Fossils have been used for over two centuries to date when events occurred in geological history. But there is a huge period of time that we are unable to date because of the absence of fossils. This is because living organisms did not have the ability to secrete hard shells until about 570 million years ago (mya). The Earth’s history has been relatively easy to record starting with the Cambrian Period which coincides with the appearance of hard shelled species. All time before that key step in evolution 570 mya is called the “Precambrian” and although it is the vast majority of Earth’s total history it is not based on fossil records. Life that existed in the Precambrian Period was comprised of microscopic organisms and a few soft-bodied larger organisms that usually decayed when they died rather than becoming fossilized. Most of the rocks from the Precambrian have existed so long that they have been (1) eroded, (2) covered by more recent ocean sediments, or (3) heated, buried, reheated — melted, eroded and reburied again and again. These processes reshaped the original sediments into metamorphic and igneous rocks that are found throughout northern Georgia.
The old Precambrian rocks would have remained deeply buried miles below the surface in Georgia; however, about 300 mya ago the North American continent collided with the African continent (the Atlantic Ocean had yet to form). The collision thrust the Precambrian rock to the surface between two faults, the Cartersville fault and the Great Smoky Fault. This tremendous crash of ancient metamorphic rocks produced some of the best collecting localities we have in Georgia in a province that the geologists called the “Blue Ridge”.
The most well know Precambrian rock in Georgia is Murphy Marble that has been mined in the town of Tate and the surrounding area for over a century. Almost half of the marble monuments in Washington DC have come from the Murphy Marble belt. This marble was made by metamorphism of one of the first limestone deposits laid down in the Precambrian ocean. Other rocks that formed in that early ocean were shale beds (mud deposits) and basalt lava flows. The shale has been metamorphosed into beautiful garnets, staurolite, and kyanite crystals that can be found in the Blue Ridge. The early lava flows were also metamorphosed, and produced green epidote crystals and hornblende (amphibole) that can be collected in the mountains north of Georgia.
The next geological province south of the Blue Ridge is called the Piedmont, and it is composed of about half Precambrian metamorphic rocks, which have been subjected to granite intrusions that are younger than Precambrian. Stone Mountain is an example of one of these younger intrusions. The Precambrian rocks of the Piedmont were once a volcanic island that was metamorphosed when the African and North American continents collided. The Precambrian rocks in this belt contain native gold and make up Georgia’s “gold belt’ which runs from Raburn County down to the southwest through Cherokee and Bartow counties. The gold found here was not in the original volcanic rocks, but the result of fractures that formed in the rocks and hot gold-bearing fluids that flowed into the fractures near the end of the Precambrian. The fluids cooled to form veins, lenses and stringer of Quartz that contained gold. Most of the gold recovered from these Precambrian rocks is not directly from the veins, but from quartz gravels that weathered out from the veins over the millions of years. Such weathered deposits of gold are common and mining operations that seek gold from old stream gravels are called “placer” operations.
Further to the south in Coastal Plain Province, a second narrow band of faults in western Georgia raised ancient Precambrian beach deposits into a high ridge called Pine Mountain. From this impressive quartzite ridge one can look down to the North on Calloway Gardens. Rain that falls in the Pine Mountain regions sinks down through the faults to the south to Warm Spring. It becomes part of the artesian springs that were enjoyed by Franklin D. Roosevelt.
The largest region in North America where Precambrian sediments were not transformed into metamorphic rocks or melted into igneous magma was in Canada and in a small region along the USA-Canadian border especially in a belt that runs across the North Shore of Lake Superior in northern Michigan and though Wisconsin, Minnesota and as far west as Montana. In this region Precambrian mounds formed by blue-green algae called stromatolites. These early one celled organisms are thought to have produced most of the oxygen in our atmosphere. Stromatolites are one of the longest living organism on Earth, and although rare they still exist living in hot springs and hyper saline bays in Australia.