INTERPRETATION OF THE GEOLOGIC HISTORY
From the two detailed sections, it is determinable that the Vicksburg epoch of the Oligocene period began, after the Forest Hill age of shallow sandy and clayey waters, with a shallow sea in which marine invertebrate life abounded. Many of these forms secreted a calcareous test, which the waves ground into fragments, as they beat against the shore, and which marine or ground water subsequently cemented into partly indurated limestone or shell marl.
The Mint Spring sea seems to have passed quietly into the Glendon sea without definite crustal disturbances. The Glendon sea was likewise shallow and likewise filled with invertebrate animals, whose calcareous tests were washed up the beach by the incoming waves and down the beach by the undertow until nearly all of the larger forms of these shells were reduced to mere fragments&$151;the matrix of the present limestone. At times slightly stronger currents brought to the area some clay, and at others even stronger currents brought sand, for both are mixed with the limestone and marl at certain elevations in the stratigraphic section.
The Byram sea followed the Glendon sea. In it thrived corals, snails, and clam-like forms, besides many other species having calcareous tests&$151;all whose fossilized shells and those of the other beds of the Vicksburg series have made this collecting area famous throughout the world. Crustal deformation, filling-up of adjoining tracts, or some other change, brought the Byram sea to an end.
The mantle rock, forming the covered interval between the Byram marl below and the sand provisionally referred to the Catahoula above, conceals the history of the change from Byram to Catahoula times. Whatever the details, the Byram sea gave way to some type of water body in which sand rather than calcareous tests accumulated. At times, too, the currents were less swift, for some clay was dropped here instead of being swept farther from the shore.
The next two ages of geologic time are less definitely known, for the clays of both the Hattiesburg and Pascagoula formations are absent. If either bed were deposited, it has been swept away by erosion; or even if both formations were laid down, both have suffered a like fate. The position which these two beds should occupy, therefore, represents an unconformity&$151;a break in depositional sequence.
The history of the next or Pliocene period is one of the most controversial of any period of geologic time. If the yellow sand and gravel of the next interval of 14 feet really belong to the Citronelle, to which they are provisionally referred, rather than to the Coastal Terrace deposits, then there is a local basis for the discussion of some of the history of this period. Here, as over a broad belt in southern Mississippi, these sands and gravels are mixed together, are partly stratified and partly interstratified in such a manner as to mask the identity of the agent which produced them. No agent other than the glaciers habitually deposits at one and the same time both coarse and fine material&$151;but the glaciers fell far short of reaching this region.
While loam, sand, gravel, and clay were being deposited in a broad belt along the Gulf of Mexico in the southern part of the State, conditions quite different obtained in the northern part of the North American continent, roughly north of the Ohio and Missouri rivers. This was the time of the great Pleistocene glaciers, when glaciers of continental extent moved outward in all directions from a center in the peninsula east of Hudson Bay, from a center west of the bay, and from a center in the Canadian Rockies. During each of the five ages of the Pleistocene; namely, during the Nebraskan, Kansan, Illinoian, Iowan, and Wisconsin, a glacier slowly formed and gradually pushed itself out from each of these centers, and just as slowly it gradually melted back from its position of maximum extension. Again it reformed, pushed out, melted, and retreated, only to repeat the sequence time and again.
It was during the advance of the Iowan that the ice ground up so much of the bedrock over which it pushed its rock-shod base, and ground up so much of the boulders it carried in transit, as it rubbed these against one another, that it had an abundance of this fine material to deliver to the streams flooded by the waters from its melting edge. This fine rock flour was carried southward by all outflowing streams and particularly by the Mississippi and its tributaries and deposited far and wide as alluvium or flood plain deposits. When the glacier melted so far back that the floods of its waters did not reach so far south, then this alluvial material was picked up by the wind and wafted over the bluffs of the Mississippi and over the uplands. As the wind's velocity was first checked at the bluffs, here most of the material was deposited, and here, consequently, are the thickest beds of Loess.
This Loess, whose particles are intermediate between sand and clay, has the peculiar physical property of standing in vertical cliffs&$151;even though the material is so loosely consolidated as to be readily crushed in the hand (Figures 2 and 3). This loess is the material that played such an important role in the investment of Vicksburg. Inasmuch as it tends to stand in vertical or even overhanging cliffs, valleys cut into it by the streams of any region are almost all deep and steep-sided. To this rule there seems to be no striking exception1.
Preceding, during, and subsequent to the time of deposition of the Loess, the waters of the Mississippi River were busy carving out of the previously accumulated sediments of the Vicksburg National Military Park area the great valley of the river. And at least some of the tributary streams in at least the lower stretches of their courses were performing a similar task. Previous to the investment, the Mississippi River flowed northward in an enormous bend and struck the east bluff of the valley at the northern edge of the city where Fort Hill is located (Frontispiece).
As is always the case on the outside of such curves, the river undercut the cliffs, forcing them farther and farther outward; in this instance, eastward. By the Mississippi thus pushing its sides farther and farther into the belt previously occupied by the lowest stretches of its tributary streams, it reduced the length of these tributaries, particularly of Mint Spring Bayou and Glass Bayou.
Inasmuch as the tributary streams in their down cutting could not keep pace with the Mississippi in its lateral cutting, these tributary valleys came to lie above the level of the Mississippi Valley. They were thus transformed into hanging valleys. At their lower end, their streams had to plunge in falls to reach the Mississippi.
Geologically, subsequent time has been so short that these tributary streams have been able to push their falls back only to their present positions (Figure 1). But even so, this lowering of their immediate outlet, has enabled them to deepen and make more rugged the lower stretch of their present valleys.
Obviously, the flood waters of the Mississippi River could not lay down as flood plain deposits the Iowan rock flour without a valley flood plain upon which to deposit it. And just as obviously the Mississippi River could not have a valley without the tributary streams having valleys. For these reasons, when the winds later picked up this dry rock flour and wafted it over the valley walls and upland, they deposited a part of it upon the side of the Mississippi Valley, a part upon the surface of the tributary valleys, and a part upon the upland as well. Subsequent erosion has, accordingly, deepened and steepened the valley coated portions, and has later formed deep and steep headward portions in the upland. Such, briefly, were the conditions at the time of the investment.
Last Updated: 18-Jan-2007