The post about the Carolina Bays that I wrote a couple of weeks ago turned out to be relatively popular (as far as this webpage goes, anyway). Carolina bays are elliptical depressions of varying size that occur along the Atlantic Coastal Plain in a band extending from New Jersey to Florida. Their limited geographic distribution and northwest-southeast orientation has given rise to many ideas about how these features were formed. Ongoing debate centers around the question of whether the bays formed as (1) the result of impacts associated with an extraterrestrial object (e.g., debris ejected by a comet strike in Saginaw Bay) or (2) through the actions of wind and water during the Pleistocene. Extraterrestrial or terrestrial?
A new paper by Chris Moore and colleagues (see full reference below) in Southeastern Geology provides more evidence that the bays were formed and modified over long periods of time by natural, terrestrial processes. You can read the paper for yourself here.
The analysis in the paper focuses on Herndon Bay, a 1-km long elliptical depression in Robeson County, North Carolina. Using a combination of detailed surface mapping, ground penetrating radar data, geomorphological analysis, and age estimates obtained using OSL, Moore et al. show that punctuated migration of Herndon Bay to the northwest from about 41 to 24 thousand years ago produced a sequence of sand rims on the southeast side of the basin. The bay held its shape and orientation as it migrated over the course of thousands of years.
A portion of Figure 3 from "The Quaternary Evolution of Herndon Bay, a Carolina Bay on the Coastal Plain of North Carolina (USA): Implications for Paleoclimate and Oriented Lake Genesis." The numbers show locations of dated sand rims left by migration of the bay (4 is the oldest, 1 is the youngest).
The evidence and analysis that Moore et al. present is a pretty strong argument against the idea that the bays were formed by a single event (i.e., an extraterrestrial impact). I encourage you to take a look at the paper. I'll just paste in a paragraph from their conclusion (pg. 168):
"The characteristics of Carolina bays, including basin shape, changes in basin orientation with latitude, and sand rims reflect long-term and pervasive environmental, climatological, and hydrological factors over millennia rather than from sudden or catastrophic events (Kaczorowski, 1977; Thom, 1977; Carver andBrook, 1989; Brooks and others, 1996; Grant and others, 1998; Brooks and others, 2001;Ivester and others, 2007, 2009; Brooks and others, 2010). The fact that practically all Carolina bays in a particular geographic region have nearly identical patterns of shape, orientation,and sand rim composition suggests similar processes working over long periods of time. This study also indicates that Carolina bays can respond rapidly, and appear to become more active during periods of climatic instability. While many nuances of bay evolution remain to be re-fined, the evidence at Herndon Bay clearly supports the concept that Carolina bays represent a regional example of a globally-occurring phenomenon: They are wind-oriented lakes shaped primarily by lacustrine processes."
Moore, Christopher M., Mark J. Brooks, David J. Mallinson, Peter R. Parham, Andrew H. Ivester, and James K. Feathers. 2016. The Quaternary Evolution of Herndon Bay, a Carolina Bay on the Coastal Plain of North Carolina (USA): Implications for Paleoclimate and Oriented Lake Genesis. Southeastern Geology 51(4): 145-171.
Addendum (3/18/2016): Let's start over with the comments. Please keep comments on the topic of the blog post (Carolina bays) or I'll delete them.
A comment on yesterday's blog post about the Middle Archaic brought up the issue of the origin and age of the Carolina bays. The bays are elliptical depressions with a northwest-southeast orientation. They vary significantly in size and occur along the Atlantic coast in a band extending from New Jersey to Florida. There is a similar set of features (with different orientations) in Nebraska and Kansas.
Carolina bays are interesting for several reasons. They're obviously peculiar, geographically-widespread features formed by some sort large scale event or natural process. There is significant disagreement as to how and when formed and, subsequently, their relationship to the early prehistory of the Eastern Woodlands. The most dramatic scenario sees the Carolina bays as impact sites from debris that rained down after an apocalyptic comet strike 12,900 years ago that triggered the Younger Dryas and caused the "extinction" of the Clovis peoples. The less dramatic scenario sees them as results of some regular terrestrial process that ran its course well before humans were even present in the region.
How did the Carolina bays form?
Today there are two main schools of thought about how the Carolina bays formed: (1) through wind-wave action associated with Pleistocene conditions unlike those of today; and (2) as impact sites of debris ejected by a comet strike in Michigan or Canada.
My impression is that the geomorphological (i.e., terrestrial) explanation enjoys a lot of support from geologists who specialize in the Pleistocene. I'm just going to paste in a paragraph from the Wikipedia entry that sums it up:
"Quaternary geologists and geomorphologists argue that the peculiar features of Carolina bays can be readily explained by known terrestrial processes and repeated modification by eolian and lacustrine processes of them over the past 70,000 to 100,000 years. Also, Quaternary geologists and geomorphologists believe to have found a correspondence in time between when the active modification of the rims of Carolina bays most commonly occurred and when adjacent sand dunes were active during the Wisconsinan glaciation between 15,000 and 40,000 years (Late Wisconsinan) and 70,000 to 80,000 years BP (Early Wisconsinan). In addition, Quaternary geologists and geomorphologists have repeatedly found that the orientations of the Carolina bays are consistent with the wind patterns which existed during the Wisconsinan glaciation as reconstructed from Pleistocene parabolic dunes, a time when the shape of the Carolina bays was being modified."
The second proposition -- that the bays were formed in connection with an extraterrestrial impact -- is the more exciting one. It has been around for a while in various forms (so far the earliest paper I've seen dates to 1933; here is a paper from 1975). Proponents of this idea point to the elliptical shape of the bays, their peculiar orientations and limited geographic distribution, and other characteristics that appear difficult to explain using the terrestrial model (why, for example, do similar features occur in Nebraska?).
This page proposes that
" . . . a catastrophic impact manifold deposited a blanket distal ejecta up to 10 meters deep in a set of butterfly arcs across the continental US. We have modeled the blanket as a ballistically deposited hydrous slurry of sand and ice originating from a cosmic impact into the Illinoisan ice sheet, and propose that Carolina bay landforms were created during the energetic deflation of steam inclusions at the time of ejecta emplacement."
In other words, an oblique comet strike on the continental ice sheet (this paper says the orientations of the bays suggest the impact site was located at Saginaw Bay, Michigan) and ejected into the air a massive load of sand and ice. That debris landed in an pair of arcs, one stretching across the Atlantic coastal plain and forming the Carolina bays.
You'll notice I have bolded the word "Illinoisan" in the quote above. That brings us to the next question.
When did the Carolina bays form?
Multiple lines of evidence suggest that, even if the Carolina bays were the result of an extraterrestrial event rather than terrestrial processes, they formed long before humans were present in eastern North America. The Illinoian stage of the Pleistocene referenced above dates to about 190-130 thousand years ago.
This paper by Mark Brooks et al. (2001) discusses stratified sequences of natural deposits in a Carolina bay that have been directly dated by radiocarbon to tens of thousands of years before the Younger Dryas (12,900 years ago) impact proposed by Firestone et al. (2007). That paper also details encroachment of a sand dune over and into a Carolina bay at around 48,000 years ago, indicating that the bay has to be older than 48,000 years.
Because many Carolina bays held water, they were attractive to both animals and humans in the regions they occurred. This 2010 paper (also with Mark Brooks as senior author) describes the presence of archaeological sites associated with Carolina bays near the Savannah River. Clovis artifacts are associated with the bays, which means that the bays could not have been the result of some event that "wiped out" the Clovis peoples: the bays were there before, during, and after the Early Paleoindian period.
I used "terrestrial or extraterrestrial" in the title of this post because I thought it would attract readers. While I'm curious about that question, however, it doesn't ultimately appear to have much bearing on the early prehistory of the Eastern Woodlands. The Carolina bays, however they were formed, predate the Paleoindian period by at least tens of thousands of years - there's a lot of positive evidence for that. Even if a comet strike at about 12,900 years ago did precipitate the Younger Dryas and cause environmental changes to which human societies would have had to adjust, that impact did not produce the Carolina bays.
No-one would have been around to experience the effects of a very ancient (e.g., Illinoian age) impact into the ice sheet. Maybe I should have titled this post "If a comet hits the ice but there's no-one around to see it, does it make a difference?" It does, of course, if it re-shaped the environment in some way that was significant to later peoples. But I don't think it is those kinds of effects that most extraterrestrial impact fans are excited about.
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