One important way that the geology of the Mississippi Embayment is different from that of most of the rest of North America is that most, but not all, of the material exposed at the landscape's surface is so young, geologically speaking, that it hasn't had time to consolidate into truly hard rock, such as limestone and sandstone. You can indeed find hard-rock outcroppings in our area, but, relative to classic limestone and sandstone, most of our rocks are soft and crumbly. I emphasize most because here and there in our area relatively recently deposited strata have developed very hard rocks indeed where local chemistry accomplished in a brief period what compaction from overlying strata didn't have time to do.
Let's imagine that we're digging a hole into, through, and out the bottom of the deep loess at Vicksburg, Mississippi. We're choosing Vicksburg because the geology there has been relatively well studied.
At Vicksburg, if you stood atop one of the higher spots on the big loess-capped bluff there, and you began digging straight down, here's what you'd find:
During the first 120 feet, you'd pass through loess. Then you'd dig your way through about forty feet of gravel, sand, and silt of the Citronelle Formation, which was deposited approximately 1.5 to 3.7 million years ago. At a depth of about 160 feet, you'd run into a chocolate-brown, gummy, partially hard clay known as the Bucatunna Formation estimated to be about 30 million years old.
Below the gummy, chocolate-brown Bucatunna, about 190 feet deep, there's the Byram Formation, with its interbedded layers of sand, shell, clay, and nodular limestone. The Byram is world-famous for its variety and quality of fossil mollusks (they pretty much look like bleached modern sea shells). Note that with the nodular limestone for the first time we're hitting real rock. Below the Byram, at about 220 feet deep, there's the Glendon Formation with its limestone (real rock indeed!) and thin, interbedded marls, and this formation is a few million years older than the last., and then we enter the Mint Springs Formation. In the Byram, Glendon and Mint Springs Formations you can find fossils of sharks, rays, bony fish, and dugongs (similar to manatees), but no whales.
And on and on we can go, through the Forest Hill Formation, and the Yazoo Clay Formation at about 350 feet, in which you can find fossils of whales and sharks, then right below the Yazoo Clay there's the Moodys Branch Formation, which also has its whales and sharks and, like the above Byram is world famous for its fossil mollusks. Note that each formation we are passing through is a bit different from the last, each has its own unique history, and, always, the deeper we dig, the longer ago it was deposited -- the older the material becomes.
The formations between the Citronelle and the Yazoo Clay Formations are known to geologists as Oligocene deposits. During the very early Oligocene -- when the deepest of the formations we've been digging through was deposited -- central Mississippi was occupied by a landscape characterized by large rivers and swamps. Sometimes large trees fell into the rivers, clogging them up. Then the rivers would cut new channels around the logjam. The old, clogged channels would then fill with sand, mud, and other material. Then minerals would seep into the fallen tree trunks, and over long periods of time those minerals would seep into the logs and become like stone -- would petrify them. The minerals, particularly silica, did not replace the woody tissue. They simply filled the empty cells in the wood.
Today you can see what remains of one such ancient logjam at the Mississippi Petrified Forest near Flora, about twenty miles northwest of downtown Jackson, on US 49. When you view the petrified logs at Flora, you are looking at relics from a time in Earth's history when mammals were undergoing an explosive period of evolution. An early native-North-American horse known as Mesohippus appeared, standing not quite three feet tall and each of its legs ending in three little hooves instead of one single large hoof. Mesohippus's descendants would one day be our modern horses. Rodents and primates were just evolving, and of course one of the branches of primate evolution would eventually lead to humans.
The petrified wood shown at the right is typical of the kind found embedded in the sand and gravel of bayou bottoms in much of Mississippi. This was found on Laurel Hill Plantation near Natchez. Notice that the wood is curiously bent. This is often the case with such petrified wood found in this area. The twisted trunks make sense when you visualize the trees growing in the braided stream network we've spoken of, where floodwater often flowed over bushes and trees growing on sandbars and islands. Today, Black Willows along streams often are similarly twisted, and likely to yield and bend when floodwater flows of them. However, this is not Black Willow wood. Probably it is an extinct species.
Before the Early Oligocene swamp, there was the warm, tropical sea in which our Moodys Branch Formation whales and sharks swam. And, of course, we can even dig below the Moody Branch...
Now we pass through many formations, each with its characteristic layers of stone or sand or clay or whatever, and each older than the last. At about 10,000 feet we find ourselves chipping through rocks born of sedimentation occurring in the bottom a warm, tropical sea occupying the area in the neighborhood of 120 to 130 million years ago. These rocks are the geologists' "Lower Cretaceous," deposited when dinosaurs dominated earth's land surface, and flowering plants were just beginning.
At maybe 14,000 feet deep -- over two and a half miles -- we burrow through rocks around 245 millions years old, rocks constituted of minerals deposited before the dinosaurs. Eventually there are rocks billions of years old, not millions, and they're not deposited by wind or water, but rather have remained deep inside the Earth ever since the Earth came into existence as a planet some 4.7 billion years ago.