Geological Maps

What is a Geological Map?

A geological map is a map that shows the geology of a given area. Geological maps are especially useful for rockhounding because they allow you to learn about the areas where you have collected specimens and to identify similar areas that may be productive for future collecting.

For example, geological maps can provide the following information:

The name and age of the geologic formations exposed at the surface.
Descriptions of those formations, often including the types of fossils and/or minerals commonly associated with them.
The locations of faults, meteorite impacts, mines, and quarries within the area.

The following sections provide information on how to use geological maps, with an emphasis on applications in Kentucky. The discussion will begin with the Kentucky Geological Survey (KGS) Interactive Map Service, followed by the KGS 1:100,000-scale maps, and finally the original 1:24,000-scale geological maps.

First....Kentucky Geological Survey Interactive Map Service Page

As noted above we are going to deal with Kentucky. First lets start off with the Kentucky Geological Survey's Interactive map for the entire state which is located at:

https://kgs.uky.edu/kygeode/geomap/

You can physically move/zoom into the area you want to view or you can click "Menu" in the upper left corner (see image to left) and then "Zoom to Location". Here you can enter:

A place name
An address
Latitude/Longitude (needs to be in decimal degrees)
ect.

For this example we are going to a site I like located at 38.008, -85.501

In our example, the map service provides the image to the left. It shows Od and Ogl as the two formations here. Being the road cut is rather deep (see photograph to the right) it is the Ogl formation we are primarily interested in.

When we click the Ogl formation it produces the pop up shown to the left. From here we can:

See the the formation is the Grant Lake Limestone.
View Geologic Description (discussed below)
View the Geologic Quadrangle for the area. These are 7.5 minute maps which are 1inch = 2,000 feet. So they are about 7 miles east to west and about 8.6 miles north south. If you are used to using normal USGS topographic maps, they are the same but for geology.
Download the "Digitally Vectorized Geologic Quadrangle" this is useful if you work with GIS.

View Geologic Description

On the popup box "View Geologic Description" and then we see it is the "Grant Lake Limestone" which is Upper Ordovician age (see image to left).

The Ordovician is a geological time period that lasted from 485–444 million years ago. Upper means the younger side and Lower would mean the older side of the date range. Upper would generally mean 458 to 444 million years ago.

It also states that the "Grant Lake Limestone" is primary a limestone / mudstone.

If we scroll down there is a description of the Grand Lake Limestone which states "..... the brachiopod Platystrophia ponderosa is abundant."

So if you found brachiopods at this location you can research Platystrophia ponderosa and see if the description matches what you found.

Handy Trick!!

If you want to save / bookmark the map at the current location and zoom,

click Menu >Bookmark>Create Bookmark

Then click "Link to this layout WITH extent"

Sidenote....Turn on layers

If you go to Menu > Layers and then scrol down to near the bottom and look for "Economic" and turn on the various layers there. This will show things like quarries, "Geological Points of Intrest" Note the more layers you turn on, the slow it is to respond.

Second....Kentucky Geological Survey 1:100,000 Geological Maps

Another useful resource is the KGS 1:100,000 Scale Geological Maps. These are 30x60 minute maps which cover approx 55 miles east/west and 35 miles north/south. But the detail is simlar to the 1:24,000 scale maps. Note the graphic at the top will show what areas are covered by which map and if clicked will give you a LOW RESOLUTION map. For a HIGH RESOLUTION PDF or djvu file, scroll down in the text and find the map you need and click. This will open a page about that map and has a link to the PDF.

Third....1:24,000 Geological Maps....Old School

These are the oldest of the maps but often they have the most detail especially about the rock descriptions in the geological column. Note that many of these are from the 1960's so roads may not be shown as they are today. If you look at the first section, you will see when you click a formation one of the things listed is the "Geological Quadrangle, Scanned". Just click that link and it will download a PDF of the 1:24,000 scale map.

Thanks to Debbie Joplin and Enrique Clark who reviewed the article for me.

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