Through the Eyes of a Geologist: Structural Geology and Mapping

For this blog post, I want to discuss some structural geology, mapping skills, and briefly touch on the geology of the Appalachian Mountains. I will first begin by showing this Google Earth image (Fig. 1) of a segment of the Appalachian Mountains in Pennsylvania.

Figure 1: Image of the Appalachian Mountains in Pennsylvania.

In this image, there are obvious "wrinkles" of the Earth that are identified as the mountains. These mountain belts were created from multiple collision events between plate boundaries and with volcanic island arcs. The first of these collisional events is called the Taconic Orogeny and occurred in the Middle Ordovician (~480 million years ago). The modern day Appalachians were created during the Alleghanian Orogeny during the Permian (~285 million years ago) which was the result of the North American Plate and African/Eurasian plate colliding. This collision also helped create the supercontinent Pangea. The landscape you see today is from further uplift and lots of erosion of the sediments.

In this image (Fig. 2) what I want to show are the folds ("wrinkles") which are called anticlines and synclines.

Figure 2: Same image from Figure 1. White arrows point to synclines and anticlines. Please note that the white arrows are pointing to the "points" of these folds.

To visualize the morphology of anticlines and synclines, take a magazine and imagine that each page is a layer of rock. The Law of Superposition tells us that originally these sediments were layered with the oldest at the bottom and the youngest at the top. If you fold the edges of the magazine down so you make an upside down U, this will create an anticline. If you would slice the top of the magazine off (or rock layers) the oldest layers would be in the middle with younger sediments on the sides. If you fold the edges of the magazine up and make a U, this will create a syncline. Here if you slice off the top you will see younger sediments in the middle with older layers on the sides (Fig. 3).

Figure 3: Example of anticlines and synclines. The image is from http://www.radford.edu/jtso/GeologyofVirginia/Photos/Structures/Foldblock.png.

In Figure 2 the folds have pointed ends, which I had pointed out with the white arrows. As you can see in Figure 3, if the beds were horizontal in the one direction, in map view you would observe continuous beds. The pointed ends of these beds indicate that the beds are dipping, or as a geologist would say, plunging (Fig. 4)

Figure 4: Example of plunging synclines and anticlines. Notice the points of the folds, similar to what I pointed out in figure 2. Image is from http://saturniancosmology.org/files/geology/Sect2_1a_files/plunging.jpg

Now we can see that the original image of the Appalachian Mountains in Pennsylvania is a series of plunging anticlines and synclines (Fig. 1). How do we know though which ones are synclines and anticlines, and how can we figure out the way the beds are dipping? These are good questions that geologists use to map geologic beds in the field. I will explain the way you can identify the types of folds and the dip of the beds. As I mentioned earlier, anticlines have the oldest beds in the middle with younger on the sides, while synclines have younger in the middle with older beds on the sides. A good resource to find the ages of beds are geologic maps. It is then as simple as looking at the ages and then determining if the fold is an anticline or syncline. To find the dip of the beds, you have to do some memorization. Anticlines will point in the direction of the dipping bed, while synclines will point in the opposite direction. You can see this principle illustrated in Figure 4.

Going back to our original Figure 1, I am now going to overlay a geologic map of the area thanks to the wonderful people at San Diego State University (Figure 5). If you have not been reading my blog, I mentioned in an earlier post about overlays you can add to Google Earth. This overlay is a geologic map of the entire United Staes.