What are these "earth pimples," as some researchers have described the circular mounds that appear in all over the Red River Valley?
Along the Red River and some of its tributaries - the Quachita, the Little River, and the Sulphur River - appear hundreds of thousands of perfectly circular mounds. They are not very tall can be indiscernible to the naked eye. They appear in alluvial plains of the rivers, sometimes within the rivers themselves, as well as in forests and on the prairies of the Cross Timbers. Apparently, their presence have confounded researchers since the 19th century. They are so plentiful, in fact, that Americans who settled along the Red River in southwestern Arkansas in the 1820s named the area around Garland City and Lafayette (no longer extant) "Mound City."
Luckily for us, I've been using LiDAR topography (caltopo), articles from Research Gate, and my own research into documented history of the past three centuries to get a better understanding of these mounds. So lemme share what I know so far.
Are they the signatures of gophers or some other burrowing animal?
One early theory is that the mounds were built by animals toiling away in the soil to find places hide. I don't buy this theory at all, but I'm adding this hypothesis for the sake of fairness (and to get it out of the way early in the article). In this theory, animals who occupied the region for millennia, and then some, did such extensive burrowing that they reshaped the landscape.
First, how BIG would these animals have to be? And secondly, where did they all go? The fossil record precludes any mammal of "monster" proportions. Geologists who have studied the mounds haven't uncovered any tunnels dug by rodents. And, when looking at the "Prairie Dog Mound City" that Randolph B. Marcy encountered along the western Red River in 1852, mounds formed from these burrowing animals are conspicuously absent from LiDAR, which would be present if the "burrowing animal theory" held any water.
So I will state for the record that I do not think that gophers, moles, armadillos, or Bugs Bunny built these mounds.
Are they geological remains of drought?
Seifert, et al (Quaternary Research 71, May 2009) explain that the mound formations, scientifically known as nebkhas (coppice dunes), comprise a geological record of a prolonged drought that occurred in the Holocene era. They cored mounds in the "Ozark Plateau, Arkansas River Valley, and Gulf of Mexico Coastal Plain" to discover that the they had similar soil characteristics. The deposits indicated build up around plant material and then, erosion from a prolonged and severe rainless period. Coppice dunes are familiar to anyone who's been in a desert or at a sandy beach, where clumps of vegetation cling to the sand.
A famous example of ancient nebkhas are the Mima Mounds that can be found in the northwestern United States. These mounds differ greatly in appearance from the mounds found in the Red River Valley, however: the Mima Mounds are clearly visible to the naked eye and are very similar in structure, almost like waves on the ground. They were made either by glacial and/or seismic activity.
Are the mounds caused by erosion?
In May 2012, Chance Robinson published a thesis that the "pimple mound formation in East-Central Texas" were caused by erosion. He studied and mapped the mounds in Leon County, Texas using LiDAR and concluded that these natural hills were the result of Pleistocene-era geology ancient river beds that left their presence in the sediments.
But because the geologists did not probe mounds in the Red River Valley, we're going to keep pontificating on them, because as usual, there's more to the story, and one version never fits all!
Might the mounds be the remains of tree copses?
Along the same vein of "coppice dunes" and erosion are "copses of trees." The Red River Valley cuts through narrow but very long forested belts -- in the central valley, they are called the Cross Timbers. Here, trees congregate on prairies in copses, an old-timey word for clumps of trees that grow on a slightly higher elevation.
During a drought, the trees in copses may build up air bubbles in their roots in their dying throes, called "hydraulic failures," and these mounds could be direct descendants of a mass-tree-die-off due to a prolonged dry period.
Droughts and especially, sudden droughts (also called "flash droughts," believe it or not) are very common in the Red River Valley. Just this year alone (2024), we experienced record rain falls in the Spring, and then, incredibly dry, dusty conditions in the Fall. All of these weather changes place a lot of stress on trees.... but guess what? We also experience flash floods here.
When the Great Raft of the Red River still existed, the back up of river water -- plus the very unpredictable heavy Spring rains, with tornadoes and hail and other damaging actions -- can also cause trees to die. The mounds might be remains of these kinds of groves that were killed in floods.
The drought theories do not factor the human element, though. For thousands of years in the Old and New world, copses of trees were planted and pruned on purpose to provide available wood for cooking and building; this practice is called "coppicing." So did humans have something to do with these mounds along the Red River?
Are the mounds remnants of the Caddo culture?
The Caddo people lived along the Red River for five thousand years or more... they are the true, original "Red River-ites." They built villages and farms along the river, and then built new villages and farms when the river shifted (as it has always been prone to do); when enemies pushed them out of an area; when they conquered another part of a territory; or when resources had been temporarily depleted and new resources had to be found. The Caddos were not nomadic but because they adapted well to the climate of the Red River Valley, they remained in this region, even if they had to move from time to time. Which means that of course, they left a profound footprint here.
Caddo homes were built from wood. It stands to reason, pun intended, that they would practice "coppicing" to stands of trees in order to have ready supplies to build their villages as they moved along the river. But these copses weren't the only mounds in the land of the Caddos; they also built mounds for themselves to keep their homes and temples on higher ground.
Since the 1920s, archeologists have discovered that many of these "pimple mounds" hide within them actual Caddo village sites, with mounds that were occupied by buildings and/or by burial grounds. Because these mounds were built deliberately, there is usually a burrow pit (where the soil was removed to build up a mound) nearby. When looking at LiDAR, even a lay observer like myself is thus able to tell a "natural mound" like a coppice dune apart from a human-made mound.
In 2024, engineer F. Andrew Dowdy published his extensive research on "Lidar and the Mystery of the Mounds" (DOI: 10.13140/RG.2.2.23667.50728) His conclusion is that "many of these mounds may have been man made" due to the eleven typologies in the soil that his probes identified: four typologies appeared to be of natural origin, but seven typologies suggested that humans had a hand in building these mounds. Using LiDAR maps, Dowdy painstakingly differentiated the mounds in areas from Missouri to Louisiana and noticed that the mounds along the Red River exhibited patterns that could not be made by geological processes: they were not random but "curvilinear, distributed, or continuous" which suggested deliberate building.
My theory... and I have one, of course.
Developing a "historical imagination" (Bailyn, 1985) allows landscapes from the past to come into focus a lot easier: if we develop pictures in our minds of what the past may have looked like using established references, we can infer and inform new conclusions. While still sparse, the scholarship on the geology of the mounds, coupled with the scholarship on the archeology of the Caddos, coupled with regional understanding of the Red River Valley's unique geo-history, can let us imagine that the mounds that are best seen on newish technology, like LiDAR, may be both natural AND human-made. One doesn't preclude the other!
Interesting article, as usual. You never mention the diameter of the mounds, or does it vary. My land, east of Denison and 1-1/2 miles from the Red River, has two strange circular places on it that are about 50 feet in diameter. They are elevated, but only 1-2 ft and you would never notice them except the vegetation is different. When I bought the land, 20+ years ago, it had had cattle on it. I have let it go back to natural forest, but no trees, other than some scrubby sumacs, survive there. Probably no relations to your mounds, but they have always mystified me.