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PART A Your friend asks how to get to Library North

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PART A

Your friend asks how to get to Library North. To help them with directions you must know locations, distances, and directions. You may also give them notes on how to navigate the topography of GSU i.e. jump over bird scooters or go below the library to avoid courtyard construction. You could speak all these directions, or it might be easier to use a map! You’re practically a cartographer in training.

Are you familiar with this map?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1: A map of Georgia State University.

Now let’s think…

                What is a map? ?

                Why is it important for us? ?

                How does a map give us information (directions, locations)?

Geologist use all sorts of maps to study different elements like, rocks, geological structures (folds, faults, mountains..), geomorphology (elevation, valleys, ridges..), water (rivers, ocean, groundwater, rain), land cover (forests, urban land, cultivated land..).

 

In this lab you will learn about two of them: topographic maps to study geomorphology and geological maps to study rocks and geological structures.

 

1.            Basics of using topographic maps:

Topographic maps are two dimensional (flat) representations of three-dimensional landscapes. That third dimension is elevation. Topographic maps are drawn as if from a bird’s eye view, looking from the sky down onto the earth.

Geologists draw contour lines to show area of equal elevation. A contour line is the map trace of an imaginary line on the ground that has a particular elevation. Any point along a contour has the same elevation. Topographic maps have many contour lines, each adjacent line differing by a constant elevation difference AKA the contour interval. Use Figure 2 to answer the following questions.

 

Figure 2: A topographic map of Arabia Mountain in DeKalb County, Georgia.

 

1.            What is the contour interval in Figure 2?

 

2.            What is the highest elevation in Figure 2? Where is it?

 

3.            Figure 2 is missing a few things. What is one thing this figure needs to be officially classified as a map? 

Contour lines follow some basic rules. If you understand contour lines, you should be able to explain the importance of each rule.

4.            Which of the following is true? Highlight all that apply.

a.            Contour lines never cross

b.            Contour lines never skip (i.e. the sequence of lines must not skip an interval i.e. 10, 20, 40 without 30 in between)

c.             Contour lines are always evenly spaced

5.            One contour line can have multiple elevations

 

 

Contour lines always have an upside and a downside (all elevations on one side of the line are higher than the line, all elevations on the other side lower). The up and downside never change along the length of the contour line. However, spacing between the contour lines can be variable; in some places on the map the contours are closely spaced and in others they are more widely spaced.

 

Contour spacing is a function of the topographic slope or gradient. Closely spaced contours represent a rapid increase in elevation i.e. steep slope. Widely spaced contours represent a gentle slope. Topographic maps can be used to calculate slope.

 

Slope = rise/run = ?h/?x

The ‘?’ symbol means change

 

 

Always be sure your units are consistent! I.e. use feet/feet or meter/meter. The units should cancel out, therefore slope is dimensionless!

 

This map shows the topography of the Stone Mountain.

Figure 3: A topographic map of Stone Mountain.

6.            If you had to hike up the top of stone mountain which side would you chose to climb? Why? (Remember your rules about contour intervals! Use the cardinal direction in your description.)

 

 

7.            You do not have enough information from this map to calculate the slope. What are you missing?

 

 

 

 

Relief is the difference in elevation between any two points on a map, and the Total (maximum) relief of a map is the difference between the highest and lowest points on the map.

 

 

Figure 4: 1 inch = 10 meter on the map

8.            What is the relief between point X and Y?

 

 

9.            What is the total relief on the map?

 

10.          If there were a river within your map’s area…  Which way would the stream flowing? Towards Y or Towards X? How do you know?

 

 

 

PART B

Now let’s learn how to interpret geological maps and understand geological structures.

Geologic maps tell us about past environments and the sequence of events. Geologic contacts are solid lines that show the boundaries of rock types. For example, a unit of sandstone will have 2 contacts. These imaginary lines differentiate the rock from the rock type below and underneath it. Each unit will have an upper and a lower boundary/contact.

REMEMBER geologic contacts and topographic contours are NOT the same thing.

1.            What is the difference between a topographic contour and a geologic contact?

 

 

i)             Maps of areas of undeformed sedimentary units

The top image in Figure 5 shows the distribution of rock types on a topographic map. The bottom image is its accompanying cross section. Cross sections help us visualize the geologic geometry below the earth’s surface. (Think about how you can see the layers in a slice of cake once you cut into it. Cross sections are like viewing a slice of the Earth). The cross section is constructed along the XY line on the map in Figure 5 (illuminated in red).

Rule: If contacts of geological units parallel topographic contours, the units must be horizontal.

On this map, notice that the contacts of each unit (the solid lines) parallel topographic contours (the dashed lines). This means that the units are horizontal.

Sedimentary units are mostly horizontal during deposition. If sedimentary layers are not horizontal, then you know stress or strain has been applied to this section of the Earth.

 

Figure 5. Geologic map and cross section.

 

1.            Based on the geometry of these units, which law does Figure 5 best represent?

a.            law of inclusion

b.            original horizontality

c.             law of super position     

d.            law of cross cutting relations

 

2.            Which unit is found in the valley of Figure 5?

 

3.            Which unit is found on the hilltops of Figure 5?

 

4.            What does it mean when sedimentary units are deformed?

5.            What is the approximate thickness of the shaded unit? Hint: Since the unit is horizontal you can find its thickness using a mm ruler and the vertical scale shown on the sides of the cross sections.

a.            200 m                    b. 50 m                 c. 400 m                d. 100 m              

 

ii)            Tilted (dipping) units

When geological units are not horizontal, their contacts do not parallel topographic contours.

The incline of units’ ranges from gentle to steep, to vertical. This is referred to as dip. If units are not horizontal, then they are dipping. The orientation of a unit, i.e. its dip angle, tells us which direction water would drip down the unit. (Think your roof shingles. The way that they are dipping is the direction that rainwater flows off your roof.) The strike is known as the ‘waterline’. The strike can be determined by imaging a giant flood. If there was an ocean in your backyard, where would the surface of the water hit your roof? Review Figure 6. 

 

Figure 6: Dipped rock unit depicting dip and strike 

 

Strike and dip symbols:

When strike and dip are provided for you, they require a specific symbol. The strike on the symbol is the long line and the dip is the short tick across the strike. The direction of the tick shows the direction and the amount of dip. For example, the following symbol shows a strike along NE and 30 degrees dip along the NW. 

 

Figure 7:  Map showing three inclined units. All units strike NW and dip 30 degrees to the SW.

 

Unfortunately, these measurements are not always provided for you. Geologist can measure strike and dip from rocks in the field using a compass. We can also calculate strike and dip by performing a little math on geologic maps.

 

 

 

Figure 8: Geological map without strike and dip measurements

 

6.            What is the strike of the shaded unit in Figure 6?

a.            N45E                      b. N60W               c. N-S                    d. E-W                   e. None of the above    

 

7.            What is the direction of dip of the shaded unit?

a.            North                    b. East                   c. South                d. West               

 

iii)           Geological map of folded areas

Rocks deform when stress is applied. Typically rocks undergo differential stress, meaning stress is not equal in all directions. If stress is greater on one side, it will change the shape of a unit. Imagine using both hands to push the pages of a notebook together (i.e. applying stress). This pressure will cause the notebook to change shape, forming a fold in the middle. When a formation changes shape without breaking this is known as ductile deformation.

8.            Folds are examples of ________ deformation while faults or fractures are types of ______ deformation.

a.            Brittle, ductile

b.            Ductile, brittle  

The axial trace (also known as fold axis) is the imaginary line that divides a fold in half. Notice when the units are dipping away from each other they form a structure called an antiform. Notice when the units on either side of the axial trace are dipping towards each other, they form a synform.

9.            Anticlines have the __________ unit in the middle of the fold.

a.            Oldest

b.            Youngest            

 

10.          Synclines have the _________ unit in the middle of the fold.

a.            Oldest

b.            Youngest            

 

Although Georgia’s landscape seems relatively flat in metro Atlanta, most of the entire state has undergone some type of deformation. This cross section of figure 9 below shows the geologic changes throughout Georgia as we walk across our 5 physiographic provinces (Coastal plain to the Appalachian Plateau). Answer the questions that follow:

 

Figure 9: adapted from Smyth, Rebecca & Hovorka, Susan & Meckel, Tip & Breton, Caroline & G. Paine, Jeffrey & R. Hill, Gerald & R. Andrews, John & Lakshminarasimhan, Srivatsan & Austin, UT. (2019). Potential Sinks for Geologic Storage of Carbon Dioxide Generated in the Carol

 

Based on Figure above please circle the correct answer for the following questions.

11.          The coastal plain is composed of

a.            Horizontal units

b.            Folded units

c.             Units that slightly inclined (not horizontal but not folded)                            

 

12.          The Valley and Ridge is composed of

a.            Horizontal units

b.            Folded units

c.             Units that slightly inclined                           

 

13.          The Appalachian Plateau is composed of

a.            Horizontal units

b.            Folded units                                      

c.             Units that slightly inclined

 

14.          Explain the difference between brittle and ductile deformation. Then, use the table below (as well as your answer from question 1) to list the 6 generalized geologic structures created by the three types of stress under brittle and ductile conditions (4 pts)

 

Stress Type                                        

Ductile Deformation                                      

Brittle Deformation