Page:Popular Science Monthly Volume 80.djvu/244

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vex 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.

Mudlump Clay

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,