From Alabama to Nova Scotia, parallel to the Atlantic coast stretches a crystalline zone of rocks that is cut by hundreds of black diabase rock dikes that was formed some 200 million or more years ago in the Triassic geological period. A dike is a vertical sheet or wall of rock of variable width or length that is extruded or cuts through older rocks. A large group of associated dikes is called a dike swarm.
The crystalline rock belt of Georgia contains a larger swarm of diabase dikes than any other eastern state. The crystalline belt of Georgia, includes 55 counties and comprises more than 17,000 square miles or slightly less than one third of the total area of the state. It includes that part of Georgia bounded by the Savannah River on the east, the coastal plain or Fall Line on the south, the Alabama state line on the west, the Paleozoic front on the northwest and the Blue Ridge highlands on the north and northeast.
There are as many of these dikes in Georgia as there are different brands of Heinz pickles, namely 57, with a total outcrop length of 256 miles. Their width varies from an inch or so to several hundred feet, to 35 miles in length. The remarkable thing about this swarm of 57 diabase dikes in Georgia is that they are all parallel and lay in a northwest to southeast direction. The diabase in these different dikes are also uniform in composition.
Diabase dikes are not an exclusive feature of the Appalachians, they are found in all the other countries of the world. There is also a great series of diabase dikes in western Scotland and Northern Ireland and England. The great Cleveland diabase dike in northern England is 110 miles long. The world’s largest known dike is in Rhodesia. Its title, “Great Dike”, is amply justified by its length of exposure, nearly 300 miles and by it average width, five miles. In South Africa, the Karoo diabase is a swarm of hundreds upon hundreds of diabase dikes and sills, that range from a few feet to over a thousand feet thick.
FORMATION OF DIKES
The diabase dikes of Georgia and all other parts of the world, are very old and all seem to be of the same geological age. The fractures occupied by the diabase were all formed by pre-cretaceous down warping of the earth’s crust. This produced great tensional cracks which are all parallel to the axis of the folding or down warping of the northwest to southeast trending synclines. This is the reason the Georgia diabase dikes are all parallel. There must have been some terrible earthquakes when the crust of the earth split open into yawning chasms – some wider and longer and deeper than the Grand Canyon. These great canyons reached 40 to 50 miles deep into huge chambers of melted diabase. The melted diabase, under great pressure surged up and filled the long fissures and then overflowed out on the surface of the earth in a mighty flood to form great basalt plateaus like the Columbia Basalt Plateau and the Decan in India. The diabase plateaus of Georgia and the Appalachians have long ago weathered and washed away leaving only the roots of the diabase dikes that fed them.
AGE OF DIKES
No exact age can be given to the diabase dikes in Georgia or the other eastern states. Well samples taken from deep wells in the Florida pan handle at a depth of 11,993 feet, below the base of the Cretaceous, and into the late Triassic period showed presence of a diabase dike that possessed essentially the same minerals as the diabase in the central zone of dikes in the Georgia Piedmont.
TRIASSIC PERIOD
The Triassic period was the first period of the Mesozoic era and was named after the three fold divisions of its rocks found in north Germany where the fossils of this age were first studied. The Triassic began 230 million years ago and lasted 50 million years.
The Triassic period was a very important age in the evolution of vertebrate animal life, because this was the age that the first shell protected eggs developed. This triumph of the egg enabled animals to invade lands far from seas, lakes, and streams, which were forbidden to the water bound amphibia. The dinosaurs and all their other reptile relatives first appeared and dominated the land during the Triassic.
THE BREVARD FAULT
The diabase dikes of the Georgia Piedmont cut into and across the Brevard Fault system. According to the “Law of crosscutting relationship”, which states that “a rock is younger than any rock it cuts across.” The Brevard Ault is therefore older than the Triassic age during which time the diabase dikes cut across it. The Brevard is the largest fault in Georgia and one of the largest in the world.
From Canada it enters the United States and passes through the Appalachian region and through Brevard, North Carolina where it gets its name. It enters Georgia from South Carolina near Toccoa and passes across the state and disappears under the Alabama Coastal plain. It confines the Chattahoochee River on its northern slope until it reaches the Alabama line. The Brevard is not a single thrust fault, but instead, it is made up of a belt of small faults that comprise the Brevard overthrust in Georgia. This great fault zone of the Brevard has ground and chewed up rocks in a belt five to ten miles wide across the state, and formed them into a new rock so fine grained that all the mineral structure is destroyed. This new rock is called Mylonite from the Greek word to mill or grind.
DIABASE
Diabase and dolorite are two different names for the same rock. They are a variety of basalt. Diabase is a fine grained, dark gray to black stone. Microscopically the rock is composed essentially of plagioclase feldspar and proxene or augite. It also contains smaller and irregular amounts of olivine and hornblende. Next to the nephrite jade, diabase is the toughest rock-like substance known and is as hard as steel. The course of the larger diabase dikes can be traced as low sinuous ridges. The surface of the ridges is covered with countless rounded boulders of varying sizes resulting from the spheroidal weathering of the diabase. The boulders have a dark brown weathered rind.
LOGANVILLE
I heard of a large diabase dike near Loganville, which is only a few miles from Atlanta. I drove over to try to find it. I asked a pulpwood cutter named John Priest if he knew where there were a lot of black round rocks. “Yeah, man, I know where there is a truck load of them. If you will buy me a can of beer, I will take you home with me and drive you over there in a four-wheel Jeep.” It was a deal, I drove to his house. He first showed me a wild cat in a wire cage. He had recently caught the wild cat by the foot in a steel trap, but with no especial injury. The wild cat had a vile savage temper and tried to jump through the cage at me when I came close to the caged and called it a nice kitty. I asked Mr. Priest how he took this piece of “dynamite” out of the steel trap. He said he threw a heavy blanket over her and tied her up.
We got into his Jeep and he drove to back country roads on the south side of Loganville to where several country dirt roads cut the same diabase dike, where there were tons of diabase boulders.
THE LOGANVILLE DIKE
To reach a most accessible dike at Loganville, go Georgia 78 to Loganville, turn right at traffic light on Georgia 20 that goes to Conyers. Go a short black and take a left on paved “Claude Brewer Road,” go about three miles and you will find diabase boulders on the right side of the paved road near the top of a hill.
TOUGH ROCK
Some thirty years ago the Georgia Highway Department cut a new road through a large dike of black diabase. The highway engineers decided to crush this black rock to make black shoulder paving. They tried to crush this strange black rock and it tore up their rock crusher. They then set up a new crusher that was guaranteed by its manufacturer to crush any rock. It was set up in the presence of a large group of highway engineers and the crusher was put into motion and a truck load of diabase rocks were dumped in the giant new crusher. But alas, it was also wrecked.
Over thirty years ago Dr. J.G. (Jim) Lester and Dr. A.T. Allen of the Emory University School of Geology made a thorough study of the diabase dikes in Georgia. Dr. Lester was well aware of the unusual toughness of the diabase boulders and always kept a few in the back yard of the Geology Building. At that time, Dr. Lester was head of the School of Geology, and he also had a devilish sense of humor. He would often ask an eager-beaver geology student to go down stairs and get a ten pound sledge hammer from Romeo and bust up one of the black rocks in the back yard for some specimens. About lunch time Dr. Lester would pass by and ask the young man for the specimens. The student would be resting and would say: “Sir, I have beat this rock all over the yard and I can’t break it. It’s the hardest rock I ever saw.” Dr. Lester would chuckle and pass on.
DIABASE TOOLS
Visitors to the vast ruins of stone temples, pyramids and walls of ancient Egypt, the Mayans, Aztecs and Incas of America, marvel at the excellent stone masonry of these ancient peoples and wonder what kind of tools they used to cut and dress such hard stones such as granite and quartzite, ages before the iron age. A careful study reveals that all these early craftsmen used celts, chisels and hammers made of diabase or dolerite. Celts or chisels made from diabase are harder and tougher than steel. I have several diabase celts or chisels used by the Mayans to cut stone.
HOW THE ANCIENT EGYPTIANS CUT STONE
Little remains of the triumphs of the ancient Egyptians, but their stone temples, obelisks and pyramids bear witness to their former skill, might and glory. The accomplishments of long ago outshone those of more recent times. The Egyptians succeeded with primitive tools and methods in extracting, moving great distances over land and water and erecting the great monoliths and obelisks more than forty centuries ago. A finished monument teaches us little of their engineering, but an unfinished piece of work may teach us much.
At Aswan, in southern Egypt is an unfinished obelisk. This piece of work failed, not through any fault of the workers but owning to the discovery of an unexpected fissure in the rock as it was nearly finished. The unfinished obelisk teaches us more about their methods of stone cutting and the tools they used than any monument in Egypt. The unfinished obelisk still lies in its quarry detached on all but its lower side. If it had been extracted it would have been 140 feet high with a base of 14 feet square. The total weight would have been 1,168 tons, heavier than any piece of stone ever handled by the ancient Egyptians. Had the stone been sound the Egyptians would have finished it, transported and erected it at some far off place down the Nile. The Egyptian engineers always finished anything they started.
Work was started on the obelisk by dressing down smooth the top surface. This was accomplished by placing red hot bricks on the rough surface to be removed. Then the surface of the hot rock was doused with cold water. The uneven rock surface fractured and was easily detachable by pounding with mauls or hammers made of diabase or dolerite. This rendered the stone reasonably smooth. The detachment of the two sides of the obelisk was one of the most delicate stages of its removal. Large balls of diabase, each weighing 12 pounds were found near the obelisk.
The ancient Egyptians called diabase, BEKHEN. Countless numbers of these diabase balls had been brought from Wadi Hammamat in the eastern desert near the Red Sea where they occur naturally, weathered out of the many diabase dikes. The black diabase balls were attached to the end of wooden rammers and were used by being struck vertically downward with great force, to cut a series of parallel vertical cuts to form a trench around the obelisk. The only tools which could produce this effect were the diabase balls. The trench and pits were therefore not cut out but rather bashed out.
Several thousand men were arranged about the obelisk in groups of three, two standing, holding and raising the diabase rammer and a third squatting below and directing the blow to the proper place. One man would sing or chant to maintain the rhythm of the blows.
Only in Roman times were iron wedges used in detaching stone and iron chisels in dressing them.
In ancient Egypt, pounding with diabase hammers or mauls was the only method used to detach and dress stone blocks. Fine carving was done with diabase celts or chisels.