Geologic Maps of the National Parks
OBJECTIVES
- Identify the features present on Geologic maps - lithology, ages and structure
- Measure and plot Strike and Dip on a geologic map
- Construct the location of layers in folded and faulted strata
MATERIALS
- Colored pencils, protractor, ruler, Geologic Map of Rocky Mountains NP, Colorado, https://ngmdb.usgs.gov/Prodesc/proddesc_736.htm
- OR http://npmaps.com/rocky-mountain/ OR http://npmaps.com/wp-content/uploads/rocky-mountain-topo-map.jpg PA Geologic map full, http://elibrary.dcnr.pa.gov/GetDocument?docId=1751215&DocName=Map1_Geo_Pa
- Geologic time scale. https://www.nps.gov/subjects/geology/time-scale.htm
- Grand canyon, Arizona geologic map. https://pubs.usgs.gov/imap/i2766/i2766_map.pdf
- https://pubs.usgs.gov/imap/i-2688/i-2688.pdf
INTRODUCTION
The Earth is an active planet shaped by dynamic internal forces. The internal heat convecting within the mantle breaks and moves the thin surface crustal plates. These forces push surface plates upward forming mountains, downward producing deep ocean trenches and spreading to form new ocean floor. Structural Geology is the study of how the Earth’s plates move and are deformed, often in a 3-dimensional manner. Structural geologists help us understand the formation of folds in mountains and faults that produce earthquakes. Geologic maps show the ages and orientation of surface and near surface rock layers, as well as folds, joints, faults, streams and weathered products. Understanding these processes and their impact on Earth materials is an essential skill for engineers, energy production, seismologist, hydrologist and many more. Geologic map reading skills involve connecting rock types, rock ages with three-dimensional features.
GEOLOGIC MAP FEATURES: Rock Type, Geologic Age & Structures
Geologic maps are a visual expression of the rock layers on the surface and below the surface. They are an essential tool for understanding the changes that have occurred to rocks over the immense span of geologic time – 4.6 billion years. The first geologic map was created by William Smith during the early 19th century (Figure 1). Geologic maps differ from topographic maps due to the addition of rock types, rock ages, and structural features. Both topographic and Geologic maps attempt to represent a two-dimensional world in three-dimensions. One of the most important aspects of a geologic map is the ability to determine rock types and if a rock unit is horizontal, folded or faulted. You can even determine the relative ages and chronological order of the rock sequences present. Geologic maps use different colors and symbols to represent the different units of rock. The term outcrop is used to represent the expression of rocks at the surface. Most geologic maps also draw a vertical line across the map to look at a cross section or cut away view at depth (figure 2). This side view shows the arrangement of rock layers below the surface as well as the topography of the land surface in profile. This profile view is essential when trying to determine how rocks are folded or faulted.
Figure 1. The first Geologic map created by William ‘Strata’ Smith of Great Britain in 1815. This map was used to connect similar rock types over large regions. For more information a book by Simon Winchester called “The map that changed the World” chronicles the life of William Smith and the development of this map. Image CC BY S.A. Wiki Commons public domain. Scan by the Library Foundation, Buffalo and Erie County Public Library. Image now at http://www.livescience.com/449-map-changed-world.html https://en.wikipedia.org/wiki/William_Smith_(geologist)#/media/File:Geological_map_Britain_William_Smith_1815.jpg
Figure 2. Cross section of a portion of the Grand Canyon NP geologic map in Arizona. The capital A and A’ at the top of the figure represents the marked cross-section A-A’ on the geologic map. Elevation on the y-axis is also given in meters above sea level. Faults are clearly marked with up and down arrows. Image NPS.gov public domain snip portion by author.
ROCK TYPES – Geologic Age
Looking carefully at a geologic map you see many colors that are used to represent different rock types and their geologic age. Using the legend at the bottom or the side of your map you can see the names of the rock units or formations, with specific details about the rocks that appear on the surface. For ease in reading a geologic map the rock unit/formation is given a symbol that incorporates the geologic time period (age), name and an abbreviation of the rock types name along with a unique corresponding color. On figures 3 and 4 below you see a small section of the geologic map of the Grand Canyon National park in Arizona. Notice the many blue, purple, green and yellow shades. Each different color represents a different lithology or rock type. Each rock type also has a unique letter symbol assigned to it. The letters represent both the geologic age (in upper case) of the rock type along with either one or two letters (lower case) representing the rock formation name.
For example find the green color with the Tgc markings on figure 3.
Figure 3. Snip of the geologic map of the Grand Canyon, Arizona. Image NPS.gov public domain.
Using figure 4 you can see that this rock labeled Tgc is a Paleozoic-clast conglomerate facies. It is listed under the rock type group for Sedimentary rocks. Use figures 3 and 4 to answer the following questions:
What color and symbol represents the rock unit Tgg?
What type of rock is Tgg?
Circle a location on the map where this unit is located.
Name an Upper Triassic rock unit.
What symbol and rock type is it?
Find the Hermit formation on figure 3. What age and rock type is this?
Figure 4. Snip of the sedimentary rocks from the geologic map of the Grand Canyon national park in Arizona. Image nps.gov public domain.
Let’s try another one using a map section from the Grand Canyon National Park, Arizona from figures 5 and 6.
Figure 5. Snip of the legend from the Grand Canyon NP, Arizona Geologic map
FIND THE FOLLOWING ROCK TYPES:
Qmrb is what type of rock?
On figure 6, find and circle Qlsb. Given the type of rock, what do you think the arrows on the map represent?
What does the upper-case Q represent?
Use the geologic time scale from figure 7 to determine the numerical age of the Basalt of Kenworthy Ranch.
Find Qp and Qp1 on the map and identify their location. What rock type and age are these units?
Figure 6. A snip section from the Grand Canyon NP, Arizona Geologic map. Image public Domain nps.gov. snip by author.
Figure 7. The Geologic Time Scale. Image NPS Geologic Resources Inventory 2018. Public Domain https://www.nps.gov/subjects/geology/time-scale.htm
Symbols: Used to represent the orientation of rock layers
Horizontal layers
Geologists take careful measure to record geological structures because they are critically important to understanding the geologic history of the region. As you have been looking at geologic maps you may have noticed a variety of symbols along with the various colors and geologic time references. One feature that is measured is the attitude or orientation of the layers (bedding). If rock units are horizontal only one unit will appear on the surface. This will appear as the same color rock spread out over a large area (figure 8). If the surface is not flat due to erosion from a stream, the layers below will appear parallel and repeating across from one another. Rivers eroding through horizontal layers form a very particular dendritic (tree branching) pattern (figure 9). Look at a snip from the Grand Canyon NP, Arizona, geologic map and notice the solid pale blue area that represent relatively horizontal rock units. Notice the yellow canyon with dark then medium blue bands paralleling the river. As the river erodes down through horizontal layer sit exposes the layers below. This produces a dendritic (branching) pattern in the exposed rocks. As you compare this to the cross section you can see that older layers are exposed as the river eroded downwards (figure 10).
Figure 8. A block model showing three horizontal layers. Only the layer on the top is exposed unless an erosional event such as a stream cuts down through it. Image:
https://commons.wvc.edu/rdawes/courseinfo/Assignments/blk1234.gif
Figure 9. A snip from the Grand Canyon NP, Arizona geologic map showing the surface pattern created by horizontal layers of rock exposed as the river cuts down through them exposing in a parallel manner exposing the layers beneath the Parashant canyon region. Image NPS.gov public domain.
Figure 10. A cross section showing the horizontal layers of the Shivwits plateau and the cut down of the Parashant Canyon exposing the older layers below. Image: NPS.gov snip
Notice the large Pennsylvanian age pale blue rock type exposed on the Pennsylvania Geologic summary map 7 by the Department of Conservation and Natural Resources (DCNR) (figure 11), and the dendritic drainage pattern clearly exposed to the north. This indicates that the rock units below are horizontal. The middle-swirled pattern indicates folded rock units, while the southeastern sections unique pattern tells us that the rocks where split and diverged. Looking at the complex cross-section of a full geologic map of Pennsylvania you can see that the rocks to the west are horizontal while the rocks in the middle portion of the state are folded (figure 12).
Figure 11. Summary geologic map of Pennsylvania. DCNR Public Domain. Image: https://www.dcnr.pa.gov/Education/GeologyEducation/Pages/default.aspx
Figure 12. Cross section of the state of Pennsylvania. Notice the horizontal flat layers to the west and the folded and faulted layers in the middle of the state. Image: http://elibrary.dcnr.pa.gov/GetDocument?docId=1751215&DocName=Map1_Geo_Pa
Inclined Strata
If the layers are no longer horizontal, we can infer that they have been tilted, folded or faulted by tectonic forces. The orientation, tilt or fold can be expressed with two values: Strike and Dip (figure 13). The strike represents the orientation of a horizontal line on the surface. The second value represents the angle at which the surface dips from the horizontal. It is always perpendicular to the strike. The dip direction of a plane is shown by the short line drawn perpendicular to the strike and pointing downhill. The number indicates the angle at which the layers dip into the earth. To determine strike, find where the dipping layer intersects the horizontal surface and draw a line parallel to this line of intersection on the top of the block (i.e. our horizontal surface). To determine dip, pretend that there is a drop of water between one bed and the next, for example, along the intersection of the green bed and the red bed (figure 14). Figure 15 shows a section of the geologic map from the Grand Canyon NP with the strike and dip symbol circled in red.
Figure 13. Strike and dip of a roof. The sloping roof of a building is a useful analogy to illustrate strike and dip. The ridge of the roof defines the strike of the roof. The roof dips away from the ridge with a characteristic angle (the dip angle). The inset in the top right corner of the figure shows the roof viewed from above with the strike and dip symbol superimposed on it.
Image: Joyce M. McBeth (2018) CC BY-SA 4.0. Satellite image from © 2018 Google Earth.
Figure 15. Grand Canyon NP, Arizona map section with a red circle around a strike and dip symbol showing a 1 degree tilt of the Pk rock unit (Kaibab formation) toward the south west. Image: public domain. https://pubs.usgs.gov/imap/i-2688/i-2688.pdf
Look on the geologic map for a symbol that may look like little “T”’s with numbers. The long top part of the T parallels the strike, which represents the trend of that plane of rocks, the shorter perpendicular dip section contains a number which equals the degrees of dip angle, as the layer dips into the earth.
Map symbols can be used for more than telling tilted, folded, vertical and horizontal strata (figure 16). Folds can tilt or plunge into the earth. Fault movement and type is also expressed. Recall that faults can be Normal (tensional stress) and Reverse (compressional stress). Carefully look at a geologic map for the variety of symbols. Then check a cross section to see a side view of the various layers.
Map Symbol | Explanation |
Strike & Dip | |
Vertical strata | |
Horizontal strata | |
Anticline axis | |
Syncline axis | |
Plunging anticline axis | |
Plunging syncline axis | |
Strike-slip fault |
Figure 16. Common map symbols used on geologic maps. Image CC BY-SA 3.0 R Harris oer.galileo.usg.edu/geo-textbooks/1
ON THE BOX MODEL TRY TO DRAW IN THE INCLINED LAYERS. USE A PROTRATOR TO INCLUDE THE STRIKE AND DIP SYMBOLS image openpress.usask.ca
Figure 17. Tilted layers.
Folded Strata
Folds are geologic structures created by plastic deformation of the Earth’s crust. Anticlinal folds form when compressional stress directions dominate and produce layers that are inclined downward away from the center fold (figure 18). This produces a fold that resembles the letter ‘A’ in a cross-section or side view. Anticlines show dip symbols away from the axis along with older rock units in the center with repeating patterns of younger rocks on either side. They can be single to form a monocline. A synclinal fold is a concave upward fold in which the layers are inclined upward. This resembles a sink or the letter U. Younger rock layers are on the inside with repeating older layers on either side (figure 19). Folds in cross section look much different than on a surface map view. In a map view the folds look like repeating units of rock type. This is seen in repeating colors on either side of a single central color. To determine the type of fold you need to look at the strike and dip symbol directions and confirm that the rock ages match. Synclinal folds show dip symbols toward the center axial line and expose younger rock units in the center with repeating older layers on either side. Anticlinal folds have strike and dip symbols pointing away from the central point (axis) and have younger rock units in the center and repeating older units on either side (figure 20). The age of the rock layers and the strike and dip symbols will help you determine the type of fold.
Figure 18. Anticlinal fold. Notice that older rock units are exposed in the center with repeating exposure of younger rocks in parallel bands. Image Public Domain original by P.S. Foresman.
Figure 19. Syncline along interstate 8 and US 40 in Maryland Sideling Hill. Image: Acroterion, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
COLOR IN THE LAYERS ON THE BLOCK MODELS (3 & 4) BELOW. Label the oldest and youngest layers. Measure and add stike and dip symbols.
Figure 20. Box number 3 is a drawing of an anticlinal fold with the older lower layers exposed at the center axis of the fold. Number 4 box shows a downward synclinal fold with repeating layers exposed around the younger central rock layer. Image: https://commons.wvc.edu/rdawes/courseinfo/Assignments/blk1234.gif
Figure 21. Box model showing anticline and synclinal folds with repeating color rock units on either side of the axis. Image: Phil Stoffer’s geology café. public domain http://www.geologycafe.com/images/folds.jpg
PRACTICE ON THE Rocky Mountain NP, Colorado GEOLOGIC MAP
Identify the park outline.
What is the scale of this map?
In what year is this map published?
Using the legend what is the oldest unit represented on this map?
What is the symbol/color for this rock type?
What is the youngest rock unit represented on this map?
Find a location on the map where they are on the surface.
Name a Cretaceous rock unit.
What rock dominates this map?
What is the symbol, age and color for the Oligocene Andesite welded tuff?
Does the unit Ysp have strike and dip symbols? Explain.
Find a few red fold symbols and explain their orientation.
Faulted Strata
Recall that rocks that behave in a brittle manner breaks, producing faults. Rocks experiencing Compressional stresses at convergent plate boundaries can produce Reverse or thrust faults. Rocks experiencing Extensional, pulling stress at Divergent boundaries can form Normal faults. Rocks that move past one another at Transform boundaries can produce Strike slip faults (figure 22). Faults on a geologic map often show sharp divisions between rocks of very different ages. Reverse and thrust faults can push older lower rock units up and on top of younger units. Strike and dip symbols are used to show direction and angle of the dipping fault plane (figure 23).
Figure 22. Faulted Strata: Normal and Reverse fault represented in block model form. Follow the black arrows to see the displacement of the layers. Image USGS.gov public domain/parks
Figure 23. Try to draw in and color the layers. Make sure to represent the fault and the dip angle of the fault. Image: CC BY SA openpress.usask.ca
PRACTICE ON THE Rocky Mountains NP, Colorado MAP
Find Dark Mountain (upper right). What is its rock type?
What is the elevation of this peak?
Measure and determine the distance from Dark Mountain peak to Lake Estes to the southeast. Show your work.
Find a few fault symbols.
Use the legend to identify a thrust fault symbol. Name one from the map.
Looking at the cross section below, try to find this fault on the map.
Figure 24. Cross section from the Geologic map of Rocky Mountains National Park, Colorado. Snip by author. Image public domain.