Ever since 8th grade geography, when I was looking at a map showing forest coverages in North America, I�ve always wondered about a curious and very large-scale formation in the American Southeast (~280 miles long). It�s a very clear crescent, almost a perfect quarter circle, over northern Mississippi and Alabama. It�s an area that stands for its relative lack of trees, as compared to the surrounding landscape.

It�s been decades since 8th grade and I�ve never heard any kind of mention or reference to this part of the country. I have a good knowledge of geophysics, and I know that there is no tectonic history in the area. Still this crescent remained a mystery to me.

So the other day as I was transitioning from Denali to Miami Beach, I saw that big crescent fly by, a taunting grin. I had to figure out what that was.

Using the Keyhole elevation readout, the formation didn�t seem to have too much of a contour in the landscape � maybe a slight dip across, and the west side was a little bit lower than the east. But the whole thing increased in altitude as it extended to the north, along with the surrounding landscape. What caused this?

So I Google searched �geology crescent Alabama Mississippi� and discovered that the area was called the Black Belt, or Black Belt Prairie. Due to the presence of chalk underground that makes the soil high in clay, it is poor for tree growth and so is naturally grassland. That explains why it looks different. By why is the chalk there?

A little further Googling and I learned about the Mississippi Embayment. In the Pleistocene era, the Gulf of Mexico extended much further inland. This crescent is the site of an ancient shoreline. [see overlay]

Apparently, this coastline had the right conditions for chalk deposits. But Googling �origin of chalk�, I learn that �chalk is composed of the skeletal remains of the bodies of minute marine animals, that can only accumulate uncontimated (sic?) by muds and other sediments in the deep ocean, away from land.� Huh? A shoreline is anything but �deep ocean, away from land.� Furthermore, �Earth movements caused � these deposits to experience extremely high temperatures and pressures, forming chalk.�

Again, how did the chalk come to be here? So far I haven�t found the answer on the internets, but I�ve found references to books that might say (not available on-line). As they say, �Questions remain.�

I have my own theory: If the chalk couldn�t have formed at that location, maybe it was deposited there after eroding from elsewhere. If you look at the overlay, you see all the rivers come from the west or the north (the Mississippi), and our crescent is an expanse on the opposite shore unbroken by rivers.. I�m imagining that these rivers brought down a lot of sediment from the Rocky Mountains, possibly including large deposits of chalk formed in an earlier epoch. From these rivers, sediments are deposited relatively undisturbed on the opposite shore. The weakness of this theory is that I don�t think there�s much evidence that any part of in the Rockies were once undersea. Hmmm.

Enjoy of my jabberin�. Anyone got any idea how the chalk got there?

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I saw that big crescent fly by, a taunting grin

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I've seen that too, but never stopped. I'm glad you did. I love this kind of stuff - keep it up.
My understanding is that chalk is mostly made up of foraminifera tests (shell-like structures). The forams live at shallow depth, up where the sunlight is. When they die, their tests fall to the sea bottom - it doesn't have to be tremendously deep, just away from strong currents. The calcareous ooze eventually gets compressed and forms chalk. The famous White Cliffs of Dover in England are chalk that has been uplifted. Here are some foram sites.

Photomicrograph of foraminiferae

Ain't they cute!! They must all be mighty upset that white boards have come along to put them out of work. :-)

The biggest mystery to me was where did the pressure and heat come from that allegedly is necessary to turn carbonate into chalk. I searched a bit more after that post and most other reference don't mention pressure and heat. So... that's that.

thanks for your responses

Chalk is sedimentary and a kind of limestone. Being sedimentary, it does not require heat to form . Pressure alone suffices to lithify previous unconsolidated sediments once they have built up on the bottom. As a matter of fact, cold bottom temperature and pressure dissolve the tests before they reach the bottom, thereby not allowing chalk to form. Another organism that adds material to the chalk is called a coccolithophore. More:

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Conditions of chalk formation
The micro-organisms that make chalk were pelagic; that is, they lived within the sea's water column, rather than on the sea bed.
On death, their remains rained down to the sea floor. If this was neither too deep nor too cold, the calcite remains collected there, where they formed a 'calcareous ooze' - a water-laden sea-floor deposit of the calcitic skeletons of pelagic organisms.
(Below a certain depth and temperature, calcite dissolves into the water so does not reach the sea floor. However, given the high temperatures and the shallow seas of the Cretaceous Period, vast amounts of calcite did accumulate on the sea floors during that time .)
The planktonic foraminifera skeletons are larger that coccoliths, but still microscopic - between 25 - 100 �m. Mixed together with the tiny coccoliths, the result is a very fine-grained, but poorly-sorted sediment. With settling, compaction and lithification, this poorly sorted sediment of microscopic remains leads to chalk - a very fine-grained, but poorly-sorted rock.
The poor sorting indicates calm conditions of deposition, as turbulence and varying energy conditions are the prime agent of sorting. The chalk is in fact so fine-grained it was once believed to be amorphous, but is now known to be cryptocrystalline.
The fine grains, and disc-like shape of the coccoliths, give the rock an exceptionally large surface area per unit volume, which makes it relatively chemically reactive. It is therefore a particularly useful form of carbonate in manufacturing )

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The quote is from this web page.

Very nice overlay and explanation. We see these features on the earth, but rarely do we find out why we see what we see.

Since we are on the subject of Chalk formations, I've attached an overlay Geological map of the famous Dover Chalk in England from nationmaster

The pic below is of the eroding chalk formation named The Needles off the Isle of Wight. If you look carefully you can see the chalk layers dipping off to the north. If you look at the geological map you would also infer that the chalk is tilted based on the narrow band of chalk formation shown in the Isle of Wight,



At the end of the Needles stands a Lighthouse Lighthouse

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In 1859 Trinity House planned a new lighthouse to be built on the outermost of the chalk rocks near sea level. It was designed by James Walker and cost �20,000. The circular granite tower has perpendicular sides and is 33.25mm high, of uniform diameter with an unevenly stepped base to break the waves mxd discourage sea sweeping up the tower. The wall varies from 1.07m in thickness at the entrance to 0.61m at the top. Much of the base rock was cut away to form the foundation, and cellars and storehouses were excavated in the chalk.

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By the way, did you know that blackboard chalk is actually Gypsum.

I am very familiar with the Black Belt in Alabama since I lived on the edge of it. I have seen alot of this chauk around the rivers such as the Alabama and Cahaba Rivers where the water cuts through the soft rock. I lived on the north edge I lived in a town called Alabaster. It was named of the limestone outcroppings that literally jutted out of the flat ground. This limestone was an alabaster color and was a very pure form that was mined by various local miners that processed it for agricultural lime and for water treatment. To further confuse your post but all of the upper 2/3 of Alabama is limestone with it getting softer as you approach Montgomery, Selma, Dempolis and Phoenix City when it turns to chauk. I do not agree that the soil is not good for trees because you can go to southeast Alabama where it is south of the Black Belt and you will find prairie grass. The geology in south Alabama is mostly sandrock and chert with limestone very deep. One other point but the limestone areas contain a very good tasting water in the aquafer. Richard Allison

This could easily have been a shoreline feature. Check out this <A Href="http://www.cretaceousfossils.com/">link</a>, you will see a shoreline did run through the area in the cretaceous period.

A very interesting thread.
At one time or another, just about all areas of the Earth that are now dry land have been under the sea. The Rockies possibly formed after the area uplifted, so I cannot speak for them. Marine fossils are found in all areas of the world. The chalk could very well have formed in the normal way right were it is.