Popular Science Monthly/Volume 31/August 1887/The Falls of the Mississippi
|THE FALLS OF THE MISSISSIPPI.|
THE Mississippi River and its tributaries, forming as they do one of the most important river systems on the globe, and draining one of the most richly-furnished continental areas, present, moreover, many interesting geological studies, and open up fields of curious inquiry to the investigator. The old discussions as to the possibility or impossibility of things has, for the most part, passed out of existence in this department of science. No one now denies the general principles of geology as at present taught; therefore new regions of investigation are to be approached on the firm foundation of the old, and difficult matters settled in conformity with established principles. That there is no new thing under the sun is a saying well worn, but in one sense correct, yet the same thing recognized as a fact in one situation may under other circumstances seem a fallacy. The Falls of Niagara are familiar to all, and came to exist through causes natural and easy of explanation, inasmuch as the whole secret lies in the character of the formations over which the river flows, viz., a crust made up of from sixty to one hundred feet of comparatively hard limestone lying in a nearly horizontal position, beneath which is a deep deposit of shales and sandstones. Whenever the river in wearing its channel back reached the point where this arrangement of rocks began, the hard limestone would naturally resist the erosive action of the waters, while the underlying shales and sandstones, offering less resistance, would be rapidly cut away, until a vertical fall such as is now seen would be the result, with a constant recession going on, leaving below the broad canon, walled on either hand by bluffs, the crests of which are preserved by the limestone crowning them.
These few reflections as to the falls and gorge of Niagara, fully demonstrated by forces now in active operation, we shall apply to the Mississippi. Here also a mighty water-way has been cut out by erosion, a fact which is universally conceded, but no definite explanation of the process has heretofore, so far as we have been able to learn, been advanced. It remained for a geology-reading inventor by the name of Robert Bates to suggest a theory which, illuminated with what little investigation we have been able to give it, promises to offer a solution of the question, or to assist in its solution. The theory briefly is, that the erosion was accomplished by means of a mighty cataract which began far down the river near its original mouth, and by gradual retrocession dug out the valley-like gorge which is so marked a feature in the upper part of its course, and left the high bluff walls on either hand, at the same time depositing heavy beds of sand at the bottom of the canon, the product of the erosion above, and that St. Anthony Falls are the ever decreasing and receding remnants of the once most stupendous cataract the world ever saw, having a perpendicular descent of perhaps six hundred feet.
Stretching over almost the entire Mississippi Valley immediately overlying the Azoic rocks lie the old and extensive beds of the Potsdam sandstone, a formation of great thickness composed of shales and friable sandstones.
From the Wisconsin River to the Falls of St. Anthony the formations through which the Mississippi has cut its way are—first, the St. Lawrence, or, as Owen has termed it, lower magnesian limestone, very analogous to the Niagara formation in density and durability.
This stratum is from one hundred and fifty to two hundred and twenty-five feet in thickness, and lies in a nearly horizontal position, dipping somewhat to the west, but not to any great extent between the Wisconsin River and St. Anthony Falls. Second, underlying this and forming a part of the Potsdam group is the St. Croix sandstone, with perhaps other sand-rock and shales local in extent. A section of the bluff in Houston County, Minnesota, gives the St. Lawrence crowning it a thickness of nearly two hundred feet, with three hundred and twenty feet of sand-rock and shale beneath.
Another measurement of a bluff in the town of Richmond, Winona County, indicates a little over one hundred and ten feet of limestone overlying four hundred feet of St. Croix sandstone. Other measurements have been made in different localities, but without doubt these already given indicate the general positions and relative thickness of the different strata. The conditions, it will here be observed, are similar to those existing at Niagara, viz., a hard limestone superimposing a soft sandstone and shale deposit.
These bluff walls rise on either hand to a height of from three hundred to five hundred and fifty feet above the water-level of the river, and have been laterally furrowed and eroded by streams flowing from the adjacent country, and in a much greater degree by glacial action, as it is a well-established fact that the river had cut its way (except for a short distance below St. Anthony) prior to the glacial epoch. On account of this lateral furrowing the bluff walls present a broken and serrated appearance, but this, when rightly considered, does not in the least militate against the correctness of the cataract theory. The valley gorge, which is from two to six miles, is at present somewhat wider at the top where the cliffs appear, and where wind and frost have been free to act, than the water originally cut it, and the débris falling below has formed a talus which, increased and modified by glacial action, has to a considerable extent effaced the wall-like appearance which is such a marked feature of the comparatively freshly-cut canon of Niagara.
Another fact which has hitherto received no satisfactory explanation is the deep accumulation of sand in the valley-bottom. There are no data sufficient to determine the depth of this deposit, but as the great river nowhere flows upon a rock-bed, but everywhere, except in its extreme northern section, has a sand bottom; and, as cities and villages are built within the bluffs on the compact accumulations at the sides of the present channel, we conclude that it must have a depth of several hundred feet.
If the gorge had been chiseled out by a process of gradual wear (which would have been the case if the strata had been of uniform resisting power), then the river should flow upon a rock-bed, and not upon sand, for the latter would, as it now does, protect the underlying strata from all wear. Upon the cataract hypothesis this peculiar condition can be met with an easy and satisfactory explanation. The descent of the Mississippi is very gradual. Directly at the base of the falls, wherever they may have originated, the sand-rock would be cut down to a depth determined by the comparative hardness of the rocks and the volume of water. For a short distance below the descent the rock would be swept clean of sand and débris, except, perhaps, the large limestone chunks fallen from above, but as the stream came to flow more evenly below the falls, sand from the erosion above would be deposited at the bottom. Two other existing conditions would assist in the deposit: first, the very slight fall of the river; and, second, the detached masses of limestone broken from the crest of the falls would help to collect and retain the sediment, and thus the accumulation would begin. All these facts, taken in connection with the further fact that no bluffs appear above St. Anthony Falls, while they are continuous below, except where broken by the lateral erosion, have a tendency to establish the theory advanced. Just how far to the south the requisite conditions of stratification exist, we have not as yet ascertained, but they probably exist wherever the limestone-capped bluffs bound the river. If the limestone formation is wanting at any place, there rapids would have taken the place of falls, and so continued until the limestone again appeared in its natural position.
There are no rock-bluffs below the confluence of the Ohio with the Mississippi, but about thirty miles above that point they abruptly rise on both sides of the river at what is called the Grand Chain, a high rock ridge traversing the region from east to west, and which lies at a considerable height above the sea-level. There, in all probability, the river had its original outlet, and there, without doubt, the cataract process began, although the character of the stratification is unknown to us. At the time the erosion was in progress, there must have been several times the present volume of water flowing in the river, spreading out wider than the bluff, even, as an occasional terrace shows. The same perpendicular rock-walls and sand-bottom characterize the Wisconsin River for a distance of eighty miles from its junction with the Mississippi; the position and character of the stratification being much the same, indicating that the same agency was active in the erosion of both channels.
St. Anthony Falls have been studied by Professor Winchell, and he has arrived at the conclusion from various data that the falls have worked their way back from a point near Fort Snelling to where they now are, a distance of about eight miles, since the glacial period, and he estimates that it has taken about five thousand years to accomplish the work, which estimate, taken as a basis of calculation, and allowing a margin of at least one third, gives us the time necessary to cut out the entire channel at, say, four hundred thousand years; and this, if, as we maintain, the cataract process wrought the gorge not only from Fort Snelling northward, but from far down the river, at or near the ancient outlet, is without doubt a conservative estimate of the time actually required.
We have thus briefly outlined the new theory of the Mississippi erosion, and this is written with the expectation that the barbed arrows of scientific criticism will be aimed toward it; but, if so, it will aid in the solution of a question hitherto little studied. Many things point toward the theory here advanced as being at least quite possible, and even probable. Its final solution will, however, necessitate painstaking investigation, with the attention directed to that especial object.