In Roger Saucier’s Recent Geomorphic History of the Pontchartrain Basin, the origins of the lakes on either side of the Isthmus of Manchac (and the Manchac Greenway) are described with the help of a series of maps showing how they grew from about 4,000 years ago to the present. Saucier shows that Lake Pontchartrain began as an irregular, half-sized body of water trapped against the Gulf Coast by the growing Cocodrie and St. Bernard Mississippi River deltas, with only the Rigolets Pass connecting it to the sea. Lake Maurepas did not originally exist at all but was, instead, an area of solid swamp and only later did the area begin to slump and fill with water to create the lake.
Expansion of Lake Maurepas eventually cut through and separated the lower reaches of the Amite and Tickfaw Rivers to, in turn, become the passes of Manchac to the sea. Both lakes have been enlarging ever since as their shorelines gradually recede through erosion, subsidence and rounding currents. Until about a thousand years ago the maps show a persistent, lingering island in Lake Pontchartrain just off the north shore, St. Tammany’s Atlantis.
Other maps in the book tell intriguing stories about patterns of subsurface faults beneath the lakes; how the Pontchartrain embayment and barrier islands formed on the Gulf shore before the Mississippi River flowed through the area; what the pattern of local rivers looked like during low sea levels during the Ice Age and the sequence of the seven most recent Mississippi River deltas that deposited glacially-eroded materials to build southeast Louisiana.
Saucier didn’t just conjure these stories from thin air; he employed incredibly astute scientific knowledge, inquiry and assiduous research. He and his team flew over the area to gather data, took core samples from numerous oil and gas explorations and road and bridge projects as well as drilled for their own. They analyzed the sediments to determine when and how they were likely deposited. In one crazy but brilliant maneuver, Saucier used taste to help determine their salt content to figure if they came from offshore, near shore or freshwater lake environments. They used radio carbon dating, looked for fossil and human artifacts and correlated the many, complex layers to determine their relationship to one another across this vast area. The maps they created in the 1960s have held true to this day and are still in use as principal reference works, making Saucier the Godfather of local geography.
The Seven Mississippi River Deltas that built southeast Louisiana in about 4,000 years.
The beach trends on the old Gulf Coast that helped form the Pontchartrain Embayment before the Mississippi River invaded the area. Note the north shore river patterns when sea level was lower.
Images from Recent Geomorphic History of the Pontchartrain Basin by Roger T. Saucier, LSU, Coastal Studies Series Number Nine.