Alluvial and Colluvial Geology of Pleistocene Georgia

Rivers and streams built all of Georgia’s coastal plain or about 1/3rd of the state.  All of Georgia’s coastal plain was under ocean water during most of the Eocene (55 million BP-33 million BP), and then periodically it was at least partially inundated with seawater transgressions until as recently as 5 million years ago.  Rivers deposited sediment from eroded rock originating in the piedmont and mountains, and gradually this fill became high and dry land.  This explains why so much sand occurs on the coastal plain.

During the Pleistocene Georgia’s rivers went through several cyclical phases that correlated with climate fluctuations.  


Diagram of changing river patterns over the last 28,000 years.  Earlier interstadials probably had some braiding rivers and some meandering rivers.

From ~28,000 BP-~15,000 BP cold arid climate caused reduced stream flow.  Lowered water tables made many small streams vanish.  Major rivers consisted of a braided pattern, and they were clogged with many sandbars and islands.  Other rivers became like a chain of isolated springs where the water table met the surface of the land.

From ~15,000 BP-~12,ooo BP sudden warming climatic trends caused massive storms and a dramatic increase in precipitation.  These super storms were of 2 types: violent tropical storms and storms caused when strong cold fronts originating from retreating glaciers clashed with warm fronts.  The frequent storms created super meanders in rivers and caused the river bends to shift wildly, resulting in lots of scroll bars.  Distinct wet and dry seasons also contributed to scroll bar formation.

From ~12,000 BP-~~5,000 BP increasing precipitation in the atmosphere caused by glacial meltwater pulses caused rivers to meander more than they do today.

From ~5,000 BP- the present modern river meandering patterns prevailed.

Colluvial environments resemble alluvial landscapes but have a different origin. Colluvial environments are shaped by rainwater eroding soil from slopes rather than from river sedimentation.  However, in some cases, rivers eroded slopes that later led to colluvial soil fill.  As I noted in the above description, during certain climatic phases of heavy precipitation and increased storm activity, rivers meandered more than they do today.  These supermeandering rivers and streams eroded slopes.  Later, when rivers returned to their present beds, rain washed soil from the slope to the flat below. 

Diagram of colluvial environments.

Colluvial flats can also exist where rainwater washes soil from rocky hills–the area adjacent to Stone Moutain in Dekalb County is an example of this.

The most common type of colluvial flat occurs along small streams and springs that at one time had a much larger flow.  Deep gulleys and steep terrain characterize colluvial lands.  Little Kettle Creek, which I visited last year, may be an example of a stream that once had a higher stream flow.  The gulley where this small shallow creek runs is at least 12 feet high in the place I visited, and on topographical maps, the gulley looks much higher than that in other places.  When it accumulated mammoth, mastodon, bison, and deer fossils, it must have been a much deeper stream with a higher flow.

View of Little Kettle Creek from the top of a bridge.  The gulley here is much deeper than the water flow.  I hypothesize Little Kettle Creek used to have a much greater flow of water.  The megafauna fossils found here must have been deposited from either alluvial or colluvial processes.

Dr. Charles Wharton listed several examples of colluvial lands–Anneewakee Creek in Douglas County, Bay Springs Creek in Carroll County, Chattahoochee State Park in Fulton County, and the southeast and northeast faces of Stone Mountain in Dekalb County.  Common trees growing on colluvial flats include beech, tulip, magnolia, white oak, mulberry, loblolly and shortleaf pine.  At Stone Mountain, mountain laurel grows in the understory.  Paw paw (See my blog entry–“Favored Fruit of the mastodons” BTW, the offer still stands–a free copy of my book in exchange for paw paw seeds or fruit)  is common (or was when Dr. Wharton checked it out 30 years ago) in the understory at Chattahoochee State Park.  Paw paw does grow best on rich soils that are found on colluvial flats.


4 Responses to “Alluvial and Colluvial Geology of Pleistocene Georgia”

  1. jamesrobertsmith Says:

    I’ve never seen a braided river, but some of my pals have backpacked in Alaska where they are common. One of my hiking companions even went backpacking in The Valley of 10,000 Smokes where he encountered a late-migrating brown bear as he and his wife (the hiker’s, not the bear’s) were climbing a mountain to get to an abandoned USGS cabin to spend the night. That’s as close to Ice Age Pleistocene as modern humans are likely to get.

    His impression (they woke the bear up by accident as they climbed the slope), was that this bear had never encountered humans. He didn’t quite know what to make of Andy and Christy. Fortunately, after a few tense moments, the bear chose to push on, away from them.

  2. markgelbart Says:

    I’ve never seen a live bear in the wild. The closest I ever came to a bear was when I visited the nature center at the Okefenokee Swamp. They had 2 big ones in an enclosed area, and they were very active, running back and forth and climbing trees.

  3. jamesrobertsmith Says:

    I have had dozens of encounters with wild bears. Mostly in North Carolina, Tennessee, and Georgia. Once in West Virginia.

    I encountered one grizzly bear when I started my hike to the summit of Avalanche Peak in Yellowstone. He wanted nothing to do with us and move quickly away. I was only able to get a few good photos.

    I’ve had one bear (in Georgia) become EXTREMELY aggressive (we thought we were about to be killed). And I’ve encountered some mildly aggressive bears in the Great Smoky Mountains National Park…those were creepy, but I never felt I was going to be killed or mauled.

  4. markgelbart Says:

    I think I told you about the time my grandparents went camping and a black bear raided their camp, forced them to retreat, and ate a week’s worth of groceries. I think he took photos. I’ll have to see, if I can locate them.

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