Roadside Geology of Georgia
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You'll be amazed at Georgia's geological diversity, from its shifting
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Roadside Geology of Georgia - Pamela J. W. Gore
Pamela J. W. Gore and
William Witherspoon
2013
Mountain Press Publishing Company
Missoula, Montana
© 2013 by Pamela J. W. Gore and
William Witherspoon
First Printing, April 2013
All rights reserved
Photos © 2013 by Pamela J. W. Gore and William Witherspoon unless otherwise credited
Geologic road maps and many of the illustrations revised by Mountain Press Publishing Company based on original drafts by the authors
Roadside Geology is a registered trademark of Mountain Press Publishing Company
Library of Congress Cataloging-in-Publication Data
Gore, Pamela J. W.
Roadside geology of Georgia / Pamela J. W. Gore and William Witherspoon.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-87842-602-7 (pbk. : alk. paper)
1. Geology—Georgia—Guidebooks. 2. Georgia—Guidebooks.
I. Witherspoon, William D. II. Title.
QE101.G67 2013
557.58—dc23
2012051766
Printed in Hong Kong by Mantec Production Company
P.O. Box 2399 • Missoula, MT 59806 • 406-728-1900
800-234-5308 • info@mtnpress.com
www.mountain-press.com
This book is dedicated to Dr. Timothy M. Chowns. A native of England, Tim came to the University of West Georgia in 1973. His contributions to the study of geology of northwest Georgia, the Georgia coast, pre-Cretaceous rocks beneath the Coastal Plain, and sedimentary rocks have been timely and fascinating. For more than twenty years, Tim and his colleague Randy Kath have brilliantly and doggedly shouldered the task of keeping the Georgia Geological Society together and its annual field trip a thriving event in which geologists, students, and rock lovers climb on rocks, argue, and build friendships.
We usually think of people as ephemeral compared to rocks, but though the oddly folded layers of carbonate rock in this 2008 Rockmart quarry photo have been blasted away, Tim, as professor emeritus, has barely slowed down. We look forward to many more years of his geologic thinking, sparkling wit, and leadership.
CONTENTS
Acknowledgments
Introduction
Geology Determines Landscape
Plate Tectonics
Georgia’s Five Landscape Provinces
Sedimentation
Geologic Timescale
A Word about Collecting
Sea Islands
Longshore Drift and Tides
Beaches
Marshes
Ancient and Modern Shorelines
ROAD GUIDES OF THE SEA ISLANDS
US 80: Savannah—Tybee Island
North Beach Area
South Beach Area
Wassaw Island and Sapelo Island
Gray’s Reef National Marine Sanctuary
St. Simons Island and Its Neighbors
Jekyll Island
Cumberland Island National Seashore
Coastal Plain
Sedimentary Formations
Physiographic Districts
Fall Line Hills
Fort Valley Plateau
Vidalia Upland
Tifton Upland and Valdosta Limesink Districts
Dougherty Plain
Bacon Terraces
Barrier Island Sequence District
Okefenokee Basin
Landforms
Pleistocene Sand Dune Fields
The Mysterious Carolina Bays
Orangeburg Escarpment
Pelham Escarpment
It Came from Outer Space
Fossils
Sharks and Other Fish
Marine Reptiles
Dinosaurs and other Reptiles
Pleistocene-Age Vertebrates
Kaolin Deposits
Heavy Mineral Sand
ROAD GUIDES OF THE COASTAL PLAIN
US 27: Columbus—Florida State Line
Providence Canyon State Outdoor Recreation Area
Lumpkin to Florida State Line
I-75: Macon—Florida State Line
Georgia 49: Byron—Americus
Andersonville and Bauxite
Georgia 24: Milledgeville—Waynesboro
Kaolin Mines
Kaolin Processing Facility
Tennille Lime Sinks
Hidden Basins
Sandersville to Waynesboro
Shell Bluff
I-16: Macon—Savannah
US 341: Fall Line—Brunswick
Oaky Woods Wildlife Management Area
Perry to McRae
Little Ocmulgee State Park
McRae to Hazlehurst
Broxton Rocks Preserve
Hazlehurst to Jesup
Griffin Ridge Wildlife Management Area
Jesup to Brunswick
US 1: South Carolina State Line—Florida State Line
Ohoopee Dunes Natural Area
Swainsboro to Waycross
Okefenokee Swamp Park
Trail Ridge
US 82: Alabama State Line—Brunswick
Albany and the Flint River
Radium Springs
Albany to Brunswick
I-95: South Carolina State Line—Florida State Line
Valley and Ridge and Appalachian Plateau
The Landscape in Relation to Anticlines and Synclines
Sedimentary Rocks and Ancient Geography
Fossils
Fossils of Cambrian Age
Fossils of Ordovician Age
Fossils of Mississippian Age
Fossils of Pennsylvanian Age
ROAD GUIDES OF THE VALLEY AND RIDGE AND APPALACHIAN PLATEAU
I-24 and I-59: Tennessee State Line—Alabama State Line
I-75: Tennessee State Line—Cartersville (US 411)
Ringgold Gap
Tunnel Hill to Dalton
Rocky Face Mountain at Dug Gap
Resaca to Cartersville
Cartersville Mining District
Ladds Quarry and Etowah Mounds
GA 136: Alabama State Line—Carters Dam
Cloudland Canyon State Park
Lookout Mountain to Carters Dam
GA 157: Tennessee State Line—Cloudland
Rock City Gardens
Along Lookout Mountain
US 27: Tennessee State Line—Cedartown
Chickamauga Battlefield
Chickamauga to Gore
Chattanooga Shale
Gore to Cedartown
Cave Spring
Slate Quarry near Cedartown
Blue Ridge–Piedmont
Premetamorphic Character of the Blue Ridge–Piedmont Rocks
Metamorphism
Igneous Intrusions
Weathering
The Patchwork of Terranes Assembled in Paleozoic Time
Earthquakes Due to Human Activity
Pull-Apart Activity during the Mesozoic Era
The Dahlonega Gold Belt: A Terrane Discovered by Prospectors
ROAD GUIDES OF THE BLUE RIDGE–PIEDMONT
I-20: Alabama State Line—Austell
Pine Mountain Gold Museum
Villa Rica to Lithia Springs
I-75: Cartersville (US 411)—Kennesaw
Cooper’s Furnace Day Use Area and Allatoona Dam
Lake Allatoona and Red Top Mountain State Park
Allatoona Lake to Kennesaw
GA 5 (I-575 and GA 515 in part): Tennessee State Line—Kennesaw
Blue Ridge to Jasper
Marble Mining District
Ball Ground to Kennesaw
GA 52: Chatsworth—Lula
Fort Mountain State Park
Fort Mountain to Amicalola Falls
Amicalola Falls State Park
Amicalola Falls to Lula
US 19: North Carolina State Line—Cumming
Brasstown Bald
Blairsville to Dahlonega
Dahlonega
Dahlonega to Cumming
GA 17 (GA 385 and Alt GA 17 in part): Hiawassee—Toccoa
Smithgall Woods State Park
Unicoi State Park and Anna Ruby Falls
Helen to Toccoa
US 23/US 441 (becoming I-985/US 23): North Carolina State Line—Lake Lanier
Black Rock Mountain State Park
Clayton to Tallulah Falls
Tallulah Gorge State Park
Tallulah Falls to I-85
Lake Lanier and Buford Dam
I-85: South Carolina State Line—I-985 (near Atlanta)
Hurricane Shoals Park
Nodoroc
Earthquakes at Dacula
US 78: Snellville—Thomson
Athens
Athens to Washington
Graves Mountain
Lincolnton Metadacite
Washington to Thomson
GA 72: Athens—South Carolina State Line
Watson Mill Bridge State Park
Elberton
Georgia Guidestones
Elberton to the South Carolina State Line
I-185: LaGrange—Columbus
GA 85: Atlanta—Columbus
The Cove
Warm Springs and Little White House Historic Site
F. D. Roosevelt State Park
Sprewell Bluff State Outdoor Recreation Area
Manchester to Columbus
Flat Rock Park
The Fall Line and Rocky Shoals at Columbus
I-75: Atlanta—Macon
Indian Springs State Park
Barnesville and Thomaston Area
High Falls State Park
Forsyth to Macon
I-20: Atlanta—Augusta
Appling Granite at Heggies Rock
Augusta
US 441/GA 24: Madison—Milledgeville
Rock Eagle
Milledgeville
Around Atlanta
Parks of Geologic Interest
Stone Mountain Park
Panola Mountain State Park
Davidson-Arabia Nature Preserve
Boat Rock Preserve
Kennesaw Mountain National Battlefield Park
Sweetwater Creek State Park
Quarries and Road Cuts
Gneiss and Schist
Quartzite
Amphibolite
Soapstone Ridge
Mylonite
Appendix: Museums and Exhibits
Glossary
References
Index
ACKNOWLEDGMENTS
The insights into Georgia geology in this book are the product of many geologists’ investigations. We are especially fortunate that six researchers took time to divide up the task of reviewing the manuscript in their regions of expertise: Clark Alexander, Burt Carter, Tim Chowns, Randy Kath, Mike Roden, and Sam Swanson.
We also owe special thanks to Dr. Robert D. Bob
Hatcher Jr. of the University of Tennessee, who has been raising the bar for understanding the geologic history of the Southeast for more than forty years. He provided digital map versions that are the basis for many of the Blue Ridge–Piedmont maps in the book and reviewed several drafts of the terrane discussion. We thank Philip Prince for his assistance in understanding landscapes and the Blue Ridge–Piedmont boundary. Other geologists who have provided comments on segments of the manuscript include Callan Bentley, Erv Garrison, David Schwimmer, and Hovey Smith. Many geoscientists have helpfully responded to our inquiries: John Anderson, Stan Bearden, Gale Bishop, Jon Bryan, Clint Barineau, Chris Capps, Jeff Chaumba, Habte Churnet, John Costello, Steve Fitzpatrick, Julian Gray, Tom Hanley, Scott Harris, Mike Higgins, Paul Huddlestun, Matt Huebner, Nan Huebner, Dick Ketelle, Tim Long, Tony Martin, Arthur Merschat, Jon Mies, Katayoun Mobasher, Billy Morris, Andy Newman, Paul Schroeder, Joe Summerour, Donald Thieme, and Nick Woodward. Thanks to the staff at Mountain Press: our editor, James Lainsbury; cartographer, Chelsea Feeney; and Jennifer Carey, who reviewed earlier drafts of this manuscript.
Additional readers have given pointers on removing stumbling blocks to the general reader: thanks to Miranda Gore, Don Lundy, Caitlyn Mayer, Marty Rosenberg, and Noah Witherspoon. Others who have helped in various ways include Robert Ables, Mike Clark, Chuck Cochran, Brad Gane, Paul Knowlton, Mary Larsen, Tony Madden, Cindy Reittinger, Jose Santamaria, and Cantey Smith.
Many people, including some of the above, have provided photos or other illustrations and are credited in individual figure captions. We would also like to thank Bill Hood and Kellyn Willis of the Elberton Granite Association for photos they provided.
We thank our employers, Georgia Perimeter College (GPC) and DeKalb County Schools’ Fernbank Science Center, and are grateful for the initial support of a Writers Institute Faculty Fellowship to Pamela Gore from GPC.
Others have helped as well, and we apologize to anyone we may have omitted. We have done our best with the mountain of information about our state, and any errors are our own.
This book began as a project of Ed Albin of Fernbank Science Center. Ed recruited each of us but was unable to continue as a coauthor. We are grateful that he also recruited Steven Jaret, who got us started in locating digital map data and moving it into a publishable format.
Our spouses, Thomas Gore and Rina Rosenberg, have provided not only advice on wording and patience with a project that seemed to roll on year after year but have also been our drivers to places all over the state, ready to find a wide spot to pull off into with ten seconds notice. Clearly this book would not exist without them.
Roads and sections of this book.
INTRODUCTION
Ask a Georgian about geologic features in the state, and the answer you get will depend on where you are. If you are in Atlanta, you will probably hear about Stone Mountain, the white whaleback of a rock you can see from downtown office windows, 14 miles away on the eastern horizon. In the Blue Ridge Mountains, you will probably hear about Amicalola Falls, one of the highest cascading waterfalls (729 feet) east of the Mississippi River, or Tallulah Gorge, one the deepest gorges in the eastern United States. In south Georgia, you will probably hear about the Okefenokee Swamp, one of the largest wetlands in North America. In west Georgia, near Columbus, you will probably hear about Providence Canyon, often called Georgia’s Little Grand Canyon, which formed due to erosion from human activity over the past 150 years. In northwestern Georgia, you will probably hear about Rock City Gardens atop Lookout Mountain, from which you are said to see seven states, or Cloudland Canyon, which is more than 800 feet deep with waterfalls. Wherever you are in Georgia, there is something spectacular to see.
In terms of museums (see the appendix), Georgia has some top honors. The Fernbank Museum of Natural History in Atlanta displays the world’s largest dinosaur, Argentinosaurus. The Tellus Science Museum in Cartersville has one of the world’s largest mineral collections and a fine fossil collection with many vertebrate skeletons. And the Georgia Aquarium in Atlanta is the largest aquarium in the world.
Georgia experienced the nation’s first gold rush. In Dahlonega, where the rush began, you can visit an underground mine and see a building that opened as a branch of the U.S. Mint in 1837. Because of its vast resource of uniform, fine-grained granite, Elberton competes with a town in Vermont for the title Granite Capital of the World. Georgia marble is also prized; many of the monuments in Washington DC are made of Georgia marble, including the big statue of seated Abraham Lincoln in the Lincoln Memorial. Today, Georgia’s richest mines are in the white clay kaolin, aptly dubbed white gold.
Georgia is the world’s largest producer of kaolin, a billion-dollar enterprise and the largest mining industry in the state, and Sandersville is the Kaolin Capital of the World. Chances are good that a glossy magazine in your house, and the pages of this book, are coated with Georgia kaolin.
The state has a long history of mining. Soapstone Ridge, southeast of Atlanta, was the source of soapstone that Native Americans carved into bowls more than 3,000 years ago. Georgia had the first aluminum mine in the United States, northeast of Rome. And nearly all of Georgia is within 50 miles of one of the thousands of iron pits that dotted North America before the mass production of steel became economically feasible.
State seal and outline of Georgia carved into Elberton Granite in a floor tile at Confederate Hall, Stone Mountain Park.
GEOLOGY DETERMINES LANDSCAPE
The systematic search for mineral resources led to the production of the first geologic maps of Georgia during the nineteenth century. The geologic maps in this book are the result of more than a century of study by field geologists. Field geologists scan the countryside for outcrops, where the bedrock pokes through the soil at the surface. Elsewhere they rely on clues in soil formed by the weathering of bedrock to determine what rock lies below. Once the boundaries of a particular rock type or pattern of rock types have been traced across the countryside, formal rock unit names, such as Nantahala Formation, Knox Group, or Stone Mountain Granite, are proposed. These formal rock units are what appear on geologic maps, with assigned colors and patterns.
When you see a geologic map for the first time, you may experience a flash of recognition, because many of the landscape’s topographical features are strongly influenced by bedrock type. Though no geologist was giving advice when Georgia’s largest cities were founded, their locations were determined by geology. Savannah began on the high ground of an ancient beach, through which the Savannah River had eroded a deep valley during the ice ages of Pleistocene time when sea level was low. As the continental glaciers melted and sea level once again rose, the inundated valley became a natural ship channel, which helped Savannah become a major port. Atlanta sits on a relatively high and dry drainage divide just 30 miles southeast of a natural gateway through the mountains, located at a bend in a great, long-dead fault. Both of these geological factors made the location an ideal spot for a rail crossroads, which is how the city got its start. Columbus, Macon, Milledgeville, and Augusta all were sited where rivers cross the Fall Line, the state’s most prominent geological boundary. The term Fall Line refers to the waterfalls and rocky shoals that developed at this geologic boundary, which separates the ancient crystalline rocks of the Piedmont to the north from the young sedimentary layers of the Coastal Plain. These cities were built at the Fall Line because large riverboats could not navigate past it, and industries arose due to the abundant waterpower provided by the falls.
PLATE TECTONICS
While geologists map rock at the surface, geophysicists learn about Earth’s interior by measuring differences in gravity, magnetism, and the time it takes for shock waves from earthquakes to pass through the Earth. At its center, Earth has an iron-nickel metal core, with a radius of some 2,160 miles, surrounded by a rocky mantle that is about 1,800 miles thick. Above the mantle is the crust, ranging from less than 4 miles thick beneath some of the oceans to as much as 40 miles thick under Earth’s highest mountains. Compared to the mantle, the crust is rich in aluminum and silicon and poor in iron and magnesium.
Rocks of the crust and uppermost mantle are brittle: under enough stress, they break, sending out earthquake waves. This brittle zone, also known as the lithosphere, varies from roughly 25 to 125 miles in thickness. Beneath the lithosphere, higher temperatures make the rock ductile, meaning it is able to bend and flow like warm taffy. This ductile zone is also known as the asthenosphere. Our recognition of the existence of the lithosphere and asthenosphere was critical to the development of geology’s unifying concept of plate tectonics.
During the first century and a half of the development of the modern science of geology, earth scientists recognized that mountains rose and eroded and oceans appeared and disappeared over millions of years, but it was not until the 1960s that they formulated the unifying theory of plate tectonics that explains why. Studies of earthquakes and the ocean floor revealed that Earth’s lithosphere is a mosaic of about twenty tectonic plates that are constantly moving relative to each other, creeping along at rates of inches per year. Plates move because, underneath the lithosphere, the mantle’s taffy-like rock in the asthenosphere is in constant, extremely slow motion as hot rock rises and cold rock sinks, like a pot of boiling soup.
Most earthquakes and volcanoes are concentrated along plate boundaries. For example, earthquake epicenters and volcanoes dot the Mid-Atlantic Ridge, an underwater feature in the middle of the Atlantic Ocean that roughly parallels the western coastline of Africa and the eastern coastlines of the Americas. This ridge marks the location along which the supercontinent Pangaea rifted apart about 170 million years ago. Since that time, the continents on either side of the rift (North America, South America, Europe, and Africa) have gradually separated as the Atlantic Ocean formed and has grown larger. Magma has risen from Earth’s mantle to fill the gap at the rift, creating hot, new seafloor. Since the younger seafloor is hotter, less dense, and more buoyant than the older, cooler seafloor on either side, it floats
higher on the Earth’s mantle, forming the ridge. Rifts like the Mid-Atlantic Ridge represent one of the major tectonic plate boundaries.
Because Earth stays the same size, geologists deduced that seafloor is being recycled elsewhere on the planet at the same rate new seafloor is created at rifts. Around the rim of the Pacific Ocean, and in a few other places, geologists have located places where older, colder, and denser seafloor is being pulled into Earth’s mantle. These tectonic plate boundaries, called subduction zones, are marked by deep-sea trenches that develop where seafloor is being subducted (meaning pulled under
) beneath another tectonic plate. As a slab of seafloor descends into the mantle at an angle, water expelled from it lowers the melting point of hot mantle rocks above, causing magma to form and rise. The magma rises to the surface more than a hundred miles from the trench and creates a chain of volcanoes that roughly parallels the plate boundary. Volcanic chains can develop on land or in the sea, depending on where the subduction zone develops. Subduction accounts for the Ring of Fire, a horseshoe-shaped region of volcanism and tsunami-causing earthquakes that encircles the Pacific Ocean. In the United States, the Cascades volcanoes are part of the ring.
Elsewhere, tectonic plates slip past one another along vertical faults, such as California’s famous San Andreas Fault, which is part of the boundary between the North American and Pacific plates. These are called transform boundaries.
Ocean basins open and ocean basins close, and when they close, continents collide. Unlike seafloor crust, continental crust is not recycled in the mantle; it is made of low-density rocks, such as granite, which are too buoyant and light to descend into a subduction zone. Instead, when continents collide, the subduction process that closed the ocean basin grinds to a halt. The force of the collision fractures the continental crust and causes it to pile up and overlap, forming mountains and high plateaus. A modern example of such a boundary is the Himalayas and Tibetan Plateau, where the formerly separate Indian and Asian continents are actively colliding.
Cross section of Earth’s interior. The white lines show a few of Earth’s plate boundaries, including the Mid-Atlantic Ridge.
The major tectonic plates of the Earth. The arrows show the relative movement of the plates at their boundaries. —Modified from the U.S. Geological Survey
All of these tectonic scenarios—continents ripping apart, oceans opening and closing, and continents colliding—occurred in Georgia and left their imprint. More than 600 million years ago, a supercontinent that existed long before Pangaea began to rift apart. Northwest Georgia had a ringside seat beside a widening ocean. After that ocean was more than 100 million years old, the regional tectonic processes shifted and subduction began to devour the seafloor. Chains of volcanic islands sprang up above the subduction zones. As subduction zones consumed the seafloor, the volcanic island chains collided with Georgia’s coast, adding new territory to North America. About 265 million years ago, Africa—which was part of the supercontinent Gondwana, along with South America and Florida—collided with Georgia. The continental collision formed the Appalachian Mountains, much as the collision of India and Asia is forming the Himalayas. With this collision all of the continents had come together once again, forming the supercontinent Pangaea. For 65 million years, Georgia lay near the center of this vast landmass. Then Pangaea began to rip apart. The crust of Pangaea cracked at hundreds of scattered locations, and magma welled up to fill each crack. Eventually one set of cracks linked up and became a continuous rift zone that separated Pangaea into the continents we recognize today. As the region continued to pull apart, the rift widened and more magma rose into it. The magma solidified into the Atlantic seafloor, and the rift zone became the Mid-Atlantic Ridge.
Some plate boundary scenarios that have affected Georgia. Arrows denote relative direction of movement. —Modified from U.S. Geological Survey
GEORGIA’S FIVE LANDSCAPE PROVINCES
Georgia’s landscape is divided into five distinct geographic regions called physiographic provinces. The southern half of Georgia lies in the Coastal Plain, the inland edge of which is the Fall Line. The central part of the state is the Piedmont, which gradually ascends northward from the Fall Line to elevations around 1,000 feet. It is cut by steep-sided stream valleys and has a few isolated summits of less than 2,000 feet, such as Stone Mountain outside of Atlanta. Piedmont means foot of the mountains,
and to the north lie three distinctive mountain provinces: the Blue Ridge, Valley and Ridge, and Appalachian Plateau. The Blue Ridge has the highest mountains, with some peaks above 4,000 feet. The Valley and Ridge consists of long, parallel ridges separated by flatlands. The Appalachian Plateau is a region of flat-topped mountains, more than 1,700 feet above sea level, interrupted by widely separated, straight valleys.
The provinces played different roles in Georgia’s plate tectonic history. The Appalachian Plateau is the eastern edge of a vast sheet of flat-lying sedimentary rocks that blanketed North America and were situated beside the ocean that preceded the development of Pangaea. The Valley and Ridge is the eastward continuation of those sedimentary rocks, which became folded and faulted when North America and Gondwana collided to form Pangaea. The Blue Ridge and Piedmont consist of metamorphic and igneous rocks that are the product of intense deformation, heating, and melting associated with the long series of events that led up to that final collision. The Coastal Plain is composed of sediments that settled on top of the southeastern portion of that deformed region after erosion had planed it down, Pangaea had been pulled apart, and the Atlantic Ocean had started forming.
Map of Georgia and the surrounding region showing the five physiographic provinces of the eastern United States. —Landscape image courtesy of U.S. Geological Survey; boundaries added from several sources
Georgia’s variation in topography is related to rock type and the history of erosion. The Coastal Plain has gentle topography because it is mainly composed of soft, easily eroded, unconsolidated sediments—that is, it lacks resistant rock to make steep slopes that can stand up to erosion. Rocks rich in the hard and insoluble mineral quartz, including sandstone, form the ridges of the Valley and Ridge and cap the Appalachian Plateau’s tabletop mountains, such as Lookout Mountain. Because of its relative resistance to erosion, it has protected these high regions as those around it have been lowered by the elements.
The quartz content of metamorphic and igneous rocks plays a role in the Piedmont’s topography, as well. The Piedmont’s few isolated mountains, which are called monadnocks, are built mainly of quartz-rich rocks, such as granite, granitic gneiss, and quartzite. The higher, more-rugged terrain of the Blue Ridge may relate, in part, to a greater proportion of quartz-dominated metamorphic rocks, such as metagraywacke and quartzite. Quartz endowed the rocks of both regions with strength that allowed them to remain as other rocks succumbed to erosion.
The distinct and partly random sequence of events by which erosion has worn this landscape down must be part of the explanation for the Blue Ridge and Piedmont topography, because the topography cannot be explained by differences in quartz content alone. In the Piedmont, for example, the isolated monadnocks are only small portions of larger regions composed of granite or granitic gneiss. For example, Stone Mountain represents only 10 percent of the mapped area of the Stone Mountain Granite; the other 90 percent is buried or appears at the surface as broad, flat pavement outcrops. Monadnocks are recognized as a kind of erosional remnant. They are what happen to be left at this particular moment in a long, ongoing process that has already planed off the surrounding landscape.
The abrupt breaks in slope, called escarpments, where the high country of the Blue Ridge steps down to the Piedmont, are other striking landscape features that reflect a snapshot in the continuous process of erosion. They represent the ongoing competition between networks of streams, in which one stream or river system takes over territory formerly drained by another.
Georgia’s landscape has been shaped mainly by stream erosion. Networks of streams can steadily reduce elevations as they cut down into underlying rock or sediment. Tributaries lengthen upstream through a process called headward erosion, in which the water gnaws away at sediment and rock at its headwaters. As stream networks compete for territory, the divides between watersheds gradually migrate. If one watershed is at a much lower elevation than its neighbor, the streams at its headwaters will periodically capture tributaries from the higher elevation watershed. This process, called stream capture, occurs when one stream nibbles through a topographical divide and diverts the stream on the other side, capturing its flow. There is evidence for recent or pending stream capture along divides all over Georgia, but it is especially dramatic along GA 52 a few miles west of Amicalola Falls, where three stream networks compete for the flows of the others. (See the GA 52: Chatsworth—Lula road guide in the Blue Ridge–Piedmont chapter for more information about Amicalola Falls and stream capture.)
Because faster-moving water has greater erosive power, a stream flowing down a steeper slope is more likely to erode the land. The stream may erode through a drainage divide and capture the flow of a tributary to another stream.
If two streams have headwaters in roughly the same area at about the same elevation, and one flows a shorter distance to the Atlantic Ocean (perhaps 400 miles) and the other flows a much longer distance to the Gulf of Mexico (perhaps 1,500 miles or more), the stream that travels the shorter distance will have a steeper slope and be more erosive (simple geometry). This is the case with two river systems that have headwaters in northeast Georgia. The Savannah River and its tributaries flow about 400 miles from the Blue Ridge Mountains to the Atlantic Ocean. The Tennessee River and its tributaries flow more than 1,500 miles from the same area of the Blue Ridge to the Gulf of Mexico. The Savannah River is more erosive and can therefore capture the tributaries of the Tennessee River and other similarly long, low-slope rivers in the area, such as the Chattahoochee and Coosa rivers. Much of the rugged landscape of the Blue Ridge, with its waterfalls, whitewater, and deep gorges (Amicalola Falls, Tallulah Gorge), can be linked to erosion and stream capture.
Stream capture in progress. A 3D perspective shows three drainage systems competing for territory near Amicalola Falls. An Amicalola Creek tributary has already captured the upper valley of a Licklog Creek tributary at A.
Anderson Creek, as the highest elevation stream, is vulnerable to capture, either by a Licklog Creek tributary at B
or an Amicalola Creek tributary at C.
GA 52 passes through both potential capture sites.
SEDIMENTATION
Sediment is at the beginning of the geologic story in most of Georgia, whether you are looking at unconsolidated sediments in the Coastal Plain, sedimentary rocks in northwest Georgia, or metamorphosed sedimentary rocks in the Blue Ridge and Piedmont. Weathering and erosion break down rocks to produce sediment of many sizes, from boulders, gravel, sand, and silt, to clay. Sediment is transported by wind or water and deposited in low areas or places where Earth’s crust is sinking, such as today’s Atlantic coast. This parade of particles gets buried, compacted, and cemented (by circulating fluids) into sedimentary rock. Gravel, sand, silt, and clay become, respectively, the sedimentary rocks conglomerate, sandstone, siltstone, and mudstone, or shale.
Many marine creatures, such as coral and mollusks, use the carbon dioxide and calcium dissolved in seawater to construct their skeletons, producing the calcium carbonate minerals calcite or aragonite. These minerals can also precipitate directly from seawater and settle on the seafloor. In arid climates, magnesium in seawater can combine with calcium carbonate to make the mineral dolomite. Calcite, aragonite, and dolomite are called carbonate minerals. Over time carbonate minerals collect on the seafloor, sometimes to great depths. They harden into sedimentary rocks called carbonates. Limestone is a carbonate sedimentary rock composed of calcite or aragonite, and dolostone is a carbonate sedimentary rock composed of dolomite.
Other marine organisms, including some sponges and microbes, make their hard parts from submicroscopic crystals of quartz, forming the sedimentary rock chert, better known as agate or flint. Groundwater may dissolve quartz as it flows through silica-rich rocks or layers of volcanic ash. The quartz (or silica) may be precipitated within limestone and other carbonate rocks, forming nodules and replacing fossils. When a carbonate rock weathers, the carbonate minerals dissolve, leaving behind a reddish soil called residuum. It is composed of any insoluble clay and iron oxide impurities from the limestone, any chert nodules, or both (also known as chert and dirt
).
A layer of any kind of sediment deposited by a single geologic event, such as sediment-laden water breaking through a natural levee during a spring flood, is