Eugene Woldemar Hilgard

“ 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. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”