Components of Geologic Maps
By
Mark A. Jirsa and Terrence J. Boerboom
2003
GEOL
3000
Objective of a Geologic Map: To interpretively portray the
spatial and temporal relationships of rocks, unconsolidated earth
materials, and landforms at the earth’s surface.
Types of Geologic Maps (on Earth)
• Complete Geologic Maps – portrays both bedrock and
unconsolidated material in areas where both are prevalent
• Bedrock Maps – portrays only the bedrock geology as best as can
be inferred from available outcrop
• Surficial (or Quaternary) Maps – portrays the distribution of
unconsolidated earth materials and landforms in areas
affected by Pleistocene glaciation
• Specialty Maps – tectonic, geohydrologic, isopach, geophysical,
mineral potential, groundwater contamination
susceptibility
Complete Geologic Maps
Scales of Geologic Maps
The scale of a map controls the amount of detail that can be
portrayed. And the amount of geologic detail controls the
appropriate scale of a geologic map.
Project/Deposit Scales (≤1:10,000)
Quadrangle (7.5’-15’)/Township* Scales
(1:12,00/20,000/24,000/50,000/48,000/63,360)
Regional/County/1°x2°/60’x30’/Scales
(1:100,000/200,000/250,000)
State/Provincial/National Scales (≥1:500,000)
* Canadian
Parts of a Geologic Map

The Map
- Base Map (type is scale-dependent)
- Map Units (w/ abbreviated labels)
- Geological Lines: Contacts, Faults, Fold axes, Structural fabric, Geophysical lineaments,
Landform outlines...
- Outcrops (at scales <1:50,000)
- Structural Feature Measurements
Planar: Bedding, Contacts, Foliation, Cleavage, Layering, Joints, Dikes, Veins, Shear
zones, Fault planes...
Linear: Mineral lineations, Cleavage-Bedding intersections, Slickensides, Fold axes,
Glacial striations...
- Other Features of Note: Drill hole/handsample locations, Mineral deposits...

The Legend
- Title/Authors/Date (again the most important part!!)
- Scale/North Arrow (w/ magnetic declination)/Location Map
- Map Unit Descriptions
- Correlation of Map Units (Younging up is the only constraint)
- Map Symbol Key (USGS recommendations: USGS OFR 99-430)
- References (previous mapping, citations from Map Unit Descriptions)

Optional Map Components
- Cross-sections - Geophysical images - Airphotos
- Source of data map
- Inset Maps showing detailed geology
- Structural/Tectonic elements map
The Map – Base Maps


Objective – Displays surface features (topography, roads, lakes &
river, cultural features) and a grid system (TRS, UTM, Lat/Lon)
that allow one to readily locate the geologic features portrayed
Best choice of base map is largely dependent on the map scale
1:24,000 scale (7.5’ USGS quadrangle)
1:100,000 scale (30’x60’ USGS sheet)
The Map – Outcrops
the “facts”
The Map – Structural Measurements
more “facts”
Planar Features - Strike and Dip
Sedimentary Bedding
Lava flow contacts
Intrusive contacts
Foliation (metamorphic and igneous)
Cleavage
Gneissic banding
Layering (modal, textural, phase)
Laminations
Dikes
Veins
Joints
Shear zones
Fault planes...
Linear Features – Trend and Plunge
Mineral lineations
Cleavage-Bedding intersections
Slickensides
Fold axes
Glacial striations...
The Map – Map Units
Definition: includes rocks or unconsolidated
materials at the earth’s surface that generally
share a common mode of origin and age. Rarely
composed of a single rock type.
“Lumping & Splitting” – the iterative process of
deciding what attributes and variables to include
in a particular map unit
The Map - Geologic Lines
Contacts, Faults, Fold axes, Structural fabric, Geophysical lineaments, Landform outlines...
Gradations in confidence:
• Certain
• Approximately located
• Inferred
• Speculative
• Concealed
The LegendTitle/Authors/Date/Scale/North
Arrow/Location Map
The Legend – Description of Map Units
•Heirarchical; arranged from
youngest to oldest
•Variable in detail
•Minimum information
will include major rock types
•Other information may
include mineralogy, texture,
structural elements, relations
to adjacent map units,
absolute age, fossil content,
etc.
Legend from the Babbitt SE quad, MN (Miller, 2005)
The Legend – Correlation of Map Units
TIME
The Only Requirement
Diagram from the Little Marais quad, MN (Miller,Boerboom, & Jerde, 2006)
The Legend – Map Symbol Legend
US Geological Survey
Recommendations for
Symbology
USGS Open-file Report 99-430
http://pubs.usgs.gov/of/1999/
of99-430/
Legend from the Duluth quad (Green & Miller, 2008)
Optional Map Components
Geologic Cross Section
Aeromagnetic Image
Mapping Responsibility Diagram