National Geographic : 1897 Dec
352 TIE DELTA OF TIlE MSSIS SIPPI RIVER Many interesting facts bearing upon the question of the geo logical formation of the Mississippi delta were brought to atten tion two years ago through the investigations and discussions connected with an engineering question which arose between the executors of the late Mr James B. Eads and the War Department as to what is the legal plane of reference for ascertaining the depths and widths of channel which Mr Eads was required by the law of the Federal Congress to maintain between the deep water of the South Pass of the Mississippi river and the Gulf of Mexico. On Belize bayou, which leads out to the Gulf from one of the now unused passes of the river, stands an old Spanish magazine, built over 200 years ago. At the time of building the jetties at the mouth of the South Pass this magazine was in a fair state of preservation. The exterior was intact and there were no cracks which would indicate settlement, the building standing perfectly level, but with the surface of the water stretching across the arch which crowned the entrance door, the sill of which must have been at least ten feet below the water. That was in the year 1877. Nineteen years later a part of the structure had been removed, but enough of the roof and arches remained to show that the subsidence had continued steadily during that period of nine teen years at about the same rate as during the preceding two hundred years. It may be stated that this rate, both from this instance and from other information, is, at the mouth of the Mississippi, about one-half of one-tenth of a foot per annum. Numerous illustrations going to prove the general subsidence of the delta lands might be stated. Not only are these lands unstable in a vertical direction, but they are often found to be so in a lateral direction. It is an interesting engineering as well as physical fact that an accurately measured base line exactly seven hundred feet in length was found, after a lapse of five years, to be 712 feet in length. It has been found impracticable to maintain with sufficient accuracy for reference purposes bench marks, level heights, and tide gauges. This subject is quite fully discussed in the Report of the Mississippi River Commis sion for 1894, pages 2794-2797, where the following important statement is made: "Discrepancies in bench-marks and level heights and gauges could only be satisfactorily accounted for by the most plausible explanation of the subsidence of the whole delta, making gauges and bench-marks at the mouth of South Pass unreliable." This remark is made by Mr J. A. Ockerson, assistant engineer to the commission.