Out of Africa

To be honest, I don’t care to travel all that much. Sure, it’s fun to see new places and meet new people, but the older I get, the more tiring travel is. Like the time I drove to Africa on business and …

Yes, I live in the United States, and yes, I drove to Africa. No plane tickets involved and I didn’t take a ship. I got into the company vehicle, set the GPS, and drove. What’s more, if you live in the United States, so can you. Set your GPS to any place in Florida and …

Yes, Florida is in North America – now. Once it was part of Africa. Of course, that was a long time ago. Return with us now to those thrilling days of yesteryear, about 350 million years ago, when giant insects ruled the earth and plants were laying down coal seams. There is a wreck of planetary proportions going on. North America and Europe are colliding with Africa, and it’s going to be a doozy. Before it’s over, one of the tallest mountain ranges that ever existed will tower over the supercontinent Pangea. But deep below the mountains the old boundary still exists, way down below the roots of the mountains, where part of one plate has dove beneath another. When Pangaea began to break up, about 200 million years ago, the old point where the plates collided millions of years before begins to split open.

Make that mostly split open. If you take two pieces of putty, press them together, then pull them apart, it seldom cleanly tears along the old boundary. It tears, with parts of each clinging to the other half. That’s what happened when Pangaea broke up. Most of the old plates when their separate ways, but a little bit of Africa remained stuck to North America. That’s pretty much the south third of Georgia and all of Florida. So anytime you drive to Florida, you’re going into what was once Africa.

The obvious question is how do we know this is so. Obviously no one found a sign that said “Welcome to Africa.” The ancient rocks in much of the Southeast below the Fall Line are buried under kilometers of sediment, which makes it impossible to compare on sight. But there are other ways. One of them are seismic surveys, which studies how waves travel through the earth. This turns up all sorts of interesting features below the soil, which is why oil companies use it to pick out likely spots to find oil. It also shows up interesting oddities. One is the Surrency Bright Spot, about 16 kilometers below Surrency, Georgia, that reflects seismic waves. This also happens to be on the ancient subduction zone where North America and Africa met millions of years ago.

What brought all this to mind, though was something called the Brunswick Magnetic Anomaly. Named after Brunswick, Georgia, this is a line of anomalous magnetism stretching across the Southeast. Like seismic surveys, mapping magnetic anomalies can tell us something about the structure of the earth far below. Such is the case of the BMA, which the study by Elias Parker, Jr, shows is likely the point where North America and Africa collided. You can find a map of it, and more information, here.

The path of the BMA sort of leaped out at me. I’m one of those folks who likes to look at Google Earth for patterns. In Georgia, the rivers roughly run toward the South – except when you come to the Ocmulgee and Altamaha. After flowing south, the Ocmulgee takes a huge turn toward the east, joins up with the Oconee, and continues on an east-southeast course to the Atlantic. There are other rivers that also flow more eastward, but none with the pronounced curve of the Ocmulgee and Altamaha rivers. The big question to me has long been why? It’s as if, after flowing south, at some point as the seas receded, it encountered high ground and flowed around it. But basically all of it is sediment from the eroding Appalachia mountains and limestone laid down by the sea. If it formed like a delta, why curve in this manner?

The BMA looked very familiar, and I found a county outline map of Georgia and made a quick overlay. The curve of the Ocmulgee River and the path of the Altamaha are a close match to the BMA. It’s as if, after the Atlantic receded, there was still a low area and the rivers took the path of least resistance.

The geologic history of the Southeastern US is complex, and there’s another wrinkle: The Apalachicola Embayment. This was basically the path of the Gulf Stream before Florida and all of Georgia was high and dry. Unlike the BMA, it’s much more shallow, maybe about 180m below the surface, and runs across it. This means even though it carved out a trench, it wasn’t pronounced enough to influence the flow of the later-day Ocmulgee and Altamaha.

If the BMA did influence the flow of rivers millions of years later, it must be a pretty substantial rift to have an effect through kilometers of sediment. Or maybe it’s a coincidence. Still, it’s a pretty big one for rivers to pretty much follow the same path.

It’s pretty cool, when you think about it. Driving from North America to ancient Africa. And if you happen to cross the Ocmulgee and Altamaha, you’re also crossing what was once the boundary between British and Spanish possessions. But that’s another story.