Blowing Rocks Preserve is a publicly accessible beach park on Jupiter Island, a barrier island on the southeast coast of Florida in Martin County. It features the most exceptional and extensive exposure of the Anastasia Formation in the state. The Anastasia Formation is a beach/bar deposit deposited along the east coast geologically recently during the Late Pleistocene. It’s positioned at the crest of the Atlantic Coastal Ridge, stretching from St. John’s County on the North Florida coast down to Palm Beach County in the south. It extends at least 20 miles inland, with its full extent still debated.
Noles1984, Public domain, via Wikimedia Commons
The formation consists of quartz sands and calcareous shell fragments, grading from calcareous sandstone to coquina, a limestone almost entirely composed of shell fragments. Coquina is the defining rock type of the Anastasia. Coquina plays a role in the human story as well, the Spanish quarried the rock to build the Castillo de San Marcos in St. Augustine and other coastal forts in the area. The poorly cemented rock absorbed cannonball blows without much damage to the structure!
The Castillo de San Marcos coastal fort in St. Augustine, Florida. Note the crossbedding still visible in the coquina bricks!
The Anastasia, being limestone, is chemically dissolved by water when it picks up carbon dioxide in the atmosphere and terrestrial sources, formed weakly acidic carbonic acid. This chemical weathering is how the Blowing Rocks got their name. As the rock dissolves it forms holes called solution pipes. Eventually the holes dissolve all the way through the rock, creating a pipe from the bottom to the surface. When waves crash against the coast, seawater shoots up the pipe, “blowing” up to 50 feet high!
Solution pipe dissolved through the Anastasia Formation.
The different chemical and physical weathering processes acting at Blowing Rocks and how they are exhibited are the most interesting aspect of the exposure to me. What’s remarkable about Blowing rocks and the smaller Anastasia outcrops along the coast is that the shoreline exposures create a somewhat rocky erosional coastline, a rare sight for the sandy beaches and calm, mangrove-laden Florida coasts.
Beach drift carved into the coastline. As waves strike the beach at an angle, they rush up the shore face and wash back to the sea at a mirrored angle, creating a zig zag pattern along the coast. While watching the waves wash up the formation, I noticed a second process increasing the concave recession in the shore face from the beach drift. The water that makes it to the top of the sea cliff and ponds there slowly flows back to the sea concentrated in channels, incising the channel into the rock.
A wave-cut platform in the formation. As the front of the cliff collapses from the wave-cut notch, it leaves a flat platform that widens as the process repeats.
A wave-cut notch forming. Waves cut a notch into the base of the sea cliff at the high tide line. As the waves cut deeper and deeper into the rock, the cliff falls away into the sea, and the cycle begins again.
Physical and chemical weathering interact and propel each other to create the sharp, jagged, angular, pockmarked surface of the exposure. The force of the impact from the spray of the waves falling down on the surface from above creates small pockmarked depressions, and the seawater collects and slowly dissolves the depressions further, creating dissolution pits. Dissolution pits join and expand at low points on the surface, and shallow rills form where the water flows back down the cliff. The exposure at Blowing Rocks is highly bioturbated. Small marine organisms burrowed into the substrate after it was deposited, creating mud lined horizontal and vertical tunnels. The mud lined burrows are harder and more firmly consolidated than the surrounding shell fragment coquina, so they remain as rod-like structures as the coquina is eroded away first. Another photo showing bioturbation and the pitted texture of the surface.A third photo showing bioturbation and the pitted surface.
Calcium carbonate crusts on the surface of the exposure. The crusts are botryoidal (globular) growths, formed from calcite crystallizing from supersaturated waters at the exposure surface. Calcite crystals precipitate around an obstruction, forming a nucleus around it. More crystals precipitate around the nucleus, forming clusters of spherical growths. The growths have since mostly eroded away, leaving just the bottom of the crusts.The exposure grades into the sand to the south. At this southern end the surface is much more rounded, due to it being low enough for the tide to continuously wash over and erode it.
John Harris 