Athro Limited, Education on the Web: Structural Geology
 Interpreting Geologic Sections 

Geological sections and maps are pictures of what rocks do. Geologic maps show the pattern of rocks on the surface of the earth. Geologic sections show the patterns of rocks as they might be exposed on the side of a road cut or on the wall of a trench. One of the things that geologic sections show very well is the geological history of an area. One can look at a section (or a map, or both) and infer the order of events that produced the section. The heart of field geology is going up to rocks and getting them to tell you their stories. Interpreting the order of events in a geological section is reading the history of that section, and is the basic tool of all field geology.

There are two basic tools for interpreting these sections: 1) superposition - generally in geology, sedimentary rocks are deposited one on top of the other, so rocks that are buried by other rocks tend to be older than them. 2) crosscutting relations - some kinds of things cut across rock units - faults break the existing rocks, folds bend them, igneous intrusions get squeezed into other rocks. Look at the sections below and try to read their histories.

You can work out the order of events in each of the geological sections listed below. Each page contains a section, a list of events, and when you have decided on the order of events, an animation of the history of the section.

Hypothetical Geologic Sections:

A fault
Folds and an intrusion
Two intrusions

Real Geologic Sections:

Gubik Formation (Neogene) at Skull Cliff, Alaska

Part of the Athro, Limited web site.
Copyright © 1998, 1999 Athro, Limited. All Rights Reserved.
Written by Paul J. Morris
Maintained by Athro Limited Date Created: 19 Jan 1998
Last Updated: 5 Jan 2000