At the close of the first Great Time--called Archæan--the continent south of the region of the great lakes, excepting a few islands, was still submerged beneath a shallow sea, and therefore no portion of the Mississippi was yet in existence. At the close of the second great geological Time--the Palæozoic--the American continent had emerged sufficiently from the ocean bed to permit the flow of the Ohio, and of the Mississippi, above the mouth of the former river, although they were not yet united.
Throughout the third great geological Time--the Mesozoic--these rivers grew in importance, and the lowest portions of the Missouri began to form a tributary of some size. Still the Ohio had not united with the Mississippi, and both of these rivers emptied into an arm of the Mexican Gulf, which then reached to a short distance above what is now their junction.
In point of time, the Ohio is probably older than the Mississippi, but the latter river grew and eventually absorbed the Ohio as a tributary.
In the early part of the fourth great geological Time--the Cenozoic--nearly the whole continent was above water. Still the Gulf of Mexico covered a considerable portion of the extreme Southern States, and one of its bays extended as far north as the mouth of the Ohio, which had not yet become a tributary of the Mississippi. The Missouri throughout its entire length was at this time a flowing river.
I told you that the earth's crust had been worked over to a depth of many miles since geological time first commenced. Subsequently, I have referred to the growth of the continent in different geological periods. All of our continents are being gradually worn down by the action of rains, rills, rivulets, and rivers, and being deposited along the sea margins, just as the Mississippi is gradually stretching out into the Gulf, by the deposition of the muds of the delta. This encroachment on the Gulf of Mexico may continue, yea, doubtless will, until that deep body of water shall have been filled up by the remains of the continent, borne down by the rivers; for the Mississippi alone carries annually 268 cubic miles of mud into the Gulf, according to Humphreys and Abbot. This represents the valley of the Mississippi losing one foot off its whole surface in 6,000 years. And were this to continue without any elevation of the land, the continent would all be buried beneath the sea in a period of about four and a half million years. But though this wasting is going on, the continent will not disappear, for the relative positions of the land and water are constantly changing; in some cases the land is undergoing elevation, in others, subsidence. Prof. Hilgard has succeeded in measuring known changes of level, in the lower Mississippi Valley, and records the continent as having been at least 450 feet higher than at present (and if we take the coast survey soundings, it seems as if we might substitute 3,000 feet as the elevation), and subsequently at more than 450 feet lower, and then the change back to the present elevation.
Let us now study the history of the great river in the last days of the Cenozoic Time, and early days of the fifth and last great Geological Time, in which we are now living--the Quaternary, or Age of Man--an epoch which I have called the "Great River Age."
It is to the condition of the Mississippi during this period and its subsequent changes to its present form that I wish particularly to call your attention. During the Great River age we know that the eastern coast of the continent stood at least 1,200 feet higher than at present. The region of the Lower Mississippi was also many hundred feet higher above the sea level than now. Although we have not the figures for knowing the exact elevation of the Upper Mississippi, yet we have the data for knowing that it was very much higher than at the present day.
The Lower Mississippi, from the Gulf to the mouth of the Ohio River, was of enormous size flowing through a valley with an average width of about fifty miles, though varying from about twenty-five to seventy miles.
In magnitude, we can have some idea, when we observe the size of the lower three or four hundred miles of the Amazon River, which has a width of about fifty miles. But its depth was great, for the waters not only filled a channel now buried to a depth of from three to five hundred feet, but stood at an elevation much higher than the broad bottom lands which now constitute those fertile alluvial flats of the Mississippi Valley, so liable to be overflowed.
From the western side, our great river received three principal tributaries--the Red River of the South, the Washita, and the Arkansas, each flowing in valleys from two to ten miles in width, but now represented only by the depauperated streams meandering from side to side, over the flat bottom lands, generally bounded by bluffs.