Geoscientist’s Toolkit: Geologic Map

Geologic map of Minnesota bedrock.  Image credit: scaled down from University of Minnesota/Minnesota Geological Survey.
Geologic map of Minnesota bedrock. Image credit: scaled down from University of Minnesota/Minnesota Geological Survey (original, 11 MB PDF).

Maps are neat. Geologic maps in particular can be quite interesting (see above, particularly the original PDF). These are the product of detailed surveys, which are undertaken both at the federal and state level, and show which rock types are found in which regions. Some of these rocks can be traced over long distances (like the sedimentary rocks of the southeastern corner of Minnesota), while others are localized.

Geologic maps give a summary of what types of rocks are in which areas. From this, you can find out search terms to get you to more information about certain rocks, or you can use the rock type to determine what used to be happening in an area. For instance, southeastern Minnesota was once covered by a warm, shallow sea, leading to sandstone, limestone, and dolostone formation. Some of the limestones are fossiliferous. Northeastern Minnesota used to be home to a volcanic rift valley (like the one presently in East Africa) and is home to volcanic rocks, such as the North Shore Volcanic Group.

In addition to the short description of the rock units, geologic maps will give the estimated age range of the rocks (if you need a refresher on geologic time, see this post). A quick glance at the time scale will show you that although you may find fossils in southeastern Minnesota, don’t expect to find any dinosaurs (they existed during the Mesozoic)!

Faults are mapped as well, either transform (offset side-to-side), thrust (compressing, one side going up), or normal (expanding, one side falling). Dikes, which are ribbon-like intrusions which cut through the local rock, are mapped as lines. Because they need to cut through the local rock, they are inherently younger than the rock which they cut through—thus a radioisotopic age for the dike will be a minimum age for the unit it intrudes.

There are also several different types of geologic maps. Bedrock maps, such as the one above, show what the primary consolidated rock is, although it may be buried beneath loosely packed, more recent sediments. Surficial maps show more recent deposits; here in Minnesota, that’s often glacial deposits of various types, but can also include features such as alluvial fans and landslide deposits.

Finding geologic maps here in the US can be a little bit tricky. The USGS has nice geologic maps (start here), but they tend to be large-area. State surveys seem to have more detailed local maps, but each state has their maps in a different location and the availability may not be consistent state to state. Montana has a nice geologic map interface on their website, while Minnesota’s geologic maps are not easily found—there are county-scale surficial geologic maps, at least for some counties, but I’ve really only been able to find them through third-party search engines. For advanced map users, the state surveys will often make the raw GIS (geographic information system) data available.

Silicic dike in the Benton Range, near Bishop, CA. Image credit: Bill Mitchell.
Silicic dike in the Benton Range, near Bishop, CA. Image credit: Bill Mitchell.
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