Mississippi State Geological Survey Bulletin 28 The Geologic History of the Vicksburg National Military Park Area

Notwithstanding the minor distribution of the Confederate and Federal armies at Vicksburg by Pemberton and Grant during the siege of the City from May 18 to July 4, 1863, the fact, nevertheless, remains that the major disposition of the forces was determined untold ages before by the Creator working through the agency of one of the Continental Glaciers in Canada and northern United States. This particular glacier, in passing over the bedrock of the region, ground some of it, as well as some of the rock material the ice was transporting, into a fine rock flour, which the flood waters from the melting glacier carried down the Mississippi and deposited as flood plain material. The westerly winds picked up this rock flour, after it had dried, carried it on to the east bluffs and uplands, and deposited it as a mantle rock that forms steep-sided valleys where streams cut into it. These valleys were the controlling factors in the distribution of the armies.

But this is beginning far along in the geologic history of the Vicksburg National Military Park region; in fact, near the closing stages of its history. Although this geologic history of the area does not stretch back to the most ancient eras of geologic time, it does reach millions of years into the past.

Perhaps this can best be demonstrated by giving all the major divisions of the geologic time scale:

Geologic time scale

Beginning with the Comanchean in late-Mesozoic, much of the subsequent material was laid down along the Atlantic and Gulf coastal plains, and is, for that reason, known as the Coastal Plains deposits. Save for some Paleozoic rocks along the stream courses in Tishomingo County, in the northeastern corner of the state, the geologic history of the beds at the surface begins in late Mesozoic time with the Cretaceous.

This was the time of the so-called Mississippi embayment, when an arm of the Gulf spread broadly over the southern part of the United States and reached as far north as the mouth of the Ohio River at Cairo, Illinois, or even slightly farther north. In this sea the thicker sediments were laid down along the old shore line which at the beginning extended almost north and south across the northeast corner of the state. As time elapsed and as the embayment became more and more filled with sediment the old shore line shifted farther toward the south and to a more nearly east and west position in the vicinity of Vicksburg. Here begins the detailed history of the Vicksburg area, but it calls for a more detailed time scale of later geologic time.

Later geologic time scale

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¹The term Vicksburg was first applied to the rocks of this region by T. A. Conrad, Acad. Nat. Sci. Phila., Proc. 3, pp. 280-299, 1847.

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Figure 1.—Falls in Mint Spring Bayou at the National Cemetery, Vicksburg. The rocks are correlated on the left and described on the right.&$151;Photographed January 9, 1935.