geology

39

7. Maps and Structure Credit is given to SERC for borrowing and modifying activities. Goals and learning objectives:

• Understand importance of both topographic and geologic maps • Read topographic and geologic maps • What information do topographic maps provide? • Identify landforms using topographic maps • Calculate slope using topographic maps • What information do geologic maps provide? • Identify geologic structures using geologic maps • Identify faults using geologic maps

Reading: Chapter 10 of the textbook Materials needed: Ruler Graph Paper Pencil String Google Earth with provided topographic and geologic map overlays Background Information: The key information here is how to read a map and what information can be obtained from a map. Identifying geologic structures on a geologic map is also an important activity. True north versus magnetic north. While not on Google maps, it is important to know the difference between true north and magnetic north. True north is the location of the pole of rotation. Magnetic north is the location of the “north pole,” where a compass points towards north. A map should show the angle between true north and magnetic north; this angle is the magnetic declination. In the field, it is important to remember that the magnetic declination must be corrected when using a map, and to check that the magnetic declination on the map is up to date for your area. Contour lines: A contour is a line that shows a specific elevation above sea level. Contour lines must never cross (crossed contour lines indicate two different elevations in one place). They must always form a closed loop, although the loop may not always be visible in the frame of the page and the lines may run into the margins. They will form a V shape pointing in the upstream direction when they cross a stream. When contour lines indicate a lower elevation, such as a dip in the surface, they will have tick marks along the downslope direction.

40

Constructing a scale: A map scale is one of the most important features you will see on a map. Scales come in a couple of forms, all of which give you information about how the map represents the real world. A scale bar is one of the most commonly used scales. In a scale bar, a specified length (the bar) represents a given distance in m, km, ft, or miles. To use a scale bar, simply use a ruler or piece of paper or string to compare the distance on the map to the length of the scale bar. For example, if your scale bar is 20 cm long and represents 100 m on the map, then a piece of string measuring 10 cm between two points shows that the points are 50 m apart.

A ratio scale is another common scale on maps. A ratio scale is just that, a ratio (i.e., 1:100). This type of scale can seem confusing, but it’s actually very simple. 1 of something measured on the map represents 100 of that same something in real life. The ratio scale is unitless, so any unit of measurement may be used as long as it is the same on each side. 1 cm measured on the paper map represents 100 cm in real life. 10 cm on the map represents 1000 cm in real life (because you multiply both sides of the ratio by 10). 1 banana on the map represents 100 bananas in real life (we will pretend all bananas are the same size)! How to construct a ratio scale. It is very easy arithmetic!

Step 1: Zoom in to Google Earth so that you can clearly see the details of the map. You can move the map around, but if you change the zoom, you will have to re-do your scale!

Step 2: Click on the ruler on the top of the page. Select the “Line” option.

Step 3: Select two points that are easy to identify (e.g., the football stadium and the Virginia Tech Airport).

Step 4: Click on your first point and then your second point. You should now have a line.

Step 5: Change your units on the line to match the units on your ruler (I prefer cm, but inches will also work).

Step 6: Measure the line on your screen using the ruler (if you have a touch screen, be careful).

Step 7: Write down the map length number that the ruler measured (you can round to a whole number).

Step 8: Make a ratio with the number in Step 6 above the number in Step 7. DO NOT DIVIDE! Example below.

Step 9: Now equate the ratio in Step 8 to the ratio to 1 over x. Example below.

Step 10: Solve for x. You now have your ratio scale 1:x. Example: I measure the distance between two points to be 199,913.48 cm (image below). I will round this number to 200,000 cm. The length of the line on the screenshot below is about 4 cm (if you have printed this document or have it on a different size screen, the length may be different).

4 cm / 200,000 cm = 1 cm / x cm centimeters cancel out, yielding:

41

4 / 200,000 = 1 / x 4x = 1*200,000 x = 200,000 / 4 x = 50,000 The map scale is 1:50,000. On this scale, 1 cm represents 50,000 cm, 1 mile represents 50,000 miles, and 1 banana represents 50,000 bananas.

Figure 7-1. Topographic map of Blacksburg. Geologic Maps Geologic structures are identified in two ways: through the physical orientation of the rocks and through the age relationships of the rocks in the region. Physical orientation of rocks is described using strike and dip. Strike shows how the rock is aligned as an azimuth on the compass; without a field compass, you may give a general direction (i.e., E-W, N-S, NW-SE). Dip is the angle of the rock units to the plane of the horizon. The orientations of rocks relative to each other defines geologic structures, which can affect the way the landscape changes over time and how materials erode.

This symbol is a strike-dip symbol. You will see them on geologic maps to indicate the orientation of the rocks. The long end runs parallel to the strike orientation, while the short end always points in the direction of dip.

Here are some common symbols used in geologic maps (Federal Geographic Data Committee Digital Cartographic Standard for Geologic Map Symbolization: Reston, Va., Federal Geographic Data Committee Document Number FGDC-STD-013-2006 [prepared by the U.S. Geological Survey]). Not

42

all of these are in the Google Earth geologic map files, but you may encounter them later in your careers.

Figure 7-2. General symbols used on a geologic map. Credit: FCDC, USGS

A syncline is a geologic feature in which rocks on either side of an axis dip towards the center of the axis. A syncline will have an axis between two or more rock units, with the rocks on either side dipping towards each other. The strike-dip symbols will be oriented so that the strike runs parallel to the rock unit’s length, while the dip is towards the center of the axis. In Google Earth, it will be easier to identify synclines by looking at the ages of the rocks: the youngest rocks will be in the center, while the oldest rocks will be along the limbs. (You can remember that the rocks dip down towards the syncline because synclines look like smiles and both start with S).

An anticline is the opposite of a syncline: the rocks on either side of the axis dip away from the center of the axis. An anticline will have dip symbols on rock units that point away from the axis. In an anticline, the rocks in the center will be older than the rocks in the limbs. (You can remember that the rocks in an anticline look a little bit like an A).

A fault occurs where there is a break in rock units due to movement. There are three main types of faults.

A strike-slip fault occurs when rocks move past each horizontally. Transform faults, such as the San Andreas Fault, are a special type of strike-slip fault that occurs at plate boundaries. A road built across a strike-slip fault will break and become offset on either side of the fault over time. These faults are right-handed (if the road is offset to your right looking across the fault) or left- handed (if the road is offset to your left looking across the fault).

A dip-slip fault occurs when rocks move up or down relative to each other. A thrust or reverse fault is caused by compressional force, while a normal fault is caused by extensional force. These

43

rocks have what is called a hanging and a foot wall. The easiest way to distinguish these is to draw a stick figure person over the fault line. The feet are on the footwall, and the head is on the hanging wall (this is because miners used to hang their lanterns on the hanging wall). (See image below). The motion of the hanging wall relative to the footwall is determined by correlating rock units on either side of the fault.

Figure 7-3. Illustrations of normal and reverse/thrust faults.

One common cause of faults is due to plate tectonics. When continents collide, they produce compressional force, which eventually overcomes the strength of the rock and forces it to break, pushing the hanging wall up over the footwall. When continents rift, tensional force causes the hanging wall to move down relative to the footwall. Additional Resources: https://serc.carleton.edu/serc/search.html?search_text=topographic%20maps&endpoint=%2Fse rc%2Fsearch.html

Hanging wall

Footwall

Normal Fault Reverse and Thrust Faults

Footwall

Hanging wall

44

Assignment: Download the Google Earth topographic and geologic map files for the state of Virginia and your home state. Since we do not have files for international geologic maps, international students should download the maps for Virginia and one other place they would like to visit in the US. Topo Map Geologic Maps of US states Rocks from above

Topographic Map Activities: 1. Construct a ratio scale using the steps above. Please show your work for full credit.

(Remember, you can move around in Google Earth, but if you zoom in or out, your scale will change!)

2. Center your map on the Brush Mountain (north-northeast of Blacksburg). a. What is the highest elevation?

b. What is the average slope (m/km) on the NE side of the mountain? SW? Which is steeper?

c. In what direction does Poverty Creek flow? How can you tell?

d. What is the ratio scale for this particular map?

e. What is the magnetic declination of this area? Does a compass needle point east or west of true north?

f. What is the contour interval on this map? What is the index contour interval?

g. Name one of the rivers/streams located within your map’s area. Which way is this stream flowing, by compass direction?

h. The term “relief” is used to describe the difference between the highest and lowest elevations in an area. What is the total relief in the quadrangle?

i. What do the small black (or purple) polygon symbols represent?

45

3. Contour the above plot to construct a topographic map

4. Now construct a topographic profile along the A-A’ line. a. Lay a piece of scrap paper along the line of the section. Mark the two ends of the section

line. b. At each point where a contour line crosses the section, make a small mark on your scrap

paper and label it with the elevation of the contour line.

46

c. Lay your scrap paper along the horizontal axis of the topographic profile (you may use graph paper, but label your scale!). Transfer your contour intersection points and elevations to this axis.

d. Using the contour value, plot a point at the correct elevation above each of your horizontal axis marks, then connect these points with a smooth line to create the profile.

5. Now look at the New River between Radford and Pembroke. a. Look at the terrain and elevation trends along the length of the river. Which part of the

river crosses the steepest terrain? Which part crosses the gentlest terrain?

b. Look at the shape of the river. How does the shape of the river channel and its valley change along its length?

c. Calculate the gradient of the river over the entire length as well as between Radford and Centerville, Centerville and Cowan, Cowan and Dry Branch, and Dry Branch and Pembroke. Look at your numbers for the river’s gradient. Where does the river have the steepest gradient? Where is its gradient gentlest?

d. Now draw a longitudinal profile. A longitudinal profile allows you to visualize the changing gradient along a river’s length. A longitudinal profile is a graph of a river’s elevation versus its length, and the slope of the profile is related to the segment gradient you calculated in the previous section of the lab.

e. Get a piece of graph paper. Draw a line parallel to the long side of the paper, and make a horizontal scale of one inch = 30,000 feet. Draw a second line parallel to the edge of the short side of the paper, and make a vertical scale of 1 inch = 600 feet. (The highest number on the scale should be larger than the elevation of the highest site along the river, and the lowest number should be smaller than the elevation of the lowest site along the river.)

f. When the vertical and horizontal scales are not the same, a profile is said to have a vertical exaggeration. To calculate vertical exaggeration, divide the vertical fractional scale by the horizontal fractional scale:

i. The vertical scale is 1 inch = 600 ft. This equals 1 inch = ___________ inches. The fractional scale is therefore 1/___________ (same number as above).

ii. The horizontal scale is 1 inch = 30,000 ft. This equals 1 inch = ___________ inches. The fractional scale is therefore 1/___________ (same number as above).

iii. Vertical exaggeration = V scale/H scale = __________________________________

47

Geologic Map Activities: Open the Geologic Map of Virginia Google Earth overlay file.

1. What Formation (by name) is found in Wolftown? ____________________________

2. What is its age? (Give the geologic Epoch.) ________________________

3. What Formation (by name) is found in Hood? ____________________________

4. What is its age? (Give the geologic Epoch.) ________________________

5. What Formation (by name) is found in Graves Mill? ____________________________

6. What is its age? (Give the geologic Epoch.) ________________________

7. Why do you think the rocks in this region appear in elongated strips? (Think back to laws such as original horizontality, cross-cutting relationships, etc., and why rocks might change. Note that the black lines on the map are faults).

8. Now look at the Culpeper region. What are the ages of the rocks around the area?

a. Do you notice any pattern in the distribution of the ages of the rocks around Culpeper?

b. What might cause the age distribution pattern of the rocks around Culpeper? Think about the geologic history of the Eastern US involving continental collisions and rifting. (Check with your TA to make sure you understand this feature!)

9. Now focus on the region between Elkton and Harrisonburg. Is this an anticline or a syncline? How do you know?

a. Massanutten Mountain is the longest mountain in Virginia. Does it make sense that a mountain is located here, given the structure you identified above? Why or why not?

b. How do you think the Massanutten Mountain was formed? (Pay attention to the rock types under and surrounding the mountain and what you know about the rocks!)

48

10. Look at the area around Clintwood. Notice the pattern of the rocks. Do you think the rocks here are flat, dipping East, or dipping West? Why do you think this?

11. Look at the geologic map located here: Geologic Map of the Pulaski Quadrangle, Virginia a. Note that the key showing the rock units puts the oldest rocks at the bottom. This is a

convention with geologic maps. The file also has a geologic cross section from A to A’. A geologic cross section shows a slice of the ground with the rock units shown as they are oriented underground. The cross section provides useful information regarding the geologic structures of a region, and can tell you how the ground is likely to erode and how water will flow through the rocks based on the structure of the rocks.

b. Look at the fault on the right side of the cross section. Is this a normal or reverse fault? How can you tell?

c. Now look at the fault on the left side of the cross section. Is this a normal or reverse fault? How can you tell?

d. What is the structure to the left of the left hand fault? The structure on the right of the fault? The structure on the left of the right hand fault? Do you think this sequence is a common sequence?

49

Appendix: A Quick Guide to Topographic Maps All maps are two-dimensional representations of a three-dimensional world. A topographic map shows you the shape of the land's surface using contour lines that connect areas of equal elevation. Most topographic maps also contain information about cultural features, such as roads and buildings, and about vegetation, locations of streams, and other natural features. In the U.S., most topographic maps are made by the U.S. Geological Survey (USGS).

Important features of topo maps: Tools for map interpretation

Scale allows you to convert map distance into real-life distance. Map scales come in three forms: A graphical scale is usually a line divided into segments showing what distance on the ground is equivalent to a distance on the map.

Example: The map distance between the “0” tick mark and the “1000” tick mark on the map represents 1000 feet on the ground.

A verbal scale states the number of feet or miles on the ground that equal one inch on the map. It can be expressed in words or as an equation.

Example: 1 inch = 1000 feet A distance of 1 inch on the map is equivalent to 1000 feet on the ground.

A fractional scale is a ratio of distance on the map to distance in the real world. It can be expressed as a ratio or as a fraction.

Example: 1:12,000; 1/12,000 A distance of 1 inch on the map is equivalent to 12,000 inches on the ground.

Any combination of the three types of scales may be found on a map, so it is important to be able to convert between them. A graphical scale can be converted to a verbal scale fairly easily, by measuring the length of the graphical scale line. Converting a fractional scale to a verbal scale is simply an exercise in unit conversion. All three examples above represent the same map scale: 1:12,000, or one inch = 1000 feet.

Example: Converting fractional scale to verbal scale.

Q: For hiking, many people use 1:24,000 scale topographic maps. How many feet in the real world are represented by one inch on the map?

A: The ratio 1:24,000 means that one inch on the map represents 24,000 inches in the real world. This question is really just a unit conversion problem: how many feet is 24,000 inches?

1 foot = 12 inches 24,000 inches x 1 foot/12 inches = 24,000/12 feet = 2000 feet

So, on a 1:24,000 scale map, one inch on the map is equivalent to 2000 feet.

Contour lines: The elevation of the land surface above sea level is represented on a topographic map by contour lines. Every point on a contour line has the same elevation. You can think of a contour line as representing a horizontal slice through the land surface. A set of contour lines tells you the shape of the land: hills are represented by concentric loops, whereas stream valleys are represented by V-shapes in contour lines. Steep slopes have closely spaced contour lines, while gentle slopes have very widely spaced contour lines.

50

The contour interval is the elevation difference between adjacent contour lines. It is important to know the contour interval in order to interpret how steep a given slope really is, or how much elevation difference is represented by a certain number of contour lines. Every fifth contour line is an index contour, drawn darker than the other lines. If an index contour is long enough, its elevation is usually written somewhere on it. If the contour interval is not stated on your map, you can determine it from the elevation difference between adjacent index contours. Map Symbols and Colors: USGS topographic maps use a standardized set of colors to designate features:

• Black – man-made features such as roads, buildings, etc. • Blue – water (lakes, rivers, streams, reservoirs, etc.) • Brown – contour lines • Green – vegetated areas such as forests • White – areas with little or no vegetation • Red – major highways; boundaries of public land areas • Purple – features added to the map since the original survey

Coordinate Systems Map-making faces the challenge of representing the Earth’s curved surface on a flat piece of paper. Different methods called map projections are chosen based on the scale and purpose of a particular map, but all projections result in some degree of distortion of the ‘ground truth’ being mapped. Regardless of projection or distortion, all maps rely on a grid system to describe the location of a point on the ground. There are several common grid systems (coordinate systems) used on maps published in the U.S. All are based on a geometric X-Y coordinate system, where X is the horizontal component and Y is the vertical component. Geographic Coordinate System (GCS) – Latitude/Longitude: In the Geographic Coordinate System, lines of latitude run parallel to the equator and divide the earth into 180 equal portions from north to south. The reference latitude is the equator (0°), and each hemisphere is divided into 90 degrees north and south. The north pole is 90°N and the south pole is 90°S. Wherever you are on the earth’s surface, the distance between lines of latitude is the same (60 nautical miles).

Lines of longitude run perpendicular to the equator and converge at the poles, and therefore do not have an equal distance between lines at all points on the globe. The reference line for longitude (0°) is the prime meridian, which runs from the North Pole to South Pole through Greenwich, England. Longitude is subsequently measured from 0–180°E or W of the prime meridian. Negative longitude values are assigned to lines west of the prime meridian.

GCS values can be stated in decimal degrees (Durango ex. -107.877, 37.287) or in degrees- minutes-seconds (Durango ex. 107°52’32”W, 37°17’9”N). Each degree can be separated into 60 minutes (’) and each minute into 60 seconds (”). The USGS maps you will use in this lab are called 7.5 minute quadrangles because each side of the map covers 7.5 minutes of latitude or longitude.

51

Universal Transverse Mercator (UTM): The Universal Transverse Mercator system is widely used because it produces the least amount of distortion for maps that cover large areas. In this system, the earth is divided into 60 north-south zones that are each 6° longitude in width. Coordinates are written as the UTM zone and an easting-northing pair in meters. The easting is the projected distance in meters east or west of the center of the UTM zone. The northing is the projected distance in meters from the equator (Durango ex. Zone 13, 244956, 4130253). Public Land Survey System (PLSS): The Public Land Survey System is used mostly in the western part of the U.S., originally to designate rural undeveloped areas. It is a grid system measured in U.S. miles, with each township being a square of 6 miles on a side. Townships are divided into 36 sections, each a square mile, and sections are divided into quarters and quarters of quarters.

The red star in the diagram would be located as NW1/4, NW1/4, sec. 14, T2S, R3W.