Urban geology field trip, Atlanta, Georgia

URBAN GEOlOGY FIElD TRIP ATlANTA, GEORGIA
by WILLIAM H. McLEMORE
in cooperation with The United States Geological Survey
and The American Planning Association
DEPARTMENT OF NATURAL RESOURCES ENVIRONMENTAL PROTECTION DIVISION
GEORGIA GEOLOGIC SURVE .f
October 1980

INTRODUCTION
At 5:30 in the afternoon on Good Friday, March 27, 1964, Anchorage, Alaska was shaken by an earthquake centered approximately 130 kilometers away in the mountainous terrain north of Prince William Sound.
Through most of the Anchorage area, the surficial materials consist of sands and gravels overlying the 11quick 11 clays. The strong ground motion of the quake resulted in liquefaction of the cl~, silt, and fine-grained sands, and triggered disastrous landslides along steep bluffs throughout Anchorage. For example, in the subdivided residential area of Turnagain Heights, lateral offsets as great as 300 meters occurred, as an area 0.5 kilometers by 2.5 kilometers slid towards the Sound resulting in excessive damage.
The earthquake cost a total of 114 human lives and approximately $310,000,000 in property damage. Within the Anchorage urban area alone, 215 residences and 157 commercial structures sustained serious damage or total destruction amounting to roughly $200,000,000. Nine people died in the city and hundreds were injured.
Unfortunately, significant life and property loss might have been prevented if existing, readily available geologic information had been utilized by local authorities in their planning and zoning considerations. The United States Geological Survey had mapped the geology of the Anchorage area in the late 1940's and in the 1950's. The maps delimited areas that trained persons would recognize as being susceptible to damage from earthquakes. In keeping with Survey policy, copies of these maps were distributed to interested officials, professionals, government agencies and libraries throughout Alaska. Apparently no one bothered to look at these maps, and habital structures were placed in areas where damages would be expected
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to occur 1n a seismically active area such as Anchorage. The tragic events at Anchorage, however, led to intensified efforts
to encourage the use of geology in land-use planning. These effort~ have grown throughout the last decade and a half. Although millions of dollars have been expended in the campaign, the majority of local governments still do not incorporate geological data in their daily conduct. Consequently. average annual property loss in the United States related to geological hazards and constraints has been estimated to exceed $5,600,000,000 (see table I).
The purpose of this field trip, therefore, is to present to planners a look at how geologic conditions have effected land-use in Atlanta, Georgia. When taking this field trip, the reader should keep in mind that Atlanta is founded on hard competent bedrock in a relatively aseismic area. Consequently, earthquakes and foundation problems would be expected to be relatively non-existent. Nevertheless, as the field trip will point out, geologic conditions have played an important role in Atlanta's development.
THE HISTORY OF URBAN GEOLOGY IN NORTH AMERICA (from Pendleton. 1978)
Isolated examples of the application of geology to land-use decisions date at least as early as the first half of the seventeenth century. Governor John Winthrop of the Massachusetts colony decided, one week after arrival in the new world, to forsake the harsh environs of Salem. The members of his party traveled south into the Boston Basin, camping at the site of modern Charlestown where they became ill from impure well water. Heeding the advice of a previous English settler, they crossed the Charles River and settled in what is now the Beacon Hill section of Boston, where soil was easily tilled and artesian ground water abundant and pure. Boston archives document that geological topics such as ground water,
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gravel resources, and reclamation continued to receive scrutiny throughout the 17th and 18th centuries.
Issachar Cozzens conducted one of the first engineering geological mapping projects in North America. He mapped and described the geology of Manhattan Island in 1843. Cozzens provided information regarding foundation materials and the relative difficulty of excavation. Charles Berkey, working for the Board of Water Supply of the City of New York, performed geological site evaluations for dam, reservoir and aqueduct projects in 1906. This is believed to be the first instance in North America in which a geologist was given administrative authority for determining the geological studies to be performed during design and construction of major public works.
In the early twentieth centur,y, the U.S. Geological Survey completed an ambitious program mapping the geology of many urbanized quadrangles, which were published as a folio mapping series. In 1946 the Survey renewed its interest in urban geology by initiating a limited program of detailed geologic mapping in cities such as Anchorage, Seattle, Omaha, Portland, Boston and Los Angeles. Bulletin 1093, dealing with the surficial geology of the Anchorage area, was published as a portion of this program in 1959. Miller and Dobrovolny specifically cautioned that the surficial materials in the vicinity of Anchorage were potentially capable of liquefaction, and disastrous landsliding might be expected if a large quake occurred. This admonition was contained in two separate sections of the report. In keeping with the Survey's rejuvenated interest in urban geology, emphasis was placed on environmental and engineering geological implications throughout the bulletin.
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During the 1950's and 1960's the Los Angeles area experienced explosive population growth, and a concurrent boom in building. Periods of high precipitation, recorded in 1952, 1957-1958, 1962, 1965 and 1969, repeatedly resulted in damage due to landsliding and debris flows. Many of these slope failures were determined to be the direct result of poor grading practices. The academic and professional geologic and engineering communities were instrumental in securing the adoption of increasing stringent grading codes in 1952 and 1962. Performance records have documented significant reductions in failure rates as a result of each code change. In the late 1950's several court decisions shocked local governments by assigning the liability for a number of serious failures related to geological phenomena to their construction regulating agencies. This assignment of legal liability was instrumental in stimulating the addition of geologists to the professional engineering staffs of both Los Angeles City and County in 1959.
The Anchorage and Los Angeles experiences, combined with federal predictions of explosive urban growth, have added considerable impetus to the movement to promote the inclusion of geological information in the burgeoning land-use planning process. Responding to this developing demand, the U.S. Geological Survey began the Urban Geology Program to develop techniques of data acquisition and presentation specifically designed for application to land-use planning. Pilot urban mapping projects were inititated in the San Francisco Bay Area, the Colorado Front Range Urban Corridor, the Greater Pittsburgh Area, the Connecticut River Valley, the WashingtonBaltimore Region, the Phoenix-Tucson Area and the Puget Sound Area.
Other public and private sector agencies responded to the same incentive. The Saskatchewan Research Council and the National Research Council of Canada jointly sponsored a geological folio directed at landuse planning in the Saskatoon area in 1970. Distribution of the folio
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to 200 major cities throughout Canada was funded by the Canadian Geological Foundation.
In the United States several state geological surveys initiated projects to stimulate the development and dissemination of what may be called geo-plann1ng technique. Map folios such as the Geological Survey of Alabama's "Environmental Geology and Hydrology--Madison County, Alabama" were compiled and distributed to local planning entities. Simultaneously, private sector agencies such as the Center for Ecological Research in Planning and Design at the University of Pennsylvania have pursued the same task, producing products such as a land-use map folio of Medford, New Jersey.
After a decade of impressive productivity many of the parties involved have paused to evaluate the result of their efforts. Unfortunately, very few local governments have incorporated geological data in their land-use planning without federal or state participation. Significant advances have been made in data acquisition and communication, but the goal of spontaneous utilization of earth science information by local government remains unattained.
Most geologists know of gee-planning concepts only from the contents of professional publication. The preponderance of research into earth science applications to land-use planning has been regionally oriented. Philosophically, regional and local land-use planning are similar; procedurally, the two can be distinctly dissimilar. Many earth scientists, assuming the two to be similar, possess misconceptions regarding the local planning process. Until these misconceptions are corrected, communications between geologists and local government will remain stifled.
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Table I. Estimated Average Annual Losses Due to Geological Hazards and Constraint*

Hazard or Constr.aint Expansive Soils Land sliding Seismic Shaking Debris Flow Debris Fan Sedimentation Mine Subsidence Loess Hydrocompaction Debris Fall
TOTAL ANNUAL LOSS

Annual Loss $4,500,000,000
500,000,000 240,000,000 150,000,000 150,000,000
60,000,000 30,000,000
.1. 000 J 000
$5,631,000,000

* From a preliminary compilation of losses due to natural hazards prepared by D.E. Jones for a natural hazards seminar, 1976.
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DEVELOPMENT OF ATLANTA
In 1937, a point near present-day Five Points was selected as the terminus for the Western and Atlantic Railroad and the settlement that developed was informally named Terminus and later formally named Marthasville. The reader might ask the question, 11 Why was this particular area selected as the terminus for a ra11raod?11 The answer is that the high rugged Blue Ridge Mountains and Great Smoky Mountains, which form a natural barrier extending northeast to Pennsylvannia, end just north of the area. Bordering this mountain barrier on the southeast is a broad flat geologic province called the Piedmont. The railroads, such as the Western and Atlantic, laid their rails from the industrial northeast southwestward through the Piedmont. Atlant~ being located at the southwestern end of the mountains,was a logical location for railroad lines to the West and Midwest (Pickering and Higgins, 1979).
Another factor that has controlled the growth of Atlanta is the cost of aggregate used in construction. Atlanta is the only major metropolitan area in the United States that depends upon a crushed granite as its sole source of aggregate. Most cities used crushed limestone or natural sand and gravel. Crushed granite is used in Atlanta because of the abundant availability of local granitic rocks. Because the price of crushed stone at quarries is relatively standard, it is the additional cost of haulage that controls marketability; thus far-away sources of other rocks are not competitive. Nineteen quarries are in the Atlanta area and the haulage distances average between 6 to 15 miles (Atkins and Power, 1978).
Indirectly, the buildings of downtown Atlanta reflect Georgias standing as a leading producer of nonmetallic minerals. Georgia is the leading producer of dimension granite and second in the production of dimension marble (White, 1978).
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Many ~uild1ngs in the downtown business district utilize granite and marble from Georgia's quarries. However. marble, granite, and limestone from other states and countries also are used for decorative purpose in buildings, statues, and so forth.
FIELD TRIP GUIDE Several areas of interest have been selected to view the urban geology of the Atlanta area. Many of the points of interest will be examined as we drive through the area, such as along the expressway. At other points, we wfll stop the bus and look at the rocks and soils. At Panola Mountain State Park, Geologic Guides to the Rock Outcrop Trail will be provided. *We will depart from the Holiday Inn at Piedmont Road and travel north on I-85. Exit at Shallowford Road and turn right. Shallowford Road intersects Briarcliff Road. Turn right onto Briarcliff Road and at the bottom of the hill, turn left onto Briarlake Road. * Locality 1 When we turn off of Briarcliff Road onto Briarlake Road, notice the change in topography. This change in topography reflects a change in the rock types. The more rugged topography to the east and ahead of us is underlain by a more resistant rock type (the rock does not break down to soils as quickly as the other rocks surrounding it). In areas such as this, some of the more expensive homes in Atlanta have been built. From observing the rock in place and from the rugged topography (round knobby hills), this rock has been mapped through the city of Atlanta. Very little industry has located on this rock probably because of the higher cost of excavation and rock removal. *Turn around on top of the hill in subdivision on left and return to I-85.* We will not stop along the expressway. However, please note the different construction methods. Since the rocks vary in composition, they weather at
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different rates. Also, the rock is highly fractured (jo1nted),wh1ch allows it to weather an~ break down to so11. These and other factors control the occurrence of hard rock or saprolite (soil that retains rock structures) in each cut. In order to locate these changes, all foundations and roadways must be drilled to determine soil and rock profiles and to collect samples for testing in the laboratories.
Where rock 1s encountered, typically, the cut slope will be 1:1 or 0.5:1. The slope angle depends upon height of the cut and soundness of the rock. For example, if the rock has fractures with a dip of 45, the cut slope cannot be 0.5:1, because the fractures in the rock would intersect the cut slope. Slumpage or landslides, therefore, would occur along the cut slope. Soil cuts typically are on a 2:1 angle, which is easier to maintain.
In building construction in the Atlanta area, the smaller buildings generally are constructed on spread footings. The saprolite usually has a bearing capacity that will support smaller buildings. However, the larger buildings commonly are set on H piles anchored in rock.
* Intersection of I-85 and I-285, turn south on I-285. * Just beyond the first interesection of I-285, you will notice a subtle rise in topography. This 1s the same rock unit that we observed at locality 1. Also, notice the larger and more expensive homes on the left. The biotite gneiss that underlies this topography has a thin soil horizon which requires blasting if larger buildings such as warehouses were built on this rock type. *Intersection of LaVista and I-285. Turn left on LaVista and immediately turn right on Cooledge. Compare the topography to the first locality.* The topography along Cooledge is flat to rolling with no rock outcrops. Also note that less expensive homes have been built in this area. *Intersection of Cooledge and Stone Mountain Expressway. Turn left onto Stone Mountain Expressway. Exit at Mountain Industrial Boulevard. Turn left and continue north. Park in Sears Parking Lot. Use caution and cross the street.*
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Locality 2 A common misconception is all soils in Georgia are red clays. The saprolite
(soils that retain rock structures) is commonly silts which are slightly coarser than clays. Also, the color of the soils in the Southeast is the result of the leaching action of rain water. The high yearly rainfall removes all elements from the soils except iron, aluminum and silica. The oxidation of iron gives the silts a red color.
If you look carefully at the saprolite, notice that these soils vary in color and composition. These soils were originally a garnet mica schist and amphibolite ( a rock that is dark green or greenish-black and fs composed of minerals that have a large amount of iron and magnesium). The schist weathers to a pale purple
I
micaceous saprolite and the amphibolite weathers to a red-orange boxwork saprolite.
Note the many buildings along Mountain Industrial Boulevard. Probably all these buildings are constructed on spread footings. No rock excavations have been noted in the immediate area. The saprolite is rippable and is conducive to simple construction techniques.
*When we turn around to continue south on Mountain Industrial Boulevard, Stone Mountain will be visible on your left. The granite will be discussed later. Cross over Stone Mountain Expressway. East Ponce De Leon intersects Mountain Industrial Boulevard. Turn left at this intersection. Continue through the town of Stone Mountain. *
Stone Mountain is much younger than the granite. Contrar,y to popular opinion, the granite is not the largest granite outcrop in the world; in fact the area underlain by the granite is fairly small (less than 10 square miles). The granite forms pavement outcrops and tan sandy silty soils. Excavation problems are encountered in areas in which the granite crops out.
After leaving the town of Stone Mountain, we will be traveling along a ridge. Also notice that a railroad will be following this ridge. In Georgia, most of the railroads follow ridge tops as construction was most simple. Once we turn onto Panola Road, we will drop off the ridge and away from the railroad.
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Panola Road will be crossing divides and does not follow a ridge. * Continue south on Stone Mountain Highway (East Ponce Deleon changes to
Stone Mountain Highway in Stone Mounta~n). Follow signs to 1-20. Turn right onto Panola Road. Cross over 1-20. Continue south on Panola Road. Panola Road ends at Salem Road. Turn left - Salem Road intersects Evan Mill Road. Turn right onto Evans Mill Road. Evans Mill Road intersects Browns Mill Road. Turn left onto Browns Mill Road. At caution light on Klondike, turn left onto Klondike Road and proceed north. Arabia Mountuin Park will be on the right.* locality 3
Arabia Mountain Park is located on a large pavement outcrop of biotite gneiss. Close examination of the rock indicates that the rock has undergone a tremendous amount of deformation. It is as if someone had swirled the rock with a large spoon.
Of interest to you, as a planner, is the lack of a thick soil horizon. What kinds of problems in construction would be encountered in areas underlain by this rock type? Since there is no soil horizon, one problem would be the emplacement of septic tanks. Another problem would be in having to remove rock if pools, basement, pipelines, etc. are planned. During the last 20 years, Atlanta has experienced an explosive rate of growth; and within this time, many parasite deve1opers planned subdivisions and commenced construction. Several of these sites were abandoned because of the high unanticipated cost of rock removal and percolation test failures due to the lack of an .adequate soil horizon
* Turn around. Return to Klondike. Continue straight. Cross over the South River. Continue West. Alexander's lake Road intersects on left just before the lake. Turn left onto Alexander's lake Road. Alexander's Lake Road intersects SR 155. Turn right on SR 155 and immediately turn right again (Entrance to Panola State Park).* locality 4
Panola Mountain State Park was established to protect the delicate ecological balance of the plants on and around Panola Mountain. In addition to the interesting ecological system, self-guided brochures are available to study the geology of the park. It should take about an hour to walk the Rock Outcrop Trail. The brochure describes the origin of the granite, its age, structures, chemical and physical weathering, and relationship between plants and the granite.
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* Follow SR 155 to I-285. SR 155 turns left after crossing Snapf1nger Creek (a 7-11 is located at the intersection}. Intersection of SR155 and I-285, Turn left and proceed west to Soapstone Ridge. Intersection of Bouldercrest and I-285. Turn right on Bouldercrest Rd. and proceed south examfng development patterns. Turn around at Church on left and return to I-285 and continue west on I-285. After I-285 crosses the South River, the expressway gradually ascends Soapstone Ridge. Exit at Moreland Avenue, turn right and travel slowly down the side of Soapstone Ridge. * Locality 5
The rocks of Soapstone Ridge formed deep within the earth and were moved into their present position by faulting. The Soapstone Ridge rocks are almost horizontal and the South River has cut through them exposing the underlying rocks in the cut behind a truck stop. The Soapstone Ridge rocks have a dark green color and the underlying metamorphic rocks have a gray to white banded color. The soil horizon is thin and expansive. Within the saprolite are numerous large round boulders formed by weathering along fractures in the rock. Blasting is required to remove these boulders. Without a thorough knowledge of the geology, especially the weathering characteristics of the rocks, many problems can occur in development. For example, in other areas, in order to determine depth to hardrock, one only has to drill with augers to refusal. However, at Soapstone Ridge, augers tend to follow (along) the fractures in the rock giving a false depth to hard rock.
An orthophoto quadrangle is in your package and Soapstone Ridge can be outlined by the lack of development and the abundance of vegetation. Only recently has development encroached onto Soapstone Ridge and that development consists mainly of truck terminals and a few subdivisions.
*Continue north on Moreland. Moreland Avenue and the McDonough Highway intersect. Turn left onto McDonough Highway.*
This is one of the older roads in Atlanta. Notice that this road runs
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along a ridge. Before the development of the expressway system in Atlanta, this was one of the main routes south from downtown Atlanta. Also. notice that a railroad follows this road.
*Turn left onto University Avenue. Intersects Stewart Avenue (U.S. 19 &41).
Turn right.* Before the development of the expressways in Atlanta, Stewart Avenue was the
main road through Atlanta to Florida. U.S. 41 comes through Atlanta from Tennessee and U.S. 19 (Peachtree Street) comes through Atlanta from North Carolina.
* We will follow U.S. 19 through Atlanta, except for a detour around
MARTA construction. Turn right on Whitehall Street. *
Whitehall is named after the first settlement in the Atlanta Metro area. Whitehall existed before Terminus and was essentially a trading post.
* Turn left on Ivy Street. Ivy Street runs parallel to Peachtree Street. * Look to your left and you will see the ridge upon which Atlanta originally developed. Directly ahead is Underground Atlanta. Underground Atlanta resulted because bridges had to be built over the railroad tracks. Eventually, these bridges were connected forming a viaduct. * Turn left on Auburn Avenue. Intersection with Peachtree Street. Turn right onto Peachtree Street. * You are now on the ridge from which Atlanta expanded. Five Points is directly behind you at the other end of Central City Park. * Peachtree Street intersects West Peachtree. Turn right and continue north on Peachtree Street. * Peachtree Street continues along the main ridge. West Peachtree Street drops off the ridge and will intersect Peachtree Street near I-85. Since Peachtree Street wanders along a ridge, West Peachtree Street was constructed to shorten the route to North Atlanta. As stated previously, all the larger building have been constructed on H piles. Near Colony Square, a tall building ( 10 stories plus) is under construction and is founded on H piles.
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* Continue north on Peachtree Street. West Peachtree intersects on left.
Intersection of Peachtree Street and I-85. Turn right on I-85 and continue north on I-85. Intersection of I-85 and Piedmont Avenue. Turn right on
Piedmont Avenue. Holiday Inn on left. *
SUMMARY Atlanta is fortunate in that little or no serious geological problems exist in the area. However, planning should take into consideration the differences in rock types, weathering characteristics, and geologic structures. Foundation cost can provide geological reports and maps of most areas. Engineering firms and universities can provide geologists who will do on-site studies.
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FIELD TRIP ROUTE
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