Perdido Key was part of the mainland of Florida until the middle of the 20th century. Perdido Key probably developed in place by
aggradation of offshore shoals consisting of
quartz sand that is likely reworked from
Pleistocene delta and shallow marine deposits. Perdido Key was shorter east to west for most of the 19th century than it is now. The central part of the island was originally a peninsula that was the southern end of a ridge on the mainland. Barrier
spits developed from the southern point of that peninsula, extending east and west. The easternmost part of the eastern barrier spit, adjacent to Pensacola Pass, may have been an island for much of the 19th century. It was known as Foster's Bank or Foster's Island throughout the 19th century. An intermittent inlet known as New Inlet, several miles west of Pensacola Pass, closed for the last time sometime between 1880 and 1900, attaching Foster's Bank to the spit and extending it to close to its present length. (In 2020, Hurricane Sally temporarily severed the eastern end of Perdido Key from the rest of the island). Perdido Pass, the western end of Perdido Key, was about east of its present location until early in the 20th century. The barrier spit on the western (Alabama) side of the inlet, Point Ornocor, overlapped what is now Perdido Key, extending for several miles parallel to and on the north side of Perdido Key. As a result, the water passage from Perdido Pass to
Perdido Bay was long, narrow and winding, passing between Perdido Key and Point Orconor, doubling the end of the point and then passing back between the point and the Florida shore on the north side. In early 1906, local residents dug a new inlet through Point Ornocor about west of the old inlet. The new inlet was widened by the
1906 Mississippi hurricane. The old inlet then silted up and closed, with the result that the portion of Point Ornocor east of the new inlet was attached to Perdido Key and the Florida mainland. The original waterway from the old Perdido Pass to Perdido Bay became known as Old River. A passage later opened from the western end of Old River to the Bayou St. John near the new Perdido Pass, separating what is now Ono Island from Perdido Key. The spit ending on the west side of Perdido Pass is now known as Alabama Point. Alabama and Florida settled the location of the state line in 1953, placing it at the location of the old Perdido Pass. In the early 1940s the
Gulf Intracoastal Waterway was constructed between Perdido Key and the mainland, including the dredging of a canal through the neck of the peninsula connecting Perdido Key to the mainland, thus turning Perdido Key into an island. Perdido Key is shifting westward due to
longshore drift. Both Pensacola Pass and Perdido Pass have historically moved westward. The westward flowing longshore current accretes sand on the up-current (eastern) sides of the passes and erodes sand from the down-current (western) sides. The site of the original Fort McRee, built in 1830 on the eastern end of Perdido Key (then known as Foster's Bank), was in the channel in the middle of Pensacola Pass by 1979. Pensacola Pass has been dredged since 1883 to maintain a channel into Pensacola Bay for United States Navy and other ships. The dredging has interrupted the natural transport of sand across the inlet from Santa Rosa Island to Perdido Key, with the result that Pensacola Pass is a net sediment sink. This has starved the eastern end of Perdido Key of sand, leading to the erosion of that part of the island. Much of the sand that makes up Perdido Key comes from the Gulf of Mexico floor offshore of the island. About of sand is deposited on the island from offshore each year. Part of that sand is carried west by longshore currents. Another part of the sand is carried, at infrequent intervals, across the island to and beyond the north side of the islands by
washovers during hurricanes. With the reduction of sand crossing Pensacola Pass due to dredging of the inlet, most of the eastern half of the island is eroding, while the western half of the islands is accreting sand. The easternmost part of the island is receiving sand dredged from the inlet in a
beach nourishment project. In the period from 1920 until 2013, the easternmost of the island lost an average of of shoreline per year. ==Hurricanes==