Stone Mountain Field Trip Worksheet

Pamela J. W. Gore
Georgia Perimeter College

© PAMELA J. W. GORE, 1993, 1997, 1998, 2004, 2007, 2009

Stone Mountain, Georgia, about 1.3 miles long and 0.6 miles wide, is located at the western end of a much larger granitic pluton (yellow). It is a rounded monadnock, which is defined as "an upstanding hill or mountain of circumdenudation rising conspicuously above the general level of a peneplain in a temperate climate" (Glossary of Geology); also called an inselberg, a German term meaning "island mountain". Several smaller monadnocks are also present in the general area (including Arabia Mountain, Panola Mountain, Pine Mountain, and others).

See also blank geologic map of Stone Mountain and vicinity for coloring, and the map key page 1 and page 2.  Maps and key are from McConnell and Abrams, 1984.

The mountain is much smaller than the granitic pluton as a whole, so it is not a mountain just because this particular type of rock is more resistant to erosion than the surrounding metamorphic rocks. The mountain lacks structural joints (although it does have jointing parallel to its surface caused by exfoliation and stress release), and it seems to be a remnant of a larger mass that is decreasing in size (elevation and diameter) due to weathering and erosion.

The mountain (and some of the other nearby monadnocks) is surrounded by three concentric annular or ring-like depressions, interpreted to be the result of major climatic changes over the last 45 million years.  Times of unusually warm, moist climate, and intense weathering led to the formation of deep saprolite, which have eroded to form the circular depressions (Dennison, 1999).

STOP 1 - WALK-UP TRAIL

The field trip begins at the walk-up trail near the West Gate. Park in the lot beside the "Confederate Hall" (#18 on map you receive at the gate to the park). Cross the railroad tracks at the walk-up trail.

All of the features we will be examining at this stop are along and to the right of the walk-up trail, between the railroad and the flagpoles, a short distance up the trail. (We will not be walking to the mountain top.)

A. Look for the white linear features that cross the trail, roughly parallel to the railroad tracks. They range in width from a few inches to a foot or more.  
 
  1. What are they? ___________________________________

     

  2. What minerals can you identify in them?

    ______________________________________________________

     

  3. Do they all have the same texture?
    What textures can you identify in the
    various veins or dikes?

    ______________________________________________________

     

  4. Follow some of these features along.
    Are they straight, or do some of them fork?
    Sketch one. 
  

 

 

 

 

B. Look closely at the rest of the rock.

  1. What is its texture? ________________________________
     
  2. a. Did it cool quickly or slowly? __________________

    b. How can you tell? _________________________________
     

  3. a. Is it intrusive (plutonic) or extrusive (volcanic)?

    ______________________________

    b. What is the evidence? ______________________________
     
  4. What minerals can you identify in this rock?

    ____________________________________________________________
     

  5. On the basis of the texture and the minerals present, what rock type is this?

    ___________________________

C. As you walk up toward the flagpoles, pause in the area near the "step" which is about 1 m high. Can you find the chisel marks or drill holes in the rock?

This part of the mountain was the West Quarry. The stone was used for building and paving.

1. Chalk lines were drawn and holes were drilled in the rock along the line. The rock was split using either wooden wedges that expanded when wet, or iron wedges. Can you find any evidence of what type of wedges were used here?

_________________________________________________________

Look for dark patches in the light-colored rock. They are several inches long.
  1. What minerals can you identify in these dark patches?

    ________________________________________
     

  2. What are these patches? What is their origin?

    _________________________________________________________

Look for evidence of faulting or offset of features in the rock.  Sketch what you find.

  

 

 

 

D. Walk to the right of the walkup trail.
  1. Why does the surface of the rock look different in this area?

    _________________________________________________________
     

  2. Look for cracks or joints in the rock. Notice in particular the joints which are more or less parallel with the mountain surface. These are exfoliation joints. They formed as a result of expansion due to release of confining pressure as the pluton that makes up Stone Mountain was uplifted and overlying rocks eroded. Approximately 36,300 ft (11,000 m) of rock has been eroded here over the past 71 million years (Dallmeyer, 1978). Exfoliation is the dominant process of physical weathering on the mountain.

    Exfoliation causes lenticular sheets of various sizes to spall (or "peel") off the mountain. As these sheets exfoliate, circular to oval depressions, referred to as solution pits, are left beneath them. The pits range in diameter from about 15 cm to a few tens of meters. Look carefully and see if you can locate some of them.
     

  3. The solution pits hold water after a rain, and this water helps to weather the rock. Look in some of these small pits to see what minerals you can identify in the sediment which coats their bottoms. List the minerals you find in the solution pits.

    ____________________________________________________________
     

  4. What minerals are missing from the sediment in the solution pits, compared with the minerals you identified earlier in the surrounding rock?

    ____________________________________________________________
     

  5. Why are they missing? What happened to them? What processes were involved?

    ____________________________________________________________
     

  6. Observe the vegetation in the solution pits. You should be able to observe several stages of an ecological succession in the pits.
    Look for pits with examples of each of the following:
    (a) lichens
    (b) mosses
    (c) grasses
    (d) weedy plants
    (e) small shrubs
    (f) trees
    How does the depth of the sediment or soil in the pits vary with successive stages of ecological colonization? Why?

    ____________________________________________________________

    ____________________________________________________________

    Lichens are one of the earliest stages of ecological succession on the rock. They are a symbiotic relationship between fungi and algae. The fungi absorb moisture and biologically weather the host rock, extracting certain chemicals from the minerals. The algae use these elements and the sun's energy to make food. Several types of lichens are found on the mountain. Quarrymen have noted that it takes about 25 years for fresh granite to weather sufficiently for lichen colonization.

    There are some very rare plants which grow in solution pits. Some are found nowhere else in the world but on the rock outcroppings on and around Stone Mountain. A small, red plant called Diamorpha smallii (or red stonecrop) can be found in the spring. Yellow Confederate Daisies bloom in the fall.

           Diamorpha smallii in bloom.  Photo by Pamela Gore.

Option: Walk part-way up the walkup trail to see some of the inscriptions, initials and dates that have been carved into the rock. Some of these inscriptions go back to the 1800's and can be used as a rough guide to the rate of weathering of the granite.

Head back to the large building, "Confederate Hall".

7.  What is the rock type of the paving stones just outside the door to the building?

______________________________________

8.  What is the rock type that makes up the outer walls of the building?

______________________________________

9.  Were either of these two rock types quarried at Stone Mountain?

______________________________________

Visit the exhibits in the Historical and Environmental Education Center in Confederate Hall. 

PROCEED BACK TO THE PARKING LOT AND DRIVE CLOCKWISE AROUND THE MOUNTAIN, TOWARD THE CARVING. PASS THE CARVING, PASS THE RIVERBOAT COMPLEX, CROSS "OLD ROUTE 78". LOOK FOR QUARRY EXHIBIT ON YOUR RIGHT. PARK IN THE LOT ON THE LEFT NEAR THE ROUNDABOUT AND GRIST MILL. (ROAD BECOMES ONE WAY AT THIS POINT. PARK, CROSS THE STREET, THEN ENTER THE QUARRY EXHIBIT BY WALKING UNDER THE RAILROAD TRACKS, AND FOLLOWING THE BOARDWALK.

STOP 2 - QUARRY EXHIBIT - "RAISING A LEDGE" - EAST QUARRY

Raising a Ledge - Stone Mountain quarrymen and the granite industry, 1850's - 1970's.

1.  What type of rock is the carved sign near the entrance to the exhibit? ______________________________________

2.  Do you think this rock was quarried at Stone Mountain? Why or why not? ______________________________________

3.  Find the list of buildings and other features made from Stone Mountain granite. What is something in your state that is made of Stone Mountain granite?  Find the name of a famous building that you recognize.

______________________________________

4.  View the Quarry Exhibit. Handouts may be available in the box.

Note the large sample of rock with coarse grains - examine the step-like flat surfaces of the white feldspar, and the irregular gray quartz.

Note the exhibits on birds, animals, wildflowers, and plant succession.

As you enter the east quarry area, look for any features in the rock which are different from those at Stop 1. In general, the rock is "fresher" and less weathered here, as a result of more recent quarrying. Many features can be seen here which are obscured elsewhere by lichen and fungi. Quarrying ceased on Stone Mountain in the early 1970's.

Looking up at the mountain, you may see a white line which has been painted on it in the distance. This is the line above which park visitors are not allowed to climb. We will not be going anywhere near the line, but it is interesting to see from a safe distance.

  1. Look carefully at the rock for the tourmaline pods, commonly known as "cat's paws". The pods are several centimeters in diameter, and consist of a cluster of black, millimeter-scale tourmaline crystals, surrounded by a white halo or "bleached zone", sometimes called a "reaction rim". The tourmaline pods are scattered through the rock, but many are near pegmatite and aplite veins and dikes, which intrude the granite. (Some of these pegmatites also contain large black tourmaline crystals.) 

      

     

     

     

      

     
    Tourmaline pods surrounded by white bleached zones. These tourmaline pods are known as cat's paws. Looking at the rock, sketch a close-up detail of one of the tourmaline
    pods in the box above. Show the shape and orientation of the tourmaline.

     

  2. Look for white dikes, up to 10-20 cm in width. Trace one out by walking along it. How far does it extend? Can you find a beginning or end to it? ____________________________________

    Are the dikes cut or interrupted by any sorts of features? Describe them. What is the black mineral in them? _________________________________________________________

    Sketch the dikes and the features which cross or interrupt them.

      
     
     
     
     

     

  3. Look for the xenoliths. These are pieces of "foreign rock" surrounded by granite.  There are at least two which are more than a foot in diameter. Walk to the left, toward a small clump of trees in the quarry area. One xenolith is up a "step" behind the trees. (There may be a pile of rocks as a marker at or near the xenoliths.)
     

    a. What rock type is the xenolith?

    ___________________________________________________________

    b. Sketch the xenolith, illustrating the layering.

     

     

     

    c. How does the granite look at the edge of the xenolith? What is different about it?

    ____________________________________________________________
     

  4. There is another large xenolith up another "step" toward the railroad tracks. Locate it.

    a. Is it the same rock type as the other one? ____________

    b. How does it differ? ___________________________________

    ____________________________________________________________

    c. Look for the quartz veins cutting the xenolith. Sketch.

      

     

     

     

     

  5. How do these xenoliths differ from the dark patches seen in the rock at the first stop?

    ____________________________________________________________
     

  6. Look for evidence of exfoliation in the East Quarry area. Exfoliation is where sheets of granite pop up due to unloading and release of confining pressure.  There is at least one spot in which a piece of rock has recently exfoliated. Sketch the evidence for exfoliation.
     

     

     

     

     

     

  7. Look for the flow banding in the granite. It is best seen in the flat areas on the side of the quarry near the trees along the railroad tracks. It looks like long sets of faint parallel streaks in the granite. Flow banding helps show that the granite was once molten and flowed like a liquid. Sketch the flow banding.
     
      

     

     

     

     

     

  8. Look at the mountain side and the rock.

    a.  Is there any water visible on the surface of the rock? ___________________

    b.  Is the water coming out of cracks in the rock, or flowing into cracks in the rock, or both? _________________________

    c.  Where does the water come from? What is the source of the water? __________________________________
     

RETURN TO THE PARKING LOT ACROSS THE RAILROAD TRACKS THE WAY YOU CAME.
OPTIONAL RESTROOM STOP AND VISIT TO OLD GRIST MILL NEAR PARKING LOT.

OPTIONAL - EXAMINING STREAM SEDIMENT

Examine the sediment in a small stream near the parking area, Grist Mill, or elsewhere in Stone Mountain Park.  Describe the composition (minerals), and texture (range of grain sizes, grain shapes, and grain sorting) in the stream sediment.  Describe your findings.

1.  What is the composition of the sediment?  Which minerals do you see?  Is there any organic matter?

_______________________________________________________

2.  What is the range of grain sizes? (gravel? sand? silt? clay?)

_______________________________________________________

3.  How are the grains shaped? (Are they angular or rounded?  Are they all about the same, or do they differ?  Describe what you see.)

______________________________________

4.  Describe the sorting of grain sizes (Are they all the same size = well sorted?  Is there a  mixture of grain sizes = poorly sorted?)

 _______________________________________________________

5.  Do you see any variations in the stream and the sediment as you follow the stream along?  Can you see any evidence of erosion anywhere?  Can you see any evidence of deposition anywhere?  Describe what you see.

 _______________________________________________________

 _______________________________________________________

 _______________________________________________________

POSSIBLE LUNCH STOP AT PICNIC TABLES.
RETURN TO VEHICLES, AND DRIVE BACK TO INTERSECTION WITH "OLD ROUTE 78". TURN RIGHT AND PARK. DO NOT BLOCK PARK ACCESS ROAD.

STOP 3 - OLD ROUTE 78

At this stop, you can observe a weathering profile on the Stone Mountain granite in the old road cuts on either side of the road. The weathered rock is called saprolite. Most of the feldspars in it have weathered to kaolinite, a white clay mineral.

  1. Describe the color, texture, composition, etc. of the saprolite.

    Color _____________________________________________________

    Texture ____________________________________________________

    Minerals ___________________________________________________

    Walk toward the lake, and as you walk observe the color of the soil. At some point, it will change. When it does, stop and look carefully at the rock along the road and in the old road cuts on the LEFT. (The right side is covered by kudzu).
     

  2. What is the color of the soil here? ____________________
     
  3. What kind of rock is found nearby in the road cut? _____________________
     
  4. How did this type of rock get here? What is its origin?

    ____________________________________________________________
     

  5. What is the dominant type of chemical weathering of this rock?

    _________________________

The fine-grained black igneous rock is a diabase dike intruding the granite. Approximately 200 million years ago, the supercontinent Pangea began to break apart. As Africa rifted away from North America, a series of tension cracks and faults formed. Along these tension cracks, basalt and diabase dikes, sills, and lava flows formed.

The dike you are looking at here is about 180 million years old (Jurassic age), and it formed at about the time of the opening of the Atlantic Ocean between Africa and North America. A great many of these dikes formed, filling tension cracks in the continental crust in the eastern and southeastern U.S. One crack became dominant, with tremendous outpourings of basaltic lava. It became the Atlantic Ocean. Had the stresses been slightly different, this crack could have become the dominant crack, and you might be standing at the edge of the Atlantic Ocean!

What we have here is a dike composed of diabase, which is similar in composition to sea floor basalt.  This is a "little sister" of the Atlantic Ocean crust basalts.

See the geologic map of the Stone Mountain area with a number of thin, linear features marked "d". These are the diabase dikes. See if you can find this one on the map (it is there, but may be faint). Map is from McConnell and Abrams, 1984.

Key to map colors:
Yellow = Stone Mountain Granite and Panola Granite
Blue = Lithonia Gneiss
Light Green= Snellville Fm (schist, gneiss, quartzite)
Dark Green = amphibolite
Red = Clairmont Fm (gneiss & amphibolite)
Purple = Promised Land Fm (granite gneiss & amphibolite)

RETURN TO THE CARS, AND PROCEED TOWARD THE CARVING ON THE MOUNTAIN. PARK BESIDE MEMORIAL HALL. WALK TO THE SIDE OF MEMORIAL HALL FACING THE MOUNTAIN.

STOP 4 - OVERVIEW OF MOUNTAIN AND CARVING

From the vantage point at Memorial Hall, you can get a good look at the mountain. The mountain rises about 700 ft, from an elevation of about 900 ft above sea level at its base, to 1683 ft at its summit. The granite is less fractured, and hence more resistant to erosion, than the rocks of the surrounding countryside. The dominant process of physical weathering at the mountain is exfoliation, which causes its rounded, dome-like shape. Sheets of rock which have spalled (or exfoliated) off the dome can be seen resting near the top of the mountain.

  1. What causes the vertical stripes on the mountain?

    ____________________________________________________________

    The body of rock which makes up Stone Mountain is somewhat larger than the mountain itself. Although popular myth holds that the granite underlies "all of Georgia", "seven states", etc., in truth, the Stone Mountain Granite is a relatively small granite body, about 8.5 mi long in an east-west direction, and up to about 2 mi wide. The western contact of the granite with surrounding metamorphic rocks is within the park.
     

  2. The exposed part of the mountain covers 25 million square feet (or 538 acres). A surveyor has figured the volume of the mountain to be 7,532,750,950 cubic feet. Granite weighs 167.9 pounds per cubic foot. Calculate the weight (in pounds) of Stone Mountain.
    ________________________________ pounds
     
  3. Over the years, approximately 7,645,700 cubic feet of granite were removed from the quarries at Stone Mountain.  This amount of stone is equivalent to paving stones 1 foot square stretching from pole to pole (12,444 miles).

    a.  What percentage of the volume of Stone Mountain is this? _________________ (You will need a calculator.)

    b.  What is the weight (in pounds) of the rocks that was removed? _____________
     
  4. The granite has been dated radiometrically. Age determinations range between 281 and 325 million years (see Grant, 1986).
    At the time that the granite body cooled, it is calculated that the land in this area stood about 10,000 ft higher than at present. Over the past 325 million years, this 10,000 ft of rock has been eroded away.

    a. Calculate the rate of erosion in feet per million years.

    ____________________________________ feet/million years

    b. Convert this erosion rate to inches per year.

    ____________________________________ in/yr

    c. Convert this erosion rate to millimeters per year.

    _____________________________________ mm/yr

    The sculpture on the face of the mountain depicts Confederate President Jefferson Davis, and Generals Robert E. Lee and Stonewall Jackson. The carving was conceived in 1915 as a Confederate Memorial, and work began in the 1920's, stopping in 1928. The mountain and surrounding property were purchased by the State in the late 1950's. The carving took six years using thermo-jet torches, and was completed in 1970.
     

  5. Now that you understanding the processes of weathering operating on the mountain, what do you predict will ultimately happen to the carving in the future?

    ____________________________________________________________

REFERENCES

Atkins, R. L. and Joyce, L. G., 1980, Geologic Guide to Stone Mountain Park, Georgia Dept. of Natural Resources, Georgia Geologic Survey, Geologic Guide 4, 29 p.

Dallmeyer, R.D., 1978, Ar 40/Ar 39 Incremental release ages of hornblende and biotite across the Georgia Inner Piedmont: Their bearing on Late Paleozoic-Early Mesozoic tectonothermal history, American Journal of Science, v. 278, p. 124-129.

Dennison, J.M., 1999, Geomorphology of Stone Mountain, Georgia - Field Trip, in Whitney, J.A., Dennison, J, and Gore, P. J. W., Geology and Geomorphology of Stone Mountain, Georgia, Field Trip Guide, Geological Society of America, Southeastern Section Meeting, Athens, GA.

Grant, W. H., 1986, Structural and petrologic features of the Stone Mountain granite pluton, Georgia, Geological Society of America Centennial Field Guide - Southeastern Section, Locality 65, p. 285-290.

McConnell, K.I. and Abrams, C.E., 1984, Geology of the Greater Atlanta Region, Georgia Geologic Survey Bulletin 96.

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Copyright Pamela J. W. Gore
Georgia Perimeter College, Clarkston Campus, GA
Page created May 23, 1997
Modified June 4, 1997
Modified November 16, 1998
Modified October 18, 2000
Modified January 15, 2004
Modified July 18, 2007
Links updated October 21, 2008