Coquina and Coastal Geology at Sebastian Inlet

The coquina at Sebastian Inlet is a soft limestone of cemented shell fragments from the Pleistocene Anastasia Formation, the same rock that underlies the nearshore reef south of the inlet and built its small jetties in 1923.

The Florida Geological Survey defines coquina as a type of limestone made up almost entirely of cemented shell fragments. Geologists file the Sebastian coast's version under the Anastasia Formation: the U.S. Geological Survey Geolex record credits E.H. Sellards with the name in 1912, lists coquina limestone as its dominant rock, and names typical exposures on Anastasia Island, near Ormond and along the coast at Rockledge in Brevard County.

The coquina along the Sebastian coast began as loose shell on Pleistocene beaches and shallow sea floors, then hardened when rainwater dissolved and redeposited the shells' calcium carbonate as a natural cement.

The Florida Atlantic University geosciences field guide cites an age of 130,000 to 100,000 years for the Anastasia, the rock formation under the Sebastian Inlet coast, and calls it a multicyclic deposit laid down during several advances of the sea. Its shell fragments are well worn, which points to a high-energy setting such as the crest of an offshore bar or the breaker zone of a beach.

The shells came largely from one animal. The National Park Service at Fort Matanzas identifies the basic component as Donax, the coquina clam, whose shells piled up in layers several feet thick over thousands of years, and notes that coquina occurs from St. Johns to Palm Beach counties, a range that takes in Brevard and Indian River counties at Sebastian Inlet. The NPS geodiversity atlas for Canaveral National Seashore, up the coast in Brevard County, describes the Anastasia as deposited in ancient beach and shallow marine environments where sand grains and shell fragments cemented together.

Near Sebastian Inlet the rock to look for is mostly underwater, as nearshore hardbottom within about 400 meters of the beaches south of the inlet, and it periodically shows or hides as sand shifts.

The Sebastian Inlet District defines hardbottom as exposed limestone bedrock in coastal water within 400 meters of shore and up to about 4 meters, or 13 feet, deep, and says the Florida peninsula is rimmed with it, more so on the east coast. These rock bands act as a natural breakwater that takes energy out of waves before they reach the beach, and they cycle through burial and exposure as sand moves. The District adds that more than 1,000 species have been recorded on Florida hardbottom and that juvenile green sea turtles, an endangered species, can stay on shallow hardbottom for years while they grow, which is the reason the rock off Sebastian Inlet is treated as habitat rather than just geology.

A University of Florida IFAS recovery plan page identifies the Anastasia formation as the rock under east coast reef communities and the backbone of the Atlantic coastal ridge. The same page says worm reefs, built by sabellariid worms from loosely cemented sand, form most often in high-energy surf zones between Martin and Brevard counties, a stretch of coast that takes in Sebastian Inlet. On land, the Geolex record's nearest named exposure is at Rockledge in Brevard County, and the FAU guide names Blowing Rocks Preserve in Martin County as the best locality for intact Anastasia fossils.

Coquina built the small jetties completed after the June 1923 opening of Sebastian Inlet, and in 2014 dredges cut 50,000 cubic yards of coquina and sand out of the inlet floor.

The Sebastian Inlet District history says the inlet was opened in June 1923 at its current location and that the jetties were completed with coquina rock carried on barges and moved by a Model T Ford converted by Roy O. Couch. A photograph in the District's photo galleries shows that converted Model T hauling coquina for the jetties and identifies the driver as Rodney Kroegel, son of the Sebastian pioneer Paul Kroegel.

The same rock sits in the channel bottom. In 2014 Orion Marine Group dredged 50,000 cubic yards of coquina rock and sand to enlarge the sand trap into a more uniform 42-acre basin, while placing more than 111,000 cubic yards of sand on the beaches downdrift (Sebastian Inlet District). Today's jetties are far larger than the small 1923 ones: after work finished in 2003 the north jetty reached 1,000 feet and the south jetty measures 575 feet (Sebastian Inlet District FAQ).

Sand pumped onto the beach south of Sebastian Inlet can bury the nearshore coquina reef, so the Sebastian Inlet District surveys the hardbottom before and after each beach placement.

For its 2019 project, which began January 20 and finished beach work on March 26 with about 110,000 cubic yards placed from near the McLarty Treasure Museum to just south of the Ambersand beach access, the Sebastian Inlet District had CSA Ocean Sciences biologists dive nine transects at 1,000-foot intervals, up to 100 feet offshore. They measured sediment depth, algae height and hardbottom relief and mapped the western edge of the nearshore rock.

The beach above the rock moves on its own as well. An April 1994 aerial photograph in the University of South Florida's Richard A. Davis coastal collection is titled Washover South of Sebastian Inlet. Between storms and sand bypassing, the coquina off the Sebastian Inlet beaches is alternately covered and bared, which is why the District compares each survey with a mapped baseline.

The jetties, beaches and nearshore rock are reached through Sebastian Inlet State Park, open 24 hours at $8 per vehicle, though bridge construction scheduled to begin in May 2026 has made south-side beach access unavailable.

Florida State Parks lists the park at 9700 South Highway A1A, Melbourne Beach, phone 321-984-4852, open 24 hours a day, 365 days a year. The Sebastian Inlet South Ranger Station switched to the standard 8 a.m. to sunset schedule on November 12, 2025; after-hours entry there is limited to registered campers and annual pass holders with a permit, and the North Ranger Station runs 24 hours. The park warns that construction on the Sebastian Inlet bridge along the Jimmy Buffett Memorial Highway was scheduled to begin in May 2026, that beach access from the south side is unavailable during the work, and that the north side can fill and close to new visitors on weekends.

While the bridge project runs, the North Ranger Station side of Sebastian Inlet is the entry that stays open around the clock, and it leads to the 1,000-foot north jetty. The beaches south of the inlet, from the McLarty Treasure Museum toward Ambersand, are where the Sebastian Inlet District maps the nearshore coquina before each sand placement.