I’ve been a member of the Nature Conservancy for 22 years but have never visited one of their preserves. I had a sudden urge to see one, and the Moody Natural Area excited my interest. The Moody Natural Area is 8200 acres of sand scrub, open pine savannah, bluff forest, river bottomland forest, and cypress/tupelo swamp along the Altamaha River southeast of Vidalia, Georgia. I’ll have to wait til fall though because summer is too !#$! hot here in Georgia, and I don’t want to drag my poor wife and child out on another long field trip. To appease my eagerness to see this remnant wonder of nature, I studied satellite photos of the site and found some interesting geology. Below is a link to the satellite photo, but it won’t link directly to the area my sketch is based on. To find it, pan to the right and enlarge.
Sketch of a satellite photo of Altamaha River adjacent to the Moody Nature Preserve. Note the oxbow lakes and paleomeanders. Both types of formations were former bends of the river that got cut off from the main channel. The paleomeanders mostly dried up and filled with vegetation. Both possibly formed as the Ice Age ended and precipitation increased making the river meander to a greater degree than it does presently.
Paleomeander scars are evident in satellite photos of the Altamaha River. The Altamaha River used to flow through these spots, creating the visible incisions. Later, the river shifted to its present location and as always is still slowly altering its position. I don’t know whether geologists have ever studied the part of the Altamaha adjacent to the Moody Natural Area, but they have examined other areas along this mighty stream. They’ve excavated braided sands in terraces that date to the Last Glacial Maximum (~30,000 BP-~15,000 BP). I’ll explain what this means below. But first, I’d like to suggest that the visible paleomeander scars in my sketch based on this location of the Altamaha may date to the end of the Ice Age (15,000 BP-10,000 BP) when the river meandered to a much greater degree than it does now. Note that some of these scars still fill with water on occasion.
Geological History of Georgia’s Rivers
36,000 BP-30,000 BP
Prior to this time period, there is only a limited amount of data studied, so I begin here. It’s likely, however, that time periods and climate phases discussed are repeated cycles that have occurred before. Climate fluctated rapidly before the LGM, alternating between stadial and interstadial. Stadials were periods of glacial expansion characterized by cold arid climate. Though glaciers never came close to what’s now Georgia, the change in climate had a significant impact on Georgia’s rivers. Long droughts lowered the water table and in many cases completely dried up tributaries. The major rivers became clogged with huge sand bars and islands. A decrease in vegetation along the rivers meant even more sandy sediment could accumulate. The surrounding landscape consisted mostly of pine and oak savannahs. Grasslands and sand scub grew in some places right up to the water’s edge. Interstadials were periods of glacial retreats characterized by cool moist climate. As the glacier to the north melted, moisture levels in the atmosphere increased as did precipitation. Though average annual temperatures increased, they were still lower than those of today. Interstadials likely had wet and dry seasons–late winter, spring, and early summer were wet; late summer, fall, and early winter were dry. Floods during the wet season accumulated river sediment; winds during the dry season converted this accumulation of sand into dunes. Because climate fluctuated during the this time period, Georgia’s rivers included a combination of meandering and braiding patterns.
30,000 BP-15,000 BP
Temperatures and precipitation rates fell dramatically during the Last Glacial Maximum as the northern glaciers expanded as far south as what today is central Ohio. This caused water tables to drop creating braided river patterns.
The Platte River in Nebraska is an example of a present day braided river pattern. Note the prevalent sparsely vegetated sand bars and islands. Georgia’s rivers looked much like this during the Ice Age rather than their present day meandering patterns.
The climate was so dry that in some places eolian sand dunes born from riverine sand deposits blew across the landscape. Thinly vegetated grassland grew to the water’s edge in some areas. Geologists find braided river sands in terraces 0-15 feet above the floodplain adjacent to the rivers. Some sand dunes formed during this era are quite large but today are covered with vegetation and more recent sediment. Others have eroded away.
15,000 BP-5,000 BP
Temperatures rose rapidly at the end of the Ice Age, albeit the rise was interrupted by a precipitous fall in in average annual temperatures during the Younger Dryas cold phase. As the Laurentide Glacier melted, there was a sudden increase in precipitation which caused massive storms and floods. Georgia’s rivers broke out of their braided pattern and began to meander to a greater extent than they do today. Flooding was more extensive as well. For the first 5,000 years of this time period scroll bars were prevalent but these decreased in abundance later. Scroll bars form when large meanders migrate and create ridges between old meander incisions. Meanders peaked in intensity about the time the great glacial Lake Agassiz in Canada broke through the ice dams and released most of its water. (See “Temporal Correlations between Lake Agassiz, the Okefenokee Swamp, and Ancient Flood Myths” from my January archives.) Rivers still meander but to a lesser extent since the atmosphere has stabilized.
“Late Quaternary Climates and River Channels of the Atlantic Coastal Plain”