The geology of the Mississippi River's Upper Basin and the Lower Basin are quite distinct. The Upper Basin is characterized by highlands, plains, and geologically diverse mountainous regions. Firmly rooted in bedrock, the highlands are gradually being eroded and the soil carried away by the river. The lower basin fills a geological crease created during the formation of the continent. Originally an inland sea it has been filled by deposition of the soils carried south from the highlands.

Geologists call the crease the "Mississippi Embayment." It stretches from Illinois to the Gulf and includes the region from from East Texas to Florida. Since the time of the dinosaurs, over hundreds of millions of years, the sea has done battle with the Mississippi River for control of the entire area. The gray on the map represents silt deposited by the Mississippi and other rivers in the embayment over the past 10,000 years. The light yellow includes deposits made up to 2 million years ago. For the past 10,000 years, since the end of the last Ice Age, the battle with the sea has taken place in Louisiana.

New Orleans  and much of the region around Lake Pontchartrain are solidly in the gray.  The land is less than 10,000 years old and solid ground is 70-100 feet below the surface. As the silt compacts the whole area is slowly sinking and must be constantly replenished by the river.

Why would anybody choose to build a city in such an inhospitable place?