0 when you purchase all four
volumes! $62 rather than $72!
To man, a hundred years is a long time; however, that perspective changes when one
realizes that the Upper Mississippi River as we know it has been in existence for 10,000
years. Knowledge of the past can help us to better understand and appreciate the present
day Mississippi River.
This is part of an ongoing series about the plants,
fish, wildlife and issues pertaining to the Mississippi River. For questions or comments,
write to: DNR Office, La Crosse, WI 54601
According to some scholars, catastrophic walls of ice broke off from the
glaciers and joined a massive run-off of melt-water, scouring out the contours of the
Mississippi River. It is envisioned that a sudden collapse of the NA ice cap produced a
massive sea-level rise with the speed of a tidal wave around the world. It is likely that
the river valley in the upper Mississippi was once 500 feet deeper than it is now, filled
as it is with gravel and sand deposited by that melt-water. The accumulated Gulf of Mexico
organisms have provided compelling evidence of a vast flood of fresh water about 11,600
Mark recently sent to us the scholarly results of his investigation into the formation
of the Mississippi River Valley: "The present lower Mississippi River Valley is underlain by an aulacogen, that is,
a rift that developed perpendicular to the larger Gulf of Mexico rift. Occasionally
earthquakes reflect continuing movement along its marginal faults, including the most
severe one in United States hiistory, at New Madrid, Missouri, in 1811-1812" (Dott
and Batten, 1988)..
This triple juction (triple junctions form at the location where supercontinents break
up forming aulacogens) may have formed sometime during the Jurrassic Period as North and
South America began separating (Dott and Batten, 1988). According to Liang (1991), major
rivers are often located at the site of a failed arm of a triple junction, with massive
deposits of sedimentary rock filling the linear basin along the trend. Therefore the lower
Mississippi River Valley was probably formed during the Jurassic period, some 208 million
years ago, as a failed arm of a triple junction.
According to Ojakangas and Matsch (1982) large glacial lakes were formed as the
glaciers retreated--the largest of the glacial lakes, Lake Agassiz, was formed as the Des
Moines Lobe of the glacier retreated some 12,000 years ago and the drainage of this
approximately 320,000 square kilometer lake may be responsible for the formation of the
Upper Mississippi River Valley--the rivers were responsible for draining the large amounts
of water from the melting glaciers. Ojakangas and Matsch indicate that Lake Agassiz
probably drained via the Minnesota River into the present day Mississippi River (1982).
more information: Dott, H.R., R.L. Batten. 1988. _Evolution of the Earth. Fourth Edition._
New York. McGraw-Hill Book Company. p463, 555. Laing, D. 1991. _The Earth System. An
Introduction to Earth Science._ Dubuque, IA. Wm. C. Brown Publishers. p. 134. Ojakangas,
R.W., C.L. Matsch. 1982. _Minnesota's Geology._ Minnesota. University of Minnesota Press.
Another Letter from our expert Readers!
I enjoy your web page. Information provided by Mark
suggesting the Lower Mississippi is millions of years old is a bit misleading.
Until relatively recently (geologically speaking), most of the Mississippi basin
drained northward toward the arctic. The modern Mississippi valley formed during
the ice ages, however subsidence in the embayment area was probably more
important than any role played by ice. Some of drainage shifts that accompanied
this crustal tilting are described by J.S. Aber at this web site: http://www.emporia.edu/earthsci/chert/chert.htm
Drainage patterns feeding sediment into the Gulf of Mexico have evolved
during the ice ages, changing from a pattern of multiple moderate size rivers
(Rio Grande, Sabine/Red, Mississippi, southern Appalachian) to a system
dominated by the modern Mississippi. There is a map which illustrates the
process published in this article: Prather, B. E