The seashore spans two distinct districts separated by more than a hundred miles of Alabama coastline that was considered for inclusion but never added. The Florida district centers on Santa Rosa Island near Pensacola, with additional areas at Perdido Key, Naval Live Oaks Reservation, and several military fortifications. All Florida areas are reachable by car. The Mississippi district is wilder and more remote. Petit Bois, Horn, East Ship, West Ship, and Cat islands are accessible only by boat, and the Gulf Islands Wilderness designation gives special protection to parts of Petit Bois and Horn islands. The Davis Bayou Area near Ocean Springs is the sole Mississippi section reachable by automobile. This geographic split gives the seashore two distinct personalities: developed recreation near Pensacola, untouched wilderness off the Mississippi coast.

Long before the National Park Service arrived in 1971, these islands served military purposes. Fort Pickens on Santa Rosa Island - a massive brick pentagonal fortification - held Geronimo as a prisoner in the 1880s and saw action during the Civil War as one of the few Southern forts that remained in Union hands. Fort Barrancas, perched on the mainland at Naval Air Station Pensacola, dates to Spanish colonial times and was rebuilt by the Americans in the 1840s. The Perdido Key Historic District preserves shore batteries that remained active through both World Wars, their concrete emplacements still angled toward a sea that once threatened invasion. Fort Massachusetts on West Ship Island was seized by Union forces in 1861 and guarded the Mississippi Sound through the Civil War. These fortifications now serve as visitor centers and hiking destinations, their thick walls cooler than the beach outside, their cannon ports framing views of the same Gulf waters they once defended.

The barrier islands began forming between 4,500 and 5,400 years ago, but the geological story runs far deeper. Beneath the Holocene sands of Santa Rosa Island lie Pleistocene formations deposited during the Sangamonian period, when sea levels stood higher than today. The Gulfport Formation preserves an ancient barrier complex; the Prairie Formation records nearshore marine and brackish lagoon environments; the Biloxi Formation holds floodplain deposits. Drill deeper and you reach the Pliocene Pensacola Clay, then Oligocene limestone. On Horn Island, the layers shift to Miocene formations - the Pascagoula and Hattiesburg deposits, remnants of ancient rivers and estuaries. The white quartz sand that makes these beaches famous is the youngest layer of a geological column stretching back tens of millions of years, each formation recording a different coastline, a different sea level, a different world.

Barrier islands are built to move. The 2004 and 2005 hurricane seasons hammered the seashore's infrastructure - roads, boardwalks, campgrounds, dune crossovers. Hurricane Sally arrived in September 2020 and flattened dunes along Perdido Key, gorging the eastern tip of the island into three small isolated fragments. Johnson Beach was hit particularly hard, its protective dune line leveled by storm surge. Restoration crews bring in sand and plant vegetation, but subsequent storms and persistent winds can undo months of work in hours. The Deepwater Horizon oil spill in 2010 delivered a different kind of damage - waves of oil and tar washed across Santa Rosa Island's beaches beginning June 1, prompting fishing and swimming bans across the seashore. Every major island in the group was affected. These events are not aberrations but the normal life cycle of barrier islands, constantly reshaped by the forces that created them.

The Mississippi islands offer something increasingly rare in the National Park system: genuine solitude. Horn Island, accessible only by private boat, has no facilities, no trails, no docks. Primitive camping is permitted, and the island's interior of pine forest and freshwater marshes feels genuinely remote. Petit Bois Island is similarly wild, protected under the Gulf Islands Wilderness designation. Contrast this with the Florida side, where the Fort Pickens campground provides water and electrical hookups among sand scrub oaks and remnant pine forest, and Pensacola Beach draws millions of visitors annually. The entrance fee is $25 per vehicle for seven days - a bargain for access to some of the whitest sand in the continental United States. The seashore contains, in its two districts, both the crowded recreation and the empty wildness that the Gulf Coast once offered everywhere.