|It happens that 20,000 years
ago the world was in the midst of an ice age. How do we know this? It's because for the
last 150 years naturalists and geologists have gathered enormous amounts of information
leading to this insight.
For example, as proof that there was an ice age, today scientists point to great lakes (including the Great Lakes) apparently gouged out by one- to two-mile thick, unimaginably heavy, slow-moving (inches a year) sheets of ice called glaciers. They can show us where these glaciers have pushed up and left hills of particular shapes, and scraped across immense territories where they left nothing but naked bedrock, and on this solid bedrock there remain today deep scars leading in the direction of the glacier's passage. At the peak of the ice age, glaciers covered 20% or more of the earth's surface. There are a number of fine web sites dealing with glaciers, including the one at www.nsidc.org/glaciers/. Also, there are several sites, such as the one sponsored by the Illinois State Museum, providing a lot of detailed information about the ice ages in general.
As further proof of what happened during the ice ages, today specialists can take us high into certain mountain valleys and into the polar regions where glaciers still are present -- where the last ice age in a sense continues. Some few of today's remaining glaciers still do a little gouging, pushing, and scouring, and even today they leave evidences on the earth's face exactly like those left by the earlier ice-age glaciers.
"Melt" is too tame a word for what happened when the ice ages ended and the glaciers began converting to flowing water. In many places the glacier to the north of our area was a sheet of ice over a mile thick, in a few places maybe even two miles thick, and at that time ice covered about a quarter of the earth's face. The southernmost boundary of the glacier to the north of us ran just to the north of today's Ohio River. Therefore, that glacier did not extend into the Lower Mississippi Valley. It did, however, profoundly affect our weather and landscape.
That doesn't mean that our entire area was tundra-like during the Ice Ages. Studies of fossilized pollen indicate that around 20,000 years ago that part of the Lower Mississippi Valley north of about Grenada Lake in northcentral Mississippi was occupied by boreal forest, consisting largely of pines, spruces, firs, birches and willows. For about 80 miles south of Grenada Lake a transition zone occurred leading from boreal forest to evergreen forests similar to today's piney woods. Moreover, average seasonal temperatures here were not tremendously different from what we have today. A detailed account of this strange environment is detailed in the 1999 work Paleoclimates, Paleovegetation, and Paleofloras of North America North of Mexico During the Late Quaternary, by Paul and Hazel Delcourt, which can be accessed online at the Flora of North America Project. The "Late Quaternary" covers our "20,000 years ago."
So, about 20,000 years ago, the glacier north of us began converting an outrageous amount of ice into flowing, charging, exploding, sea-like rivers of meltwater which rampaged toward us, gushing down an immense, wild-looking wasteland-channel that one day would tame and become known as "the lazy Mississippi."
When such an unthinkable quantity of water as we're speaking of roars across the landscape, it doesn't take the time to develop scenic meanders, or even to scour for itself comfortably deep channels. Nor does it satisfy itself with carrying mere silt. It scours the landscape like a tidal wave, carrying away and shifting around sand, gravel, even boulders, and anything else that gets into its way.
When such a flood subsides, its remaining waters do not recede into simple, river-like streams. The waters subside and flow through an interconnecting maze of channels, some stream-like, others river-like, none very deep, and between the channels lie islands, some tiny and some large. Such interconnecting channels are referred to technically as braided streams. You can see pictures of some braided streams here.
Braided streams today are mostly found below melting glaciers, and upon alluvial fans at the base of mountains which issue large amounts of meltwater when winter's snows melt during the spring. In the 21st century no braided streams appear anywhere in the lower Mississippi Valley, but you can still fly over the San Gabriel Mountains north of Cucamonga, California and see nice ones.
So, 20,000 years ago the lower Mississippi Valley was occupied by a tremendous braided stream network. According to H.N. Fisk, one of the Grand Old Men of lower Mississippi Valley geology, the Mississippi River remained braided until about 5,000 years ago, when at last North America's continental glacier had melted, and the sea level had more or less risen to today's level.
For, during ice ages, sea levels drop. In fact, 20,000 years ago, so much of the earth's water was invested in the glacier to the north that the sea level had dropped 350 to 400 feet lower than at present. The old familiar shapes of the Earth's continental coastlines were much different than they are now.
There's one more thing to say about braided streams, something very important for us, and that is that dust sometimes rises from the dried-out mud coating the sand and gravel bars rising between the streams in the braided network...
During the 1930s a geologist by the name of Ralph Tuck studied glaciers in Alaska's Matanuska Valley, just northeast of Anchorage. Conditions there were similar to those that may have existed during the time of loess deposition in the lower Mississippi Valley. Glaciers stood twenty to forty-five miles up the valley from Tuck's observation point, and they issued floods of meltwater into braided streams. The meltwater in those streams was milky-white with silt. This silt resulted from glaciers pulverizing the rocks they moved across. When the milky meltwater pooled on sand and gravel bars during periods of low outflow, mud flats occurred as water left silt behind. This mud dried, and dust rose from the dried-out mud when the wind stirred...
In fact, Tuck described a pall of dust that during dry weather obscured the skies over the town of Palmer at the mouth of the valley. Surveyors' stakes driven into the ground in 1913 were found smothered by several inches of dust in 1935.
Here's the point: "Several inches" of accumulated dust over a period of twenty-two years may not sound like much. However, at our C14 and... snails link we see that in our area loess was deposited during a long period between about 25,000 and 18,000 years ago -- some 7,000 years.
Therefore, even if only one inch of loess-dust is deposited a year, then in 7,000 years there can be 7,000 inches of loess. That's 583 feet. If you remember that during the last 18,000 years there must have been a great deal of erosion of loess, even from spots where loess is thickest now, then 583 feet sounds like a comfortable guess for how much loess was originally deposited, for nowadays our deepest deposits range from about 100 to 200 feet in depth.
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