THE MISSISSIPPI DELTA 237 



A NEW DEVELOPMENT IN THE MISSISSIPPI DELTA 



By Professor E. W. HILGARD 



university of california 



Introduction" 



T N 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 in- 

 volved, of course, a general examination of the coast formations of 

 Louisiana, and among them, of the Passes of the Mississippi, and of 

 tlie puzzling phenomena of " mudlump " upheaval in the Passes, which, 

 at times, seriously obstructed commerce, but the origin of which re- 

 mained a matter of conjecture. It had, to some extent, been investi- 

 gated by Sir Charles Lyell (1858) and is commented upon in the tenth 

 edition of his "Elements of Geology"; it was also conjecturally dis- 

 cussed 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 Lov^ter Mississippi Delta not a Normal One 



The bird-foot shape of the lower Mississippi delta, with deep embay- 

 ments 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, carry- 

 ing in the bar sands; and only suhordinately hy river overflow. The 

 river in this lower delta region is for 50 miles below Fort Jackson bor- 

 dered 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.) 



