The earliest part of the Cenozoic Era is also called the Paleogene Period, which includes the Paleocene, Eocene, and Oligocene epochs. The extinction of the dinosaurs at the end of the Cretaceous left room for mammals to grow and diversify throughout the Paleogene. Both marine (sea) and terrestrial (land) environments were present in Mississippi during this time. Marine environments were found to the south while the northern portion of the landscape was covered with dense forests and swamps.
The middle part of the Cenozoic Era is called the Neogene Period, which includes the Miocene and Pliocene epochs. During this time the seas began to recede a bit, only to come back as glaciers melted during the late Cenozoic.
The late part of the Cenozoic Era is called the Quaternary Period, which includes the Pleistocene, the Holocene, and perhaps soon the Anthropocene (recent) epochs. The landscape during these times saw further development of the Mississippi River Valley, higher sea levels with a shoreline that looks much like it does today.