A geological map is the single most useful document an explorer can hold before fieldwork, and most of the good ones are free. This guide walks through the national and state geology datasets in the United States, Australia and South Africa, how to put them into Google Earth or a GIS, and how to read a flat map as a 3D picture of the ground, down to working out how deep a mapped contact sits below your licence.
Sources USGS · GSWA · GSNSW · GSV · CGS
Formats KMZ · SHP · GDB · PDF
Markets Australia · USA · Canada
A 3D geologic map shows rock units and structures as volumes and surfaces below the ground, not just as coloured patches on it. Most explorers start with the 2D version: a bedrock geology map or a regional geological map from a government survey. With strike and dip readings, a few drill holes and some simple geometry, that flat map becomes the first 3D geologic map of a licence.
Try it: Depth-to-contact and true thickness from a geological map →
This matters because mineral deposits are three-dimensional. A gold-bearing shear, a porphyry intrusion or a lithium pegmatite dips somewhere below the outcrop. Reading the map in 3D tells you where a drill hole should start and how deep it has to go, before anyone pays for a metre of drilling. For gold ground, our guide to gold map sources, from government data to old mines covers the occurrence layers that sit on top of the geology.
“The USGS state compilation stitches 48 state geologic maps, from 1:50,000 to 1:1,000,000 scale, into one database.”
A 1:1,000,000 regional map can place a contact kilometres from where it really is on your licence. Find the largest-scale sheet that covers your ground first, then use the regional map only for context.
Types of geological map and what each is for
| Map type | What it shows | Typical scale | Best use in exploration |
|---|---|---|---|
| Regional geology map | Major units, belts and faults across a state or country | 1:250,000 to 1:1,000,000+ | Choosing which belt or province to work in |
| Bedrock geology map | Solid rock with soil and young cover stripped away, often interpreted | 1:100,000 to 1:500,000 | Tracing host rocks and structures under cover |
| Surface or surficial geology map | What is actually at the surface, including sand, soil and regolith | 1:24,000 to 1:250,000 | Planning sampling and understanding transported cover |
| Map of mineral resources / mineral deposit map | Mines, deposits and occurrences, often over simplified geology | Any | Seeing where known mineralisation sits in the geology |
| 3D geologic map or model | Surfaces and volumes of units and faults at depth | Regional to deposit | Depth, drill planning and cover thickness |
Two questions settle which map you need. How big is the area you are deciding about, and how deep does the target sit? A regional geological map answers the first question for a whole belt. Only a larger-scale map, drilling or a 3D geologic map answers the second. For more on field mapping at licence scale, see our guide to geological mapping methods and scales.
How to read a geological map legend
Most mistakes with a borrowed map come from the legend, not the geology. Before interpreting anything, work through the margin of the sheet.
- Unit order. Legends list units from youngest at the top to oldest at the bottom. Knowing which rock is older tells you which intruded or overlies which.
- Letter codes. The first letter or symbol usually gives the age, the rest the rock name. Two units with the same colour but different codes are different rocks.
- Line styles. Solid contacts were observed; dashed ones are approximate; dotted ones are concealed under cover. On a covered licence, most contacts may be dotted.
- Structure symbols. Strike and dip ticks, fold axes and fault symbols carry the 3D information. Note whether dips are measured or inferred.
- Cross-sections. The section lines printed on the map are the author’s own 3D geologic map in two slices. Read them before drawing your own.
- Source and date. The reliability diagram or source index shows which parts were mapped in detail and which were compiled from older or smaller-scale work.
Surveys follow their own conventions, and the USGS, Australian state surveys and the Council for Geoscience all differ in detail. The principle is the same everywhere: the legend tells you how much of the map is observation and how much is interpretation. A licence covered mostly by dotted contacts and inferred units is a licence where a satellite, geophysical or drilling layer will change the picture most.
United States: USGS geologic maps and USGS mineral maps
The National Geologic Map Database
The National Geologic Map Database (NGMDB) is the national archive of US geologic maps. The USGS describes it as a Congressionally mandated function under the Geologic Mapping Act of 1995, built with the Association of American State Geologists, covering geology, geochemistry, geophysics, paleontology and geochronology. It is the place to find old geological maps as well as new ones, because it catalogues published maps back through the history of both the USGS and state surveys.
State geologic maps in one database, and in Google Earth
For an earth geological map of the whole conterminous US, the USGS State Geologic Map Compilation (Data Series 1052) joins 48 state geologic maps, ranging from 1:50,000 to 1:1,000,000 scale, into one continuous database with consistent lithology and age attributes. The USGS geologic maps of US states page offers it as a file geodatabase (415 MB), shapefiles (634 MB), per-state shapefiles and per-state compressed KML (KMZ) for Google Earth. California’s KMZ, for example, is about 14 MB. Alaska, Hawaii and Puerto Rico have separate maps.
This is the simplest Google Earth geology overlay there is. Open the state KMZ, set the transparency to around 50%, and the geology drapes over Google Earth’s terrain, giving a rough 3D geologic map of the terrain for free. It is not a subsurface model, but seeing units wrap around ridges and valleys already tells you a lot about their dip.
USGS mineral maps
The USGS mineral maps most explorers use are its occurrence databases, the Mineral Resources Data System and the USMIN mine features layer, served from the same Mineral Resources Online Spatial Data site as the state geology. Layering them on the state geology produces a quick map of mineral resources for any US district: deposits as points, host rocks as colours, faults as lines.
The USGS 3D geologic framework
The USGS has also published a national-scale 3D geologic map. Its 3D Geologic Framework for the National Crustal Model (version 2.0, September 2025) is built from 1:250,000 to 1:1,000,000 state and continental geologic maps plus depths to subsurface boundaries, extrapolating rock type and age downward and constraining the depth of bedrock and of sedimentary cover with geophysics in many areas. It was made for seismic hazard work, so it is coarse for exploration, but it is a good first read on cover thickness in basins.
A national compilation joins maps of very different scales and ages. Across a state line, or between two source sheets, the detail can change abruptly. Read the source-map index for your area before trusting a contact.
Australia: geological maps of WA, NSW, Victoria and South Australia
Geological map of Western Australia
The Geological Survey of Western Australia publishes the state map in layers. The 1:500,000 State interpreted bedrock geology (DMIRS-016) is one of three layers of the 1:500,000 geological map of Western Australia, alongside Cenozoic geology and linear structures. Its metadata says it was compiled from the 1:100,000 and 1:250,000 Geological Series maps and regional maps up to 1:1,000,000, then adjusted using magnetics, radiometrics, gravity, ASTER and Landsat imagery, seismic, drilling and unpublished mapping. It is released under a Creative Commons Attribution licence.
That description is worth reading closely. An interpreted bedrock geology map of a covered craton is already partly a model: geophysics and drilling have been used to guess what lies under the sand. It is the right map for regional targeting, and the wrong one for siting a single drill hole.
Geological map NSW: MinView and Seamless Geology
The Geological Survey of NSW runs the NSW Seamless Geology project, a statewide compilation organised into time slices of the major rock packages. Version 2.6 of the data package was released in May 2026 with upgraded mapping of the Rockley-Gulgong Volcanic Belt and the Canbelego 1:100,000 sheet area. It downloads for ArcGIS Pro, MapInfo and QGIS, in GDA94 and GDA2020, and can be viewed on phones through MinView. A single-layer version shows surface geology only. The survey notes it dropped the ArcMap format after Esri retired ArcMap on 1 March 2026.
Geological maps Victoria: GeoVic and 3D models
The Geological Survey of Victoria provides GeoVic, a free online mapping application for geology and earth resources data, and publishes 3D geological models. Resources Victoria’s page on 3D geological modelling describes a Whole of Victoria model covering the full crustal thickness, a more detailed Stavely model for the Grampians-Stavely Zone of western Victoria, and basin models for the Otway and Gippsland basins. Victoria is one of the few places where a free 3D geologic map exists at a scale an explorer can use regionally.
Geological map South Australia: SARIG
South Australia’s SARIG portal delivers statewide geology, geophysics, regolith, paleodrainage and tenement data free, with no login. Geological maps download as preset PDF sheets or GIS layers, including 1:250,000 and 1:100,000 surface geology sheets.
NSW packages come in GDA94 and GDA2020, and older maps may use AGD66 or AGD84. The shift between datums can matter at licence scale, so set every layer to the same datum before comparing geology with your tenement.
Geological maps South Africa and other countries
South Africa’s Council for Geoscience, which houses the national geological survey, offers free national-scale PDFs on its CGS geological maps page: a mineral map, a geological map and a chronostratigraphic map of the country. More detailed sheets, including the 1:250,000 series, are requested or bought from the Council. For prospecting and mining rights, the cadastre sits with the Department of Mineral and Petroleum Resources; see our South Africa mining licence guide.
For countries with thinner online coverage, the OneGeology initiative was the main attempt at a single world geology portal. Its factsheet counts 116 participating nations, each represented by its geological survey, covering more than 100 million km², about 70% of the Earth’s land surface. The OneGeology site now says the initiative has closed after 18 years, with its resources still available and a new host being sought for the portal. For many African and Latin American countries, the national survey remains the only source of detailed geology, often on request or for a fee.
Reading a flat map as a 3D geologic map
Every geological map carries 3D information. The strike and dip symbols, the way contacts bend across valleys, and the cross-sections printed in the margin all describe how units continue below the surface. Turning that into a working 3D geologic map of a licence takes three steps.
- Read dips. Record strike and dip symbols near your target, and note whether they are measured or inferred.
- Project down-dip. For a contact of known dip, depth below a point is the horizontal distance from the outcrop, measured in the dip direction, multiplied by the tangent of the dip.
- Check with drilling or geophysics. Any existing drill hole that crosses the contact tests the projection. Where there are none, magnetics or gravity often show whether the unit continues.
The geometry is taught in every structural geology course. Athabasca University’s GEOL 319 lab on depth and thickness works the classic case: a bed dipping 32° crops out 100 m from a property line, so a vertical hole at the line meets it at 100 × tan(32°), about 62.4 m. For true thickness on flat ground, Steve Dutch’s structural geology notes give thickness = outcrop width × sin(dip), and for a vertical hole, thickness = vertical interval × cos(dip).
Depth-to-contact and true thickness from a geological map
Assumptions: planar contact with constant dip, flat ground, distance measured perpendicular to strike in the dip direction. Depth = distance × tan(dip); distance along the dip = distance ÷ cos(dip); true thickness = outcrop width × sin(dip), or vertical interval × cos(dip). Formulas as taught in Athabasca University GEOL 319 Lab Unit 4 and Steve Dutch’s notes. If you measure the distance at an angle to the dip direction you are using an apparent dip and the depth will be wrong. Folds, faults and changing dip break the result; confirm with drilling.
From a hand projection to a 3D geologic map model
Once a licence has more than a handful of drill holes, the hand projection gives way to software. Modelling packages build surfaces from contacts, dips and drill intercepts and let you slice the result in any direction. Our comparison of 3D geological modelling software covers the main options, and our QGIS guide for geologists covers the free GIS route for the 2D layers that feed any 3D geologic map.
A pre-fieldwork checklist for any geological map
Before a map goes into a work programme, it is worth ten minutes of checks. They take less time than one wasted field day.
- Scale and date. Is this the largest-scale sheet for the area, and how old is the mapping behind it?
- Datum and projection. Does it line up with your licence boundary and your GPS, or is there a shift?
- Observed versus inferred. How much of your ground is covered by solid contacts, and how much by dashed or dotted ones?
- Structures. Which faults cross the licence, and are they mapped at surface or interpreted from geophysics?
- Occurrences. Which recorded mineral occurrences sit on or near the ground, and in which unit?
- Depth. For the unit you care about, what do the dips and cross-sections say about how deep it goes below your target area?
Interactive geologic maps, Google Earth and Google Maps
Most surveys now offer an interactive geologic map in the browser: MinView in NSW, GeoVIEW.WA in Western Australia, GeoVic in Victoria, SARIG in South Australia and the USGS interactive state map. They are the fastest way to look, and the slowest way to work, because you cannot easily combine them with your own data. For real work, download the layers. Our round-up of geological survey agencies and the free data they hold lists where to find them.
For a quick view, Google Earth remains the easiest option. Its terrain gives relief for free, so a state KMZ draped as a Google Earth geologic map already behaves like a rough 3D geologic map of the surface. Google Maps is different: it has no geology layer of its own, so searches for “google maps geology” or “google geological maps” end up at third-party overlays or back at Google Earth. Some university projects publish interactive world geology in the browser; treat them as context and go back to the survey data for anything that matters.
- For looking: the survey’s interactive viewer.
- For layering with your licence: Google Earth with the survey KMZ plus your boundary KML.
- For analysis: QGIS or ArcGIS with shapefile or geodatabase downloads in one datum.
- For depth: dips, cross-sections and drill data in a 3D modelling package.
Old and historical geological maps
Old geological maps still earn their place. They often record outcrops, workings and field observations that were later generalised away, and they show how interpretations changed. The NGMDB catalogues historical USGS and state maps, and most Australian surveys keep scanned older sheets. Use them for observations, like the location of an outcrop or a shaft, rather than for interpretation, which may be decades out of date.
A project summary that shows a regional map at 1:1,000,000 over a small licence is not showing the licence’s geology in any useful detail. Ask for the largest-scale map, the mapping date and whether contacts have been drill-tested. General information, not investment advice.
Adding a satellite layer to your geological map
Government geology tells you which rocks and structures cross your ground. It rarely tells you where alteration has changed those rocks, which is where many deposits sit. Our satellite-based mineral detection analyses multispectral and hyperspectral data over your boundary to flag mineralised target zones, alteration halos, faults and fractures, delivered as georeferenced GIS files that drop straight onto the survey geology. The result is a ranked list of exploration targets for fieldwork, not resources.
- Input: coordinates, KML/KMZ or a polygon, plus country and target mineral.
- Output: high-potential zones, prospectivity heatmaps, estimated location and depth ranges, and geological interpretation of faults, alteration and host rock, in PDF and GIS formats.
- Premium+: TargetMax™ drilling-angle recommendations and interactive 3D subsurface models of vein structures and mineral distribution, a satellite-driven complement to a 3D geologic map.
- Timing and cost: 5–20 business days; early-exploration timelines cut from months to days, and costs lowered by up to 80–85%.
We have scanned 100,000+ hectares for 20+ mineral types in 25+ countries. Draw your licence on mining.farmonaut.com (Map Your Mining Site), request a price through the mining query form, or see a sample of satellite-driven 3D mineral prospectivity mapping.
Every drill pad clears ground. A 3D geologic map that projects contacts correctly, checked against satellite and geophysical data, puts the first holes in the right place and reduces the number needed to test a target.
Put a target layer on top of the survey geology
Send us your licence boundary. We return ranked target zones with structural and alteration interpretation as GIS files, ready to overlay on the bedrock and regional maps you already use.
Frequently asked questions
What is a 3D geologic map?
It is a map or model showing rock units and faults as surfaces and volumes below the ground. Surveys publish some at regional scale, such as the USGS national 3D framework and Victoria’s 3D models. At licence scale, explorers build their own from mapped dips, cross-sections and drill holes.
Where can I find a bedrock geology map?
From the state or national geological survey. In Western Australia, the 1:500,000 interpreted bedrock geology is free under Creative Commons. In the US, state surveys and the NGMDB list bedrock maps, and the USGS state compilation covers the conterminous states.
What is the difference between a regional geological map and a regional geology map?
None; the terms mean the same thing. Both describe small-scale maps, usually 1:250,000 or smaller, that show belts, major units and faults across a large area. They are for choosing where to work, not for siting drill holes.
Is there a Google Earth geology overlay?
Yes. The USGS offers each US state’s geology as a compressed KML (KMZ) file for Google Earth, and several Australian and Canadian surveys publish KML layers. Draped on Google Earth terrain, they give a quick view of how units sit across hills and valleys.
How do I work out how deep a mapped contact is?
Multiply the horizontal distance from the outcrop, measured in the dip direction, by the tangent of the dip. A 32° contact 100 m away is about 62 m deep. The calculator above does this and also converts outcrop width to true thickness.
Where can I get geological maps of South Africa?
The Council for Geoscience offers free national geological, mineral and chronostratigraphic maps as PDFs. Detailed sheets such as the 1:250,000 series are requested or bought from the Council.
Reviewed September 2026 against the USGS National Geologic Map Database, USGS Data Series 1052 and the USGS geologic maps of US states page, the USGS 3D Geologic Framework data release, GSWA’s DMIRS-016 metadata, the Geological Survey of NSW Seamless Geology project page, Resources Victoria’s 3D geological modelling page, the Council for Geoscience maps page, the OneGeology site and factsheet, Athabasca University’s GEOL 319 Lab Unit 4 and Steve Dutch’s structural geology notes.
Dataset versions, file sizes and download counts change; check each survey’s page for the current release. The calculator assumes planar geometry and is a planning aid, not a substitute for drilling. Nothing here is investment advice. Satellite targets are exploration targets, not mineral resources, and need sampling and drilling to confirm.
