Units A 1 and B 1 have the largest aerial distribution. Back-barrier deposits and shell reefs partially filled the open estuary. Deltaic sediments enclosed the Pontchartrain Basin to the south and eventually covered the Pine Island Barrier, forming Lake Pontchartrain. At that time, the basin began to accumulate prodelta, delta front, and crevasse deposits. Cypress swamps and fresh water marshes formed in the upper basin and intermediate to saline marshes formed in the lower basin.
Many similarities exist among the gulf estuaries.
The same processes that formed and flooded the incised valleys of Mobile Bay to the east and Sabine Lake to the west formed Lake Pontchartrain. Each of these estuaries was an open bay that was closed or partially closed by westward movement of shoreline sands that formed spits or cheniers. Mobile Bay was partially closed by sands forming Morgan Peninsula. Sabine Lake was fully closed by the sands forming the chenier plain of western Louisiana. Lake Pontchartrain was partially closed by sands forming the Pine Island Barrier Trend.