As the seas have risen and fallen over geologic time, the river and the sea have combined to fill the trough with sediment, and the trough has sagged under the weight. Thus the rocks that line the Mississipi Embayment are bent downward in its midsection and exposed along its edges, older the farther east you go.
In only two places are there deposits not related to the embayment: along the Gulf coast, where short-lived sandbars and lagoons are regularly swept away and sculpted by hurricanes, and in the extreme northeast where a tiny edge is exposed of the continental platform deposits that dominate the Midwest.
The most distinctive landforms in Mississippi arise along the stripes of rocks. Gently dipping strata that are harder than the rest are left by erosion as low, level ridges, broken off steeply on one face and ramping gently into the ground on the other. These are called cuestas.
More Mississippi resources on About.com:
Mississippi Geography, State Symbols & Facts
Mississippi National Parks
Mississippi State Parks
Mississippi Scenic Roads
Mississippi Saltwater Fishing