An explanation for the depositional history of the Gosport Sand has proven to be as puzzling as its diverse mollusk fossils is beguiling. We propose that shifting delta lobes produced patchy environments which accumulated to form the Gosport Sand and likely other glauconite- and fossil-rich shell beds of the Gulf Coastal Plain of the Paleogene. We present facies and mineralogical data to suggest glauconite in the Gosport Sand was deposited during relatively stable environmental conditions. Increased glauconite maturity and taphonomic alteration in the Gosport Sand are attributed to eustatic transgression, resulting in decreased deposition, enhanced winnowing, and increased exposure at the sediment–water interface. Increased fossil diversity and abundance likely result from time-averaging and accumulation of shells from ancient patchy habitats analogous to modern “inner shallow shelf” and “open sound” environments. Delta progradation and subsidence would have determined the migration of these environments. We evoke a similar depositional model for glauconite-rich shell beds distributed throughout the Paleocene and Eocene of the Gulf Coastal Plain based on their transgressive nature and roughly equivalent facies. These shifting environmental conditions may have played a partial role in generating the high molluscan diversity of the Gulf Coastal Plain of the Paleogene.