North Georgia is a natural showcase for geomorphology, the science of landforms. A tour around small towns like Tiger, Lakemont or Rabun Beach shows splendid examples for the development of mountain landscapes and valleys.
Our Appalachian Chain is very old. It became a mountain range 470 million years ago when two continents collided and heaved up material between them. Ever since the mountains first began their slow rise, taking millions of years, rain and running water have been wearing them down. Little channels on the surface become grooves as the runoff removes loose stones and sand. Grooves turn into tiny valleys. Water running down the side slopes makes them wider and deeper.
Meanwhile, groundwater coming from inside the rock mass is emerging at the bottom of the small valley. Soon a permanent stream replaces the rain runoff. Now there’s plenty of force available to continue the erosion process. Fed by the groundwater, the stream is carving a V-shaped valley into the landscape. Smaller streams form tributary valleys. At the top of each valley, more material is removed, lengthening the valley in an upstream direction.
In some places, the stream merges with one on the other side of the mountain, and a water gap is formed. It looks like a river broke straight through the mountain, but what happened is simply that two streams on opposite sides of the mountain deepened their valleys until the landforms merged into one large cut. York Creek, which feeds Anna Ruby Falls, passes through such a water gap before reaching the falls. Some of the most spectacular water gaps can be seen in Pennsylvania. Just north of Harrisburg, the Susquehanna River cuts through successive ridges at right angles like a bandsaw. The water didn’t “climb up” to the tops of the ridges, of course. It simply kept cutting down as the rocks below it were rising.
When a river has flattened its landscape sufficiently, it deposits solid matter, and a level floodplain forms. It consists of sand, gravel, pebbles, boulders, and whatever else the water could drag there. Floodplains are abundant in the North Georgia mountains. Just look for a flat, grassy valley floor with slopes rising on both sides. Few people are bold enough to put houses on a floodplain next to the river. Flood water has been there before, and it will visit again.
Rudi Kiefer, Ph.D., is a professor at Brenau University, teaching physical and health sciences on Brenau’s Georgia campuses and in China. His column appears Sundays and at gainesvilletimes.com.