Chapter 8. Geological Structures
Overview of Geological Structures Part 1: Strike, Dip, and Structural Cross-Sections
Adapted by Joyce M. McBeth, Karla Panchuk, Lyndsay R. Hauber, Tim C. Prokopiuk, & Sean W. Lacey (2018) University of Saskatchewan from Deline B, Harris R & Tefend K. (2015) “Laboratory Manual for Introductory Geology”. First Edition. Chapter 12 “Crustal Deformation” by Randa Harris and Bradley Deline, CC BY-SA 4.0. View source. Last edited: 8 Jan 2020
In Part I of geological structures, students will learn how to interpret strike and dip information from a geological map, prepare a geological cross-section from a plan-view geological map, and measure the thicknesses of geological units.
8.2 STRIKE & DIP
To learn many of the concepts associated with structural geology, it is useful to look at block diagrams and block models. Block diagrams are images based on three-dimensional (3-D) block models, which are blocks of wood or paper with geological structures marked on them. Block models and block diagrams assist in visualizing how 3-D geological structures in the real world can be represented in two dimensions on a map or in a geological cross-section.
As you examine the block diagrams in the figures in this section, note the different ways that you can view them: from above, or from the sides. If you look at a block from along the side, you are seeing the cross-section view. This is the view of geological structures you also see when you drive through the mountains and the roads have been cut through the rocks, exposing structures in the rock that you wouldn’t see otherwise. If you look at the block from directly above, you are looking at the map or plan view (Figure 8.3).
As you look at geological maps and drawings and try to figure out about how rocks have changed when they are tilted and/or deformed, it is useful to remember how they were deposited in the first place. Let’s briefly review some of the geological laws that you learned in the chapter on relative and absolute dating. Sedimentary rocks, under the influence of gravity, will deposit in horizontal layers (principle of original horizontality). The oldest rocks will be on the bottom (because they had to be there first for the others to deposit on top of them), and are numbered with the oldest being #1 (law of superposition). The wooden block in Figure 8.4 (a cross-section view of sedimentary layers) provides an example of the principle of original horizontality and the law of superposition.
Each of the boundaries between the colored rock units in Figure 8.4 represents a geological contact, which is the planar surface between two adjacent rock units. Earth’s rock layers are often complicated: rock layers are often tilted at an angle, not horizontal – this indicates that changes have occurred since deposition (e.g., the rocks have been uplifted by tectonic activity and tilted). Figure 8.5 is a block model example of tilted rocks. Which color bed in the block model is the old