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Among the thoughts I’ve wondered as I walked on the Three Rivers Greenway: Is the large, straight ridge just inland of the trail between the Thomas Newman Boat Landing and the Cayce wastewater treatment plant a man-made structure or the result of natural river processes?

It’s hard to imagine a reason for people long ago to have gone to the trouble of pushing up that much dirt to protect what historically has been either farm fields or timber tracts. But the historic Old State Road is only a few hundred yards away. Maybe the berm was constructed to protect a major traffic artery.

Also, the giant oaks now rooted in the top of the ridge along the Congaree River are about the same size (and I assume age) as trees that have grown on the smaller man-made earthworks less than a mile away along Congaree Creek. For those who don’t know, those were constructed, mostly by conscripted African-Americans, near the end of the Civil War to provide cover for Confederate troops in an effort to slow the march of Union Gen. William T. Sherman up Old State Road toward Columbia.

Could the ridge on the river’s edge have some connection to those earthworks? Seems unlikely, but I still wondered. So I asked John Jameson, a heritage consultant with the City of Cayce who coordinates efforts for the 12,000 Year History Park. He asked others, and Robert Hawks, a GIS analyst with Cayce’s utilities department, offered his thoughts in an email.

“While I will not pretend to be an environmental science expert, I may be able to shed some light on this. My assumption is that the features described are the natural flood levees along the Congaree River. Those banks are built up by the relatively heavier sedimentation available immediately downstream of the outlet from the fall line further north. This heavier sedimentation keeps the river straight for several miles beyond what would normally be expected in a lowland, potentially meandering floodplain. Further south, these embankments begin to taper off, and the river begins meandering.”

Hawks’ explanation, based on geologic science, makes perfect sense. Using a terrain map that can be found at Lexington County’s OneMap service, he noted another relatively straight ridge further inland that marks the edge of the floodplain. I love how rivers can so often meander but occasionally create unnatural-looking straight lines.                                                                                                                                                                                           

And Hawks is backed up by archaeology. A trench survey of the ridge done by Neal W. Ackerley in 1976 as part of a SCE&G power line project found more than a dozen distinct stratified layers created over thousands of years.

While the ridge certainly isn’t man-made, it might hold some hints of human occupation of the region. That 1976 study found a quartz flake about 7 feet below the surface and ceramic-bearing deposits in a layer with a high level of organic matter about 5 feet below the surface. Both could be indications of human occupation of the region, with the quartz flake layer estimated to be more than 2,000 years old.

Work on the other side of the river found similarly stratified layers, an indication that the river in this section has wandered little from side to side over the past 1,000 years. So this short section where the Congaree River runs straight has been in place a long time, long enough for deposits left by intermittent floods to push up a large ridge.

OK, now I wonder if the smaller ridge that runs from the Knox Abbott/Blossom Street bridge to the railroad trestle on the Cayce Riverwalk has a similar backstory. Or is it related to canal-and-locks projects designed to boost trade on the river more than 100 years ago?

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