Geology

Sidell

ON CERTAIN GEOLOGICAL PHENOMENA OF THE MISSISSIPPI DELTA

1861

From Corps of Topographic Engineers, U.S.A., Professional Paper 4, Appendix A, Washington, 1861.

The observations taken to ascertain changes of the shores, etc., during the progress of the work [survey of the mouths of the Mississippi], could not be very extensive, but, by inquiry and observation, much information was elicited respecting changes that had occurred in times past and those now in progress. Nevertheless, changes of importance, though not of great extent, occurred during the time of the work. The one most worthy of notice was this: The boat passed over a certain place on the Northeast bar, at the commencement of operations, on which there was about 2 feet water. Before their termination a lump at this place projected 2 or 3 feet above water; a change which, by comparison with other known points, was shown to proceed from a rise of the bottom.

This phenomenon is not uncommon, but, on the contrary, occurs frequently. A channel of entrance may be destroyed by this means, and, until another channel is formed, the bar will be impassable. The pilots and captains of tow-boats give innumerable instances of it. Ballast stones and anchors, which have been thrown overboard or lost, have been brought to the surface. The lumps appear to be forced through the ordinary bottom by some power acting from below, but what may be the cause which produces effects so wonderful, future researches must determine.

Another curious circumstance relative to these lumps and salt springs is, that they are only formed in the immediate vicinity of the bars or next to the gulf. The only instance noted in which a spring came up through the marsh, was at a place near the bayou running past the northeast light-house. . . . With this exception, and that of those lying near the mouth of the Balize bayou (which, by-the-by, was once the main pass), the lumps and salt springs are all found near the mouths of the principal passes.

It is perhaps proper to mention in this place some experiments that were made to determine if the deposit of sediment were owing solely to the check of velocity of the current on meeting the outside waters. The conclusion was that the effect was not owing solely to this cause. Proper vessels had been provided for the experiments, and in these as many fit substances as were at hand were dissolved in a mixture with the water, each in a separate vessel. These substances were common salt, epsom salt, alum, sea-water, brine from the salt-springs, and sulphuric acid. The river water alone took from ten to fourteen days to settle, while the solutions became perfectly limpid in from fourteen to eighteen hours, or from one-fifteenth to one-eighteenth part of the time. I know not to what cause to attribute the effect, unless it be action of these substances on the vegetable matter contained in the water, which aids in the suspension of the earthy matter. . . .

However, from these experiments we may conclude that the earthy matter is deposited more suddenly than would be the case if it depended on the check of velocity alone; that the bars will be formed just at the debouchés, or where the salt-water is first met; and that the greater the quantity of water brought down, the sooner, on account of the sudden precipitation, will the bars be formed at the debouchés.