventions can help, but they are not universal.
If you are wondering what all the colors and abbreviations mean, the practical answer is map-specific: find the unit key, match labels to descriptions, and check the symbol explanation before interpreting lines or marks. A familiar-looking map symbol can have a different use on a special-purpose map, so keep the map’s scale, date, source, and theme in view.
What does a surface geologic map show?
A surface geologic map represents the distribution of geologic units at the surface. A colored or patterned area marks a unit as defined by that map’s explanation. The unit may be described there by its name, age, rock type, and other distinguishing characteristics. The map shows where the unit is mapped; it is not, by itself, a picture of everything beneath the ground.
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The word “surface” matters: the map is a representation of mapped geology at the ground surface, not a complete inventory of buried layers. A boundary drawn on the page is also a mapped interpretation, and its style may indicate that its exact position is uncertain or concealed. The map’s scale affects how much detail can be shown, so do not expect a small-scale map to resolve every local feature.
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A map may also include a cross section: a geologist’s interpretation of how units and structures relate below the surface. That interpretation is limited by available knowledge and by the map’s scale. Read a cross section as an explanatory model, not as a direct view underground.
Where should you start?
Find the map explanation and unit key
Locate the explanation, legend, or map-unit key, often printed alongside the map or in its accompanying text. Match each abbreviation or label printed in a colored area to its entry in the key. Use that entry to learn the unit’s exact name, stated age, lithology, and any map-specific qualifications.
Do not assume an abbreviation has the same meaning on every map. Nor should you infer a unit’s age or rock type from its color alone. The key is authoritative for the map in front of you, including any conventions that differ from general symbol standards.
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Check the map’s source, date, and scale
When comparing maps or judging how much detail to expect, note the scale, publication date and source, unit definitions, and symbol explanation. Also check whether the map depicts bedrock, surficial deposits, or a special-purpose theme. Those details determine what the mapped areas represent and which conventions apply; there is no universal ranking that makes one map best for every use.
Two maps of the same place can differ because they were made at different scales, dates, or for different purposes. Before treating a difference as a changed geologic interpretation, check whether one map is showing bedrock while another emphasizes surficial deposits, or whether their unit definitions and symbol keys differ. Compare like with like where possible, and rely on each map’s own explanation rather than transferring a color or symbol meaning from one sheet to another.
How do you identify a mapped rock unit?
- Find the area of interest. Locate it on the map and note the color or pattern, along with the label or abbreviation printed in the area.
- Look up the label in the map-unit key. Read the complete description rather than relying on a shortened label. Record the name, age, lithology, and any qualifications the map gives.
- Check for special conventions. Read the explanation for the map’s treatment of colors, patterns, and labels. A special-purpose map may depart from familiar conventions, and the map’s own explanation takes precedence.
- Follow the unit’s boundary. Trace the line around the area and inspect its style. The line may be a contact, a fault, or another feature identified in the legend.
The result is a map-specific identification: this area is assigned to the unit described by this map’s key. It does not mean that every map uses the same color for that unit, or that the mapped boundary is equally certain everywhere.
When reading a description, keep the distinctions in the map’s wording. A short abbreviation on the map is a pointer to the longer unit entry, not a complete description in itself. The key may specify age and lithology, and can include qualifications that distinguish the mapped unit from another with a similar label or appearance. If a unit has more than one part or special notation, follow the printed explanation rather than simplifying it based on the patch’s color.
How can you tell a contact from a fault?
A contact marks the boundary between geologic units. A fault is a break along which displacement has occurred. Both may appear as lines separating areas on a map, so use the line’s symbol and the map explanation—not its position alone—to tell them apart.
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USGS and FGDC materials show contacts and faults as separate line-feature categories. A line’s style can also indicate how its position is known or whether it is concealed. Common conventions distinguish a solid trace from dashed, short-dashed, or dotted traces. The exact meaning must still be checked in the individual map’s key.
| Line style or category | Common convention | How to read it |
|---|---|---|
| Contact | Boundary between geologic units | Use the map key to identify the units it separates and any qualifications on the trace. |
| Fault | Break along which displacement occurred | Use the fault symbol and key to determine the represented fault type and any movement information. |
| Dashed trace | Commonly indicates an approximately located feature | Check the map explanation to confirm how the author uses the dash pattern. |
| Short-dashed trace | Commonly indicates an inferred feature | Read the key; do not treat the convention as a guarantee that every map uses it identically. |
| Dotted trace | Commonly indicates a concealed feature | Use the map’s definition, especially if the line is a fault or contact. |
These are common conventions listed in USGS symbol guidance, not a substitute for the legend. In particular, line style can say something about location certainty or concealment while the feature category—contact or fault—says what kind of boundary or structure the line represents.
Read those two kinds of information separately. A dashed line does not, by itself, tell you whether the feature is a contact or a fault; the symbol category does that. The dash pattern commonly describes how confidently or directly the trace is located. For example, USGS guidance uses dashed for approximately located features, short-dashed for inferred ones, and dotted for concealed ones. These are useful starting points, but the map’s own key settles the reading.
When a boundary is hard to classify, look for its symbol in the explanation and see whether it is listed under contacts, faults, or another line category. Then note any fault marks along the trace. Do not call every line between differently colored areas a fault: a contact can separate mapped units without representing a break with displacement.
What do common fault symbols mean?
Fault symbols can encode the type of fault, the side that moved down, or relative movement. Look for small marks drawn on or beside the fault trace, then match them to the symbol explanation. Do not interpret arrows or paired marks from appearance alone: their meaning depends on the map’s key.
Normal faults and hachures
In a common USGS convention, hachures mark the downthrown side of a normal fault. The hachures help indicate which side moved downward relative to the other. Confirm the symbol on the particular map before using that reading.
When you encounter hachures, first confirm that the line is identified as a normal fault in the key; then use their placement to identify the downthrown side under that convention. The marks indicate a relative side of movement, not the fault’s exact history or a complete account of how much displacement occurred.
Thrust faults and sawteeth
Sawteeth commonly mark the upper plate of a thrust fault. Their placement is meaningful: use the side on which the teeth are drawn, as defined by the legend, rather than treating them as decoration or as a direction arrow.
Check the key for both the fault category and the placement convention. The sawteeth help show which side is the upper plate; do not transfer that interpretation to a different kind of fault mark or assume that every triangular-looking mark on a map is a thrust-fault symbol.
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Ball-and-bar symbols
A ball-and-bar may mark the downthrown side of a fault when the symbol indicates dip. Check the key for the exact meaning and orientation of the marks. A ball-and-bar is not interchangeable with every arrow or paired mark used for movement.
Other movement marks
USGS symbol guidance also includes conventions for relative horizontal movement. The map’s symbol key is needed to read the direction or relationship that a particular arrow or paired mark encodes. If the key is missing or unclear, do not infer a specific movement sense solely from the mark’s shape.
The safest sequence is to identify the fault trace, locate the mark in the legend, and then read the side or direction it encodes. The mark may report a particular movement relationship; it does not replace the map’s explanation or provide every detail about the structure. The USGS symbol sheet includes normal and thrust faults, relative horizontal movement, and other structural symbols as distinct entries.
How do strike-and-dip symbols work?
Strike-and-dip symbols record the measured orientation of bedding and other planar geologic features. The symbol’s orientation and accompanying number or numbers convey the mapped attitude. The line and its associated marks are meaningful together, so do not read a number without checking which symbol it belongs to.
Map explanations may distinguish inclined, horizontal, vertical, or overturned beds. They may also include symbols for foliation, cleavage, joints, and other features. Consult the legend for the exact forms used on the map; a familiar-looking mark should not be assumed to have a universal meaning.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problems- Identify the symbol family. Check whether the mark represents bedding or another planar feature, such as foliation or cleavage.
- Read its orientation and number. Use the symbol explanation to interpret the line’s orientation and any accompanying value or values.
- Check the special cases. Confirm how the map depicts inclined, horizontal, vertical, or overturned beds before interpreting an individual mark.
These marks are observations of orientation represented on the map, not labels for the surrounding color patch. Start by deciding what planar feature is being measured. Then read the symbol’s orientation and its accompanying number or numbers together, using the explanation to distinguish the map’s forms for different attitudes. If the same map includes foliation, cleavage, joints, or lineation marks, do not assume that all of them are strike-and-dip symbols for bedding.
Do colors have standard meanings?
Geologic maps use colors and patterns, including palettes associated with geologic age. Those conventions can provide clues, but they are not a universal code that replaces the unit key. Map-specific choices and special-purpose maps can differ. A color patch identifies a mapped unit only according to the explanation for that map.
For that reason, do not identify an unfamiliar rock, formation, or age from color alone. First match the area’s label to the map-unit description. Then use the map’s color and pattern conventions as a visual aid for following that unit across the page.
USGS’s Selection of Colors and Patterns for Geologic Maps of the U.S. Geological Survey discusses color and pattern conventions, and the FGDC Geologic Map Symbol Standard includes color and pattern symbols. These resources help explain common approaches; they do not make colors universal across every map or remove the need to consult the particular map’s unit key.
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What if the unit name is unfamiliar?
If you have a location or unit name but need more context, the Cooperative National Geologic Map, developed by the USGS and State Geological Surveys, can help you look up mapped information. Its map lets users click or tap a location to see a mapped unit and its materials, and its free-text search can find source map units by name.
Treat that resource as a route to further information, not a replacement for the source map’s own legend. The source map defines the abbreviations and conventions used on that map, and its unit description may include details that a broader lookup does not convey.
A practical lookup can begin either way: use the location-based map when you know where the area is, or search by unit name when you have an unfamiliar label. In either case, compare what you find with the source map and its unit description. The broader resource can help you identify mapped materials or locate source-map information, but only the original legend explains exactly how that map used its abbreviations, colors, and symbols.
How should you use a cross section?
A surface map locates mapped units and structures at the surface. A cross section presents an interpretation of subsurface relations as far as they are known and as the scale permits. Read the two together: the map establishes where the surface features are shown, while the section helps explain one interpretation of how those features may relate below ground.
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When a map includes a section, use it as an explanatory companion to the surface view, not as evidence that every feature continues unchanged beneath the map. Its interpretation is bounded by what is known and by the scale at which the map and section were made. King and Beikman’s 1974 USGS Explanatory text to accompany the Geologic map of the United States is one reference for the distinction between surface mapping and interpreted subsurface relations.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What are the most common reading mistakes?
- Guessing a unit from its color. Look up its label in the unit key; palettes and special-purpose choices can vary.
- Calling every boundary a fault. A contact separates units; a fault is a break with displacement. Check the line category in the legend.
- Treating line styles as decoration. Dashed, short-dashed, or dotted traces may convey approximate location, inference, or concealment. Confirm the map’s definitions.
- Reading fault marks without their key. Hachures, sawteeth, balls, bars, arrows, and paired marks can indicate different things. Use the exact map explanation.
- Assuming orientation symbols all mean bedding. The map may show several planar or linear features. Identify the symbol family first.
- Taking a cross section as a literal view underground. It is an interpretation, constrained by knowledge and scale.
- Comparing maps as if they had identical conventions. Check each map’s scale, date and source, unit definitions, symbol explanation, and mapped theme before comparing their details.
One further pitfall is to treat the style of a trace as a statement about the kind of feature. A dashed line may indicate approximate location, but it does not by itself distinguish a contact from a fault. Likewise, a symbol that looks familiar is not enough to establish a movement direction or bed attitude without checking its map-specific entry.
Which sources explain the conventions?
The Federal Geographic Data Committee’s Geologic Map Symbol Standard describes line, point, color, and pattern symbols, with examples and usage notes. It aims to make geologic map products more consistent while recognizing that special-purpose products may need clearly explained variations. USGS’s Geologic Map Symbols Commonly Used on Maps of the United States Geological Survey presents common symbol conventions, including contacts, faults, folds, and orientation marks. These references are useful for understanding familiar symbols; the individual map’s explanation remains the authority for that map.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallFor background on the distinction between mapped surface geology and interpreted subsurface relations, King and Beikman’s 1974 USGS Explanatory text to accompany the Geologic map of the United States describes the role of geologic maps and cross sections. USGS’s Selection of Colors and Patterns for Geologic Maps of the U.S. Geological Survey discusses color and pattern conventions. Neither general reference changes the need to read the key printed with the map you are using.
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These are digital reference resources for understanding conventions, not substitutes for a map-specific explanation. Optional field tools are not required to read a legend, and a printed regional map or manual depends on the location and edition a reader needs. For a general map-reading task, the essential steps are to consult the map and its key, then use the official symbol references as background when a convention is unfamiliar.
Frequently Asked Questions
How do I find out what all the colors and abbreviations on a geologic map mean?
Match each area’s label or abbreviation to the map-unit key, then read the full unit description for its name, age, lithology, and qualifications. Check the map’s color and pattern explanation as well; color alone is not a reliable identifier.
How can I tell whether a line is a contact or a fault?
A contact marks a boundary between geologic units; a fault is a break along which displacement occurred. Use the line category in the map’s symbol explanation. Dashed, short-dashed, or dotted styling may separately indicate approximate location, inference, or concealment.
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What do hachures and sawteeth on a fault line indicate?
In common USGS conventions, hachures mark the downthrown side of a normal fault, while sawteeth mark the upper plate of a thrust fault. Verify their placement and meaning in the particular map’s key.
Can I identify a rock unit from its color?
Not reliably. Colors and patterns can follow age-related conventions, but map-specific choices and special-purpose maps can differ. Use the label and unit key to identify the mapped unit.
What does a strike-and-dip symbol tell me?
It records the measured orientation of bedding or another planar geologic feature. Read the symbol’s orientation and accompanying number or numbers together, and check the key for distinctions such as inclined, horizontal, vertical, or overturned beds.
Where can I look up an unfamiliar mapped unit?
The Cooperative National Geologic Map, developed by the USGS and State Geological Surveys, lets users click or tap a location to see a mapped unit and its materials; its free-text search can find source map units by name. Confirm details against the source map’s own legend.
Is a geologic map cross section a literal picture of what is underground?
No. It is an interpretation of subsurface relations as far as known and as the scale permits. Use it alongside the surface map, while keeping its interpretive and scale limits in mind.
The Bottom Line
Read the map’s own explanation first: use its unit key for colors and abbreviations, and its symbol key to distinguish contacts, faults, and structural marks. General USGS and FGDC conventions are helpful guides, but the legend, scale, source, date, and mapped theme determine what a particular map shows.
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