Popular Science Monthly/Volume 80/March 1912/A New Development in the Mississippi Delta
|A NEW DEVELOPMENT IN THE MISSISSIPPI DELTA|
By Professor E. W. HILGARD
UNIVERSITY OF CALIFORNIA
IN 1867 the writer was commissioned by the Smithsonian Institution to determine, if possible, the geological age and mode of formation of the rock salt deposit on Petite Anse Island, Louisiana. This involved, of course, a general examination of the coast formations of Louisiana, and among them, of the Passes of the Mississippi, and of the puzzling phenomena of "mudlump" upheaval in the Passes, which, at times, seriously obstructed commerce, but the origin of which remained a matter of conjecture. It had, to some extent, been investigated by Sir Charles Lyell (1858) and is commented upon in the tenth edition of his "Elements of Geology"; it was also conjecturally discussed by General A. A. Humphreys and other engineers connected with the Mississippi River Commission. My results, so far as the salt deposit is concerned, were published as Memoir No. 248 of the Smithsonian Institution; while the full report of my investigations of the Mississippi mouths and the mudlumps was published in the American Journal of Science in 1871-72.
As this work and its publication dates back so many years, and the latest publications on American geology and hydrography have wholly omitted any mention of it; and since a new phase of the subject has lately arisen confirmative of the views expressed and forecast made by me in 1872, it seems appropriate to recall that work to mind, and direct attention to the unfortunate fulfilment of a former prediction.
The Lower Mississippi Delta not a Normal One
The bird-foot shape of the lower Mississippi delta, with deep embayments in between, is unexampled in any other large river delta in the world. The bays between the delta-fingers ("Passes") are being very slowly shallowed, chiefly by wave and tidal action from the Gulf, carrying in the bar sands; and only subordinately by river overflow. The river in this lower delta region is for 50 miles below Fort Jackson bordered by narrow banks of unyielding gray clay, between which is carried the entire volume of the river through the narrow-banked "Neck," until it reaches a common point of divergence, the "Head of the Passes," whence similarly narrow-banked channels diverge, unbranched, in bird-foot form. (See the map accompanying this paper.)
The usual shape of a normal delta is a convex protrusion beyond the main shore-line, with usually slight protrusions at the mouths of the distributaries; as can be seen by an inspection of the maps of the deltas of any of the larger rivers, such as the Nile, Ganges, Brahmaputra, Danube, Volga, Lena and others. Within the delta-areas of these streams, large and small distributaries form a complex network, frequently changing at times of high water. No such changes are shown by the narrow-banked, diverging arms of the lower Mississippi delta, which steadily advance into the Gulf singly, and without any permanent distributaries being formed. The only approach to the form and structure of an ordinary delta occurs about three miles above the Head of the Passes, on the east side, where small and shallow channels connect with the main river through Cubit's Gap, a shallow lateral outlet.
Notwithstanding these facts, the Mississippi delta is figured in the latest edition of Chamberlin and Salisbury's extended work on geology, apparently as an example of a "normal" delta, and its formation is somewhat elaborately, but unconvincingly explained on the basis of the formation of the ordinary river deltas. The explanations do not, unfortunately, fit the facts as observable by any one examining the banks of the Mississippi Passes; nor is any mention whatever made of the existence and formation of the "mudlumps," which have proved of such vital importance to the commerce passing through the mouths of the Mississippi, that they have been called the "evil geniuses of the Passes."
Considering that these mudlumps have for many years been known to, and discussed by pilots, navigators and United States engineers, and have been somewhat elaborately treated of by Lyell many years ago, it is remarkable that their existence, and the part they have so obviously played in the régime of the Mississippi Passes, should have been wholly ignored by writers on general geology, and even in the standard work of Russell on the "Rivers of the United States."
As my detailed investigation of the subject, made in 1867, is not even mentioned among the references given by Chamberlin and Salisbury, it seems proper to recapitulate that investigation in print, especially since recent events in the delta seem to have strikingly confirmed my results.
The Port Hudson Clay ("Blue Delta Clay" of former Writers)
The investigations of Humphreys and Abbott had established the fact that the sands and silts of the true Mississippi delta, at least from Baton Rouge to the mouths, are underlaid at comparatively shallow depths by a stratum of blue clay, 10 to 20 feet in thickness, practically impervious to water, and almost inerodable by water alone. This is called the "blue delta clay" by Humphreys and Abbott; it will here be called the Port Hudson Clay, because it is entirely independent of the modern delta formation built up by the river.
In this stratum, when exposed in-shore or in shallow water, there frequently appear stumps of the deciduous cypress, suggesting that in former geological times a cypress swamp extended out gulfward, perhaps to the deep-water line at the edge of the continental shelf. Beneath this Port Hudson clay stratum lie formations materially different, and of such a character, both physical and biological, as clearly proves them to be not river alluvium, but of marine, brackish and paludal origin. But these formations, as well as the Port Hudson Clay, have nothing to do with the present problems of the delta, beyond serving as the floor on which it is built forward. The depth of the sands and silts of the true delta is practically from thirty to forty feet, and rarely reaches above sixty feet. That so great a river should show so small a depth of alluvium, when compared with such rivers as the Nile, Ganges, Hoangho and others, at first appears incredible; but it becomes intelligible when considered in connection with the existence of the underlying Port Hudson clay stratum, and the extraordinarily rapid extension of the mouth of the river towards the Gulf; the advance of the bar at the mouth of the Southwest Pass being, at the time mentioned, about 340 feet per annum. That this advance, however, is not made by the usual process of delta formation, is clearly shown on the accompanying map of the mouths of the Mississippi, and the "normal" delta of the Volga.
As is well known, a continental shelf, covered by a comparatively shallow depth of water, runs out for about thirty miles beyond the present mouths of the Mississippi Elver, then breaks off into the deep waters of the Gulf. The original surface stratum of this shelf is the Port Hudson clay (the "blue delta clay" of Humphreys and Abbott); but it is now coming to be gradually covered with the delta deposits of river sediment; and it would be natural to connect the shallow-lying shelf with the unusually rapid advance of the river mouths.
It is not easy to see at first sight why even the existence of the Port Hudson clay stratum should interfere with the ordinary, merely convex form of the delta protrusion; nor why there should he the sudden diversion of the several main distributaries from one permanent point, viz., the Head of the Passes, and a failure of the Passes themselves to give out numerous minor distributaries, as is the case in all other known deltas.
Moreover, the existence of the "Neck," a single, narrow-banked channel carrying the main river from below Forts Jackson and St. Philip to the Head of the Passes without breaking through the narrow embankment into Grand and Bird Island Bays, is precisely analogous to the fingers of the lower delta.
Even a cursory examination of the material causing this division and obstinately resisting the impact of the main current of the river at the Head of the Passes, shows that it is wholly distinct in character from the ordinary sandy and silty river sediment, and very different also from the Port Hudson clay; being a compact, impervious gray clay, and corresponding exactly to the material constituting the mudlumps. So long as it remains submerged or fully wetted, this clay resists erosion to a remarkable degree.
As we descend either of the Passes, an examination of their banks shows that these are formed of this same gray clay, and not of sandy or silty river deposit, which usually covers the clay only to the depth of a few feet. Hence, even a rise of the river does not wash out such lateral channels as Chamberlin and Salisbury speak of, connecting the river current with the adjacent bays. Even where such channels exist they have occasionally to be dug out by hunters or fishermen in order to reach the intervening bays, as they tend to fill up with the debris of mudlumps, and with river deposit. As we approach the mouths of the Passes, the banks are found to consist of small islets with small, shallow channels between, which, however, are also being rapidly filled in, progressively, partly by river deposit, but chiefly by disintegrated clay of the mudlump masses that have been raised above the water level. For when this clay of which the mudlump masses consist has been exposed to repeated partial drying and wetting, it crumbles into a loose mass, which is washed by rains into the shallow channels intervening between the mudlump islets and there settles into a mass very resistant to erosion.
Active Mudlump Cones
Still farther downstream we come, in all hut the South Pass, to mudlump islets obstructing the channel, historically known to have been upheaved from the river bottom, and frequently exhibiting low cones from the apices of which there is a steady flow of semi-liquid mud, accompanied by an occasional bubble of combustible gas. Remnants of such active mudlump cones have successively obstructed the Pass à l'Outre, its branchy the Northeast Pass, and the Southwest Pass. Rod soundings in active mudlump craters have reached a depth of 24 feet, but no solid bottom.
On measuring the proportion between the volume of gas and mud, I found the former to range from about one twenty-fifth to one thirtieth of the mud flow; the uniformity of which clearly indicates a steady pressure or vis-a-tergo. The mud flow, according to the universal testimony of river pilots, varies with the stages of the river, becoming much more lively at times of flood. Clearly, the gas is a wholly subordinate feature and not the cause of the mud flow; as has already been stated by Sir Charles Lyell. The latter attributed the ascent of the mud to the pressure of the sands and silts of the bar at the mouth of the Pass upon a mud stratum lying beneath them and under the bed of the river, the origin of which he, however, did not try to explain, but which is now to be considered.
Mud Layer Formed by Flocculation Beyond the Bar
My investigations of the peculiarities of colloidal clay, begun about 1869, led me to conjecture that the stratum of liquid mud was due to the precipitation of such clay from its diffusion in the turbid clay-water passing over the bar, by intermixture with the saline sea water; and that a layer of gelatinous, semi-fluid mud should, therefore, be found to seaward of the bar. That such is actually the case was proved by numerous reports received from pilots of sea-going vessels, who stated that at varying distances outside of the bar the sounding-lead begins to sink more slowly before it comes to a final stop on solid sea bottom, usually the Port Hudson clay. I could not obtain any definite estimate of the depth of the mudlayer, but the pilots said it might be from five to fifteen feet, according to the distance out from the bar.
I have, unfortunately, been unable to obtain an authentic sample of this mud from outside the bar. But of its existence there can be no doubt, and the huge scale upon which clay precipitation by flocculation occurs at mouths of all turbid streams emptying into the sea or saline basins, clearly shows that flocculation is certainly not the "limited and obscure" phenomenon that Chamberlin and Salisbury declare it to be ("Geology," Vol. I., p. 360). As the bar is built forward, the river sands and silts are spread on the mud stratum, so as to bury it under a broad cover. There would then tend to form on the surface of the mud stratum, by upward filtration, a thin but compact crust of tough clay constituting the bed of the river beneath the sands and silts which are continually shifting with the current. Under this crust, however, the semifluid mud-stratum would remain, and would also underlie the river channel inside of the bar, on both sides of the channel, and farther upstream to an unknown distance, in one connected layer; and would be subjected everywhere to the pressure of the marshes and overflow materials.
Mechanism of Mudlump Upheaval
Taking the existence of this semi-fluid mud-stratum resting upon the Port Hudson clay and buried by the delta sands and silts throughout the lower delta, for granted, the mechanism of mudlump upheaval at once suggests itself; for the rapid advance of the heavy and extended load of river sands and silts of the bar over the mud stratum would not permit the escape of the slow-moving mud to seaward.
A striking confirmation of the presumption that the pressure on the mudlayer is exerted by the accumulation of sediment and vegetation in the marshes, and of the existence of the mud-layer itself even under the older marsh formation, is the occurrence of a large mudlump-cone in full activity in 1867, in the marsh seven miles above the mouth of the Southwest Pass, and between it and West Bay. From the Pass, it appeared as a slightly irregular conical hill, which, judging from the extent to which it projected above the highest reeds, was about 18 to 20 feet in height. A glittering mud stream on the south slope could readily be distinguished by the field glass. The lump was inaccessible at the time of my visit, but had previously been fully described from a personal visit by pilot Ben Morgan.
Mudlumps commonly arise in a channel or pass immediately inside of the steep upstream slope of the bar, in or alongside of the main current, where the depth is greatest, and where the bottom therefore can most readily yield. Soundings show that (doubtless owing to the impact and consequent scooping action of the river current as it is forced to ascend to the crest of the bar) there is nearly always a maximum depth just at that point, in the course of the main channel. This would seem to mean that as the weight of the superincumbent river sands and silts is thus relieved, the pressure of the great area of marshes lying upstream, of the delta deposits on either side, and to some extent, perhaps, the pressure of the bar itself, causes the upheaval of the river bed and in many, though not in all, cases produces an extrusion of the semi-fluid mud which is but slowly washed away by the current. The vents are formed near the water surface at first, but are then built up from the outflowing mud, which partly consolidates by loss of water, until small mud volcanoes, rising usually from three to four, but sometimes as much as twelve feet above the water surface, are formed.
A comparison of the mud ejected with that forming the narrow banks of resistant clay bordering the Passes, as above outlined, and also the same resistant clay banks at the Head of the Passes which cause their divergence, shows the materials to be undistinguishable. The conclusion is inevitable that the entire bird-foot delta at least, and doubtless also the narrow Neck in which the main river flows below Forts Jackson and St. Philip, are the outcome of the formation and destruction of mudlumps as the river progressed; and that these clay banks constitute the normal mode of progression of the emerged portion of the lower delta.
The extraordinary resistance of the mudlump clay, when once consolidated in the channel banks, to erosion by water, serves, together with the somewhat similar characteristics of the underlying Port Hudson clay, to explain the exceptional form of the lower delta of the Mississippi.
I should add that a microscopic, physical and chemical examination of the mudlump mud, and of the clay from the banks of the Passes, bear precisely the biological characteristics to be expected under the conditions outlined. There is an intermixture of fresh-water and brackish marine organisms; while the water forming the mud is manifestly sea-water in a condition of considerable dilution, and changed by maceration with the organic débris brought down by the river. As a result of such reductive maceration, the sulphates in sea-water have been largely eliminated in the form of minute crystals of iron pyrites, and the lime as carbonate; while the ratios of the chlorids have suffered little change. The details of this investigation are set forth in my paper in the Journal of Science, already referred to.
Eads's Proposition to Open the South Pass
In the early 70's of the past century, even the widest of all, the Southwest Pass, had become so obstructed by mudlumps that deep-sea navigation was very difficult to maintain, despite the most active dredging on the part of the government, and the construction of tugs of enormous power, designed to pull deep-drawing vessels through the upheaved mud. Captain James Eads, the builder of the St. Louis bridge, then conceived the idea that, as the South Pass was unobstructed by mudlumps, it might be made the main and permanently navigable channel if sills of willow mattresses were placed across the entrance of the other distributaries (the Southwest Pass and Pass à l'Outre), and if jetties were constructed at its mouth to maintain a current so strong as to carry away the obstacles caused by river deposits of all kinds. Accordingly, a bill was introduced into Congress for the construction of these improvements. When this came to my knowledge, I wrote to Captain Eads calling his attention to my investigations of the Passes and of the origin of mudlumps, and suggesting that, so sure as the main current of the river was turned into the South Pass, mudlumps would necessarily arise; not at once, but within a period of probably twenty to thirty years. Captain Eads replied that, while he appreciated the force of my objection, even a surcease of twenty to thirty years would be of great service to American commerce, and that it would be unfortunate to bring the matter to public notice at that time. To this suggestion I agreed; and so soon as the bill passed Congress, the work was begun with his wonted energy by the distinguished engineer. He, unfortunately, died before its completion, but it was carried out according to his plans by his successors.
A Mudlump Appears in South Pass, Confirming Author's Theory
For about twenty-five years after the completion of the jetties there was little difficulty in keeping up the required channel depth of 26 feet, 200 feet wide in the South Pass and over the bar beyond the jetties. Within the last six or seven years, however, obstructions began to appear in the channel, which the dredges found great difficulty in removing and which, when removed, seemed to be promptly renewed at the original point. Within the last year it was definitely stated in the newspapers that the obstruction was a "mud bubble upheaved by the gas from the bottom of the river; the mud being so stiff that the dredges could not handle it, and a navigable channel had to be carried around it." I, thereupon, communicated with the engineers in charge of the South Pass navigation and promptly received, from Assistant Engineer C. Donovan, a blueprint of a map showing the soundings in the jetties, and beyond and across the bar; which, by this time, has become a pretty definite ridge, as in the case of the other Passes. This map shows that the current impinges directly against a mudlump mass which has arisen across its course, but has not yet reached the water surface; and a scrutiny of the soundings shows that a maximum depth exists, even now, right at the upstream slope of the newly risen lump. The highest portions of the mudlump are still about twenty feet below the water's surface; whether naturally, or kept so by dredging, I have not learned. No eruption of liquid mud has thus far been reported, although at such a depth it might easily have escaped observation.
Possible Forestalling of Farther Upheavals
This unfortunate verification of my prediction of the event would seem to confirm pretty definitely my theory of the origin of mudlumps in general. As this confirmation is of considerable interest for the future of deep-sea navigation to New Orleans, I think it desirable to call attention to it at the present time, in order that, if possible, measures may be taken to prevent a still farther obstruction from mudlump upheaval, by persistent cutting-away of the present lump as it rises; so as to relieve the pressure at the point already upheaved, and thus, perhaps, prevent the rising of additional obstructions by giving vent to the mud column at the initial point. How far this is practicable I will not venture to discuss. It is an engineering problem of no mean difficulty, considering that, in past experience, the utmost efforts of specially constructed dredges have failed to maintain a proper depth for more than a few hours or days, where mudlump upheavals had occurred in the Southwest Pass. In the South Pass there has thus far been only a single upheaval to deal with, whereas in the Southwest Pass a succession of these upheavals rendered the maintenance of the main channel extremely difficult. When we consider that in many cases the shock of the grounding of a vessel on a mudlump was sufficient to cause a quick upward movement (in one instance lifting the bow of the vessel above the water over-night), showing a state of very unstable equilibrium, it is not at all inconceivable that in a comparatively narrow channel, as is the South Pass at the present time, diligent and effective dredging might serve to cause the upheaving force to continue to spend itself on the one point where it has now acted, and so to prevent, or at least retard, the formation of new upheavals.
It is to be noted that at the present time the dredges maintain, in the seaward channel, a depth of as much as thirty-seven feet. "Whether this depth, apparently excessive for the present requirements of navigation, will prove an incitement to new upheavals of the bottom is, perhaps, a question worth considering. In any case, the mudlump now lying across the mouth of the main channel of the Mississippi has doubtless come to stay, and no amount of dredging will suffice to do away with it.
- ↑ In view of the many phenomena indicating that the present course of the Mississippi River is comparatively young, and that in times not far remote its waters flowed toward the Arctic Ocean, as contended by Professor G. H. Tight, such a condition of things would simply indicate a temporary cessation of an oscillation which, taking into consideration the deep submerged channels of western Louisiana and the present elevation of the Loess hills of Mississippi and Louisiana above the level of the river, as discussed by me (Am. Journ. Sc, Vol. 48, November, 1869, p. 335) would amount to more than 800 feet.