I’ve gotten a little farther in Bearss’ Volume II of the Vicksburg Campaign. Finally, after months of feints and attempts to get around the city, Grant is putting into action his plan to get his army up onto the level ground behind the city, where he will not have to face the bluffs, swamps, and fortifications as well as Pemberton’s army.
While Pemberton was occupied trying to clutch after Grierson’s cavalry, who were riding across the bottom right of the map at this same time, Grant was moving his army south, where the navy ships that ran the batteries could transport them over. The first thought was to take Grand Gulf, the red circled area in the lower left. It turned out this was too well fortified to take, so the outflanking operation had to move even farther south, to Bruinsburg.
Pemberton didn’t profit from the warning, and when Grant pushed inland only a small force from Grand Gulf was available to stop him. They met outside of Port Gibson.
The country outside of Vicksburg is very odd. The ground is made of a material called loess, that holds its structure well against gravity but erodes quickly. So the streams in the area cut deep gorges into the soil with incredibly steep sides. I’ve seen these in person, and one gorge between the Union and Confederate lines seemed to be as deep as it was wide. These gorges usually fill up with tangled vegetation making it very hard to operate off the ridges between streams, which is where the roads are.
So a small force can block a road against a larger force pretty well, but if all the roads are not blocked going cross-country to follow the enemy using the road might be impossible. The Rebel force managed to get across the two roads the two Union corps were using, and it took all day to wear them out and open up the road again. But because of the problems in blocking alternate routes, it was felt that the force had to retire back to Grand Gulf, destroying the bridges over the Bayou Pierre to slow down Grant.
Grant soon found a way across, and Grand Gulf was threatened from the rear. The post was abandoned, and the defenders retired across the next river line, the Big Black. This meant they were safe for the moment, but now Grant had a decent supply base on the River at Grand Gulf, and an open field ahead of him. What to do next?
The map shows Grant’s decision — to quickly move to separate the defenders of Vicksburg from possible reinforcements by taking Jackson and destroying the railroads. With the two sides based in opposite directions, and he in the middle, he should have the upper had for as long as he could keep the initiative.