EarthCaches do not contain a physical container or logbook. Instead, they teach you about geology through real‑world observations. For this Earthcache you will take a short, easy hike in the Helen Cooper Floyd Park (1 mile out and back).

• 烈 Mosquitoes are common in this coastal environment, especially in warm months — consider insect protection.

• ☀️ Heat and humidity can be intense; bring water, take breaks, and be mindful of sun exposure.

• ‍♀️ Stay on established paths and designated viewing areas to protect both yourself and the fragile shoreline which may hide uneven ground or wildlife.

•  Use caution near the water’s edge and do not attempt during flood, tropical storm or hurricane warnings.

Logging Tasks

To claim this EarthCache  , you must first visit the posted coordinates and then Send Answers via the geocaching.com Message Center to the following questions based on your observations and the information provided in the short Earthcache Lesson below:

1. Shoreline Geomorphology: Describe the shape of the shoreline at your location (straight, curved, irregular, eroded, built‑up). What does this shape suggest about the underlying geology?

2. Evidence of Ancient Shorelines: Look for clues such as terraces, steps in the landscape, dune‑like ridges, or abrupt changes in vegetation. Do you see any features that might represent former shorelines or relict dunes?

3. Human Modification vs. Natural Geology: Identify any visible human‑altered features (dredged banks, riprap, jetties, channel markers). How do these modifications differ visually from natural geological formations in the area?

A photo of yourself or your GPS is optional.

Landforms Through Time: AICW–St. Johns Junction

At the intersection of the Atlantic Intracoastal Waterway (AICW) and the St. Johns River, you are standing at a geological crossroads shaped by millions of years of coastal evolution. This location is not only a major navigational junction — from here, a boater could travel inland or continue south along the AICW all the way to Miami — but also a window into the deep geological past of northeast Florida. The landscape you see today is the product of shifting sea levels, migrating barrier islands, ancient dunes, and the slow carving of a north‑flowing river system.

Understanding how this landscape formed requires stepping back through several major geological periods, from the Pliocene through the Pleistocene and into the Holocene. Each stage left behind landforms and sediments that remain visible in the region’s physiography.

1. Barrier Island Formation (Late Pleistocene to Holocene)

The modern coastline of northeast Florida owes its shape to the formation of barrier islands, which developed during repeated sea‑level rises over the last 120,000 years. During warm interglacial periods, rising seas pushed inland, allowing waves and longshore currents to build elongated sand islands parallel to the coast. These islands — including today’s Amelia, Talbot, and other nearby barriers — consist largely of quartz sand transported from the Appalachian Mountains by rivers and redistributed by coastal processes.

These barrier islands created protected lagoons and estuaries behind them, influencing the development of the St. Johns River system. Their dune ridges, some still visible today, record ancient shoreline positions.

2. Sea Level Fall and Inland Water Trapping (Late Pleistocene)

During the last glacial maximum around 20,000 years ago, sea level dropped dramatically — more than 100 meters lower than today. The coastline extended far eastward, and the barrier islands became connected to the mainland. As sea level fell, the dune systems blocked eastward drainage, trapping freshwater in broad, low‑lying basins.

This trapped water set the stage for the next major transformation: the creation of a north‑flowing river.

3. Development of the North‑Flowing St. Johns River (Late Pleistocene to Early Holocene)

With eastward drainage blocked, water sought the lowest available path — which, in this region, sloped gently northward. Over thousands of years, the trapped waters carved a shallow valley, establishing the early course of the St. Johns River.

By the early Holocene (8,000 years ago), rising sea levels stabilized the drainage pattern. The St. Johns became one of the few major rivers in North America that flows north, a direct result of the subtle tilt of the coastal plain.

4. Marine Terraces and Ancient Dunes (Pliocene–Pleistocene)

Long before the modern river existed, earlier high sea levels created marine terraces — broad, flat surfaces marking ancient shorelines. These terraces formed during multiple highstands over the last 3 million years. Each time the ocean rose, it carved a new shoreline; each time it fell, it left behind a terrace.

These terraces, along with older dune ridges, form the underlying framework of the region. They influence the river’s meanders, the elevation of nearby land, and the distribution of sediments you can observe today.

5. Cutting the Modern Outlet to the Atlantic (Holocene)

As sea level rose toward modern levels, the St. Johns River eventually breached a low point in the coastal dune system and carved an outlet toward what is now Mayport. This created the modern estuary and allowed tidal processes to shape the lower river valley.

The Atlantic flooded the lower basin, transforming the river into a tidal estuary influenced by twice‑daily ocean tides.

6. Tidal Widening and Estuary Evolution (Holocene to Present)

Once connected to the ocean, tidal forces widened and deepened the river mouth. Natural tidal scouring shaped the estuary for thousands of years. In the last 150 years, human activities — dredging, channel straightening, jetty construction — have accelerated these changes.

Even so, the estuary remains fundamentally a product of Holocene sea‑level rise and coastal geomorphology.

Sources:

This Earthcache utilized Microsoft Copilot to sprinkle in emojis and make text easier for kids and families to read, helping everyone enjoy the park even more! .  For more detailed information on these topics visit these websites:

https://en.wikipedia.org/wiki/Geology_of_Florida

https://en.wikipedia.org/wiki/St._Johns_River 

https://www.sjrwmd.com/waterways/st-johns-river/

https://sjrr.domains.unf.edu/st-johns-river-basin-landscape/

https://indianriverswcd.org/docs/References/SJ90-10_TechPub.pdf 

https://sjrda.stuchalk.domains.unf.edu/files/content/sjrda_120.pdf