June 2022

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Hunting Island is a barrier island along the southern coast of South Carolina. Due to coastal development, it is rare to see a barrier island in such a natural state! The island is a real treasure, but its days are numbered, Hunting Island is eroding away into the sea. Barrier islands are highly dynamic systems, constantly forming and disappearing as currents, waves, and storms erode away and re-deposit sand.

Barrier islands are elongate strips of sand that form parallel to the coastline, often occurring in chains separated by narrow inlets. Lowlands separating the raised sand dunes of the barrier islands from the mainland flood and turn into narrow, protected, low-energy bays, where tidal flats, salt marshes, and lagoons form. Barrier islands play a critical role in protecting the coastline from storms. However, that defense is lost when the heaviest development occurs on the barrier island itself, as so often is the case along the Atlantic and Gulf coasts where barrier islands occur.

The park map of Hunting Island above shows, moving from the coast towards the mainland: beach – dune sequence – salt marsh and tidal creeks. On the southern end of the island, you can see where a new barrier island sequence is forming on the seaward side of the barrier island itself! The sea has filled into the topographic low between dune ridges, creating a lagoon. The spit of land between the lagoon and the sea was breached by recent hurricanes, forming an inlet cutting off the bottom corner of the island, creating a new barrier island called Little Hunting Island. Eventually the spit will be breached again, forming more islands.

While Hunting Island has largely been free to follow its natural processes, some evidence of human interference to slow longshore drift can be observed. Waves do not strike the coastline head-on, but rather at an oblique angle. This results in a net movement of sediment in a prevailing direction parallel to the coastline.

At Hunting Island the prevailing longshore drift direction is to the north, so the island is eroding away to the south and depositing to the north. So does that mean the island is growing to the north? Normally it would, but strong seaward currents from St. Helena Sound along the northern end of the island flush the sediment migrating north from longshore drift directly out to sea, depositing it miles away into the Atlantic Ocean. Currents moving towards the shore are not strong enough to transport this sediment back to the island, resulting in erosion of both ends of the island, and its eventual disappearance. The location of the Hunting Island Lighthouse on the north side of the island has moved south twice since its construction in 1859 due to this erosion, with the original site of the lighthouse being more than a mile north of the current northern extent of the island!

While barrier islands are ephemeral features, this is not ideal for development, and most barrier islands have extensive engineering structures and beach replenishment programs in order to restrict their movement. As is typically the case, these measures are more of a delay than a solution, and often have unintended consequences. By retaining sediment in one area other areas are starved, and by continuing to develop on areas that would be underwater without artificial controls, the inevitable flooding and beach loss are delayed and worsened.

Groins and jetties are two common structures used to control sediment transport along the coast. Groins are barriers constructed perpendicular to the coastline to restrict longshore drift. Sand piles up against updrift side of the groin, while the downdrift side is starved of sand and erodes away. In order to counteract this beach loss on the downdrift side, another groin is emplaced, resulting in an inwardly curved coastline and eventual beach loss at the termination of the groin sequence. A groin sequence is present along the beaches of Hunting Island, and can be seen below. Jetties are similar to groins, but emplaced adjacent to tidal inlets to stabilize the inlet and keep it from migrating.

The temporary nature of barrier islands is quite evident at the southern end of Hunting Island, where natural processes have not been interfered with. The lagoon was breached by Hurricanes Matthew (2016) and Irma (2017), cutting off the southern tip and creating Little Hunting Island. When we visited in June of 2022, the breach was actually partially filled in with sand, turning Little Hunting Island back into a spit. The breach will likely be reestablished with hurricane season coming soon, turning it back into an island. Breaches are not created by overwash during the storm event, but by backwash after the storm. The water level in the lagoon rises during the storm, eventually overtopping its banks and flooding over at the path of least resistance, breaching the spit and creating an island.

One of the most striking features of erosion on the island, and something I had never witnessed before at that scale, can be seen in an area on the southern portion of the island called Boneyard Beach. As the beach has eroded away and the sea has crept further and further up the shore, the saltwater has choked the life out of the trees behind the beach. These trees, with their weakened dead roots, are then toppled and strewn about, creating a “boneyard” of these dead and denuded giants.

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I returned to the falls in early June, hoping to capture some different perspectives with my newly acquired drone! I received my Part 107 commercial drone license in March and had been very eager to get my hands on a working drone to finally fly. The falls were running strong but clear with a relatively low sediment load, indicating primarily subsurface input rather than runoff carrying sediment. This occurs when rain recharges the aquifer, but enough time has passed since the rain event that overland runoff heavy with sediment has slowed. Precipitation that has infiltrated into the subsurface of the drainage basin feeding the creek now dominates, its water less sediment-laden due to filtration through the soil and soft bedrock. The level of the creek is still fairly low at the start of summer and the heavy rain it brings.

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I visited the falls in late September, and the creek was running strong and high with very turbid discharge. Turbidity is the measure of the clarity of the water, indicating the level of suspended solids such as clays and silts. An increase in turbidity is most often seen after heavy rains, as surface drainage flows into the stream, carrying sediment eroded from the land surface.

During this visit I tried to find the swallow hole where the creek disappears underground, before reemerging and flowing into the Suwanee River. I left the park and crossed NW Falling Creek Rd, and followed its course north of Dicks Rd. Slashing through the thick undergrowth and swatting away the swarming mosquitoes, I found the creek flowing very slowly northwest through wide, flooded, well-defined meanders, the water nearly stagnant. I eventually had to give up the search as the vegetation became too thick and the banks too steep and muddy. A small boat would have been my best bet to follow the meanders and find where the creek ends.

July 2021

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June 2021

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Falling Creek Falls is located in a small county park about a mile north of Lake City in North Florida. The park, nestled in the wooded countryside of rural Columbia County, includes a boardwalk that leads to the falls about a quarter mile down the trail. The creek drops 10 feet over a clayey limestone lip at the falls, and continues downstream through a narrowing ravine. Falling Creek, a tributary of the storied Suwanee River, is an intermittent stream fed by seasonal rain and shallow groundwater from a drainage basin extending to the east. It flows west for a few miles, down the falls, then disappears down a swallow hole in the karst topography, flows underground, then reemerges and joins the Suwanee River to the northwest.

Falling Creek’s water level varies drastically depending on the season and recent precipitation. During my visit in June in the midst of an uncharacteristically dry start to the summer, the creek was mostly dry, with the stream bed fully exposed (perfect for viewing the geology), and barely a trickle coming down the falls. When I returned at the end of a rainy July, the entire creek was flooded above its banks and into the floodplain of the surrounding woods. Water was flowing full and fast above the ravine, flooding the boardwalk. The water level was so high you wouldn’t have known there was a waterfall there!

The lithology of the area, exposed at the falls and the stream bed when the creek is dry, consists of the Coosawhatchie Formation of the Hawthorn Group. The Coosawhatchie Formation here is made up of gray-green sandy clays interbedded with phosphatic poorly consolidated limestones and sands. It was deposited during the Miocene Epoch, roughly 5-23 million years ago, in a low-energy shallow marine environment.

The falls are formed where the denser and more consolidated gray member that forms the creek bottom upstream overlies the softer and less consolidated light green member that make up the lithology downstream. The stream cuts down through the softer layers where it meets them, the flowing water eroding them away. Eventually those softer layers break away to the edge of the more resistant rock, and then continue to get cut away under the overlying resistant rock, resulting in an overhanging lip that water cascades down vertically, a waterfall. The waterfall will slowly retreat upstream as the cycle continues.

Karst topography is very evident in the more consolidated, gray, calcareous stream bed upstream of the falls. Small sinks and fractures are abundant in the clay/rock beds where mildly acidic surface water has dissolved the rock. Moving downstream, the walls of the ravine and plunge pool beneath the falls show the less consolidated gray/green/yellow phosphatic clay/sand/rock beds. The stream bed and banks are lined and littered with cobbles and boulders from these eroded beds. Vegetation covers much of the surface of the ravine walls, showing that the creek often has minimal to no discharge. Karst topography is still present below the falls, but the dissolved sinks and fractures are mostly covered by the boulders and cobbles. About a quarter-mile downstream of the falls, the creek disappears into one of those sinks.

The Coosawhatchie Formation here is semi-confining, the low permeability from the tight, clayey, lithology capping the highly permeable and productive Floridan Aquifer below it, slowing flow in and out. I believe the confining nature of the formation plays a role in the highly variable water level of the creek and its susceptibility to flooding. Surface water from the drainage basin flowing into the creek after rainy periods cannot easily infiltrate through the low permeability lithology that makes up the stream bed, so the water level rises and rises. As it drains downstream it may also encounter a choke point at the swallow hole, the heavy discharge backing up as it tries to flow down the tighter sink, like a partially clogged drain. Increased sediment load from the heavy discharge might clog up the swallow hole, further reducing drainage, resulting in increased flooding of the banks. I would like to visit the creek again and see if I can find a way down to the swallow hole to check it out.

July 2021

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Ocean Reef Park is a beach park on Singer Island, a barrier island/peninsula on the east coast of Florida in Palm Beach County. Like Blowing Rocks Preserve, it also has a notable exposure of the Anastasia Formation. It is roughly 15 miles south of Blowing Rocks. An overview of the Anastasia Formation and map of its extent are in my post on Blowing Rocks Preserve. The Anastasia here is significantly more weathered and eroded than Blowing Rocks. The pockmarked texture is much more rounded, similar to the southern end of the Blowing Rocks exposure where it grades into the sand. The rock appears to consist of more quartz sand grains and less shell fragments, and the shell fragments that are present are finer. The distinctive rod-like bioturbation, so pervasive at Blowing Rocks, is absent here.

The dissolution pits have expanded, eroding away much of the surrounding rock and leaving only small peaks between pits.

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April 2021

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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.

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 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!

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.

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