Know before you go:
At Ground Zero of this EarthCache, you will see an enormous limestone platform, with the Devil’s Bridge visible in the distance. Please use caution around the rock at your discretion. For safety, remain a safe distance from both the bridge and the ocean’s edge. The Atlantic waves here can be extremely powerful, and the wave splash frequently surges up onto the limestone platform, especially during rough seas. Sudden waves can reach areas that appear dry only moments before. All EarthCache questions can be observed from a safe location hundreds of feet away from the cliff edge, so there is no need to approach the bridge or the cliff to complete this EarthCache.
To claim your find for this EarthCache, please complete the following logging tasks and send your answers to me through Message Center. The goal is to observe and learn from the geology around you, so have fun exploring and answer each question to the best of your ability! You may log the EarthCache immediately after sending your answers; there’s no need to wait for a response from me. If I have any concerns or questions about your answers, I’ll reach out to you.
Logging Tasks:
- At the GZ, observe the limestone you are standing on and the rock formations nearby. What are some visible signs of erosion? Describe what you see.
- While viewing the potholes from a safe distance away, estimate the average diameter of the potholes at the GZ. What shape(s) do the potholes appear to be (circles, ovals,
- Based on what your observations at the GZ and what you’ve learned about potholes in the description, explain in your own words how wave sediment could gradually carve potholes into limestone over time.
- Imagine how the landscape may change over thousands of years. Based on the erosion you observe today, how do you think the landscape will change (the limestone platform and Devil’s Bridge arch) if erosion continues?
- As of June 2019, earthcaches contain required photo logging tasks. Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site. Please post this in your log.
Geology of Antigua:
The geology of Antigua varies across the island due to its volcanic origins and to limestone formed from corals in the Late Oligocene. Igneous volcanic rock is heavily present in the mountainous southwest. In the central part of the island and on the north side near St. Johns, sedimentary rock is common, including limestone, siliciclastic rock, and fossils such as gastropods preserved in chert. The northeast region of the island, which this EarthCache is a part of, is known as the Antigua Formation. Millions of years of accumulation and compaction of marine debris, such as scleractinian corals, crinoids, sponges, and mollusks, have formed the limestone present in this region today. The Devil’s Bridge is composed of limestone.
What are potholes?
Geological potholes are depressions that form in solid bedrock through the persistent action of moving water and sediment. They develop when water currents trap small rocks, pebbles, or sand within a natural crack or shallow depression in the rock. As water moves, these sediments can swirl and slowly grind against the bedrock through abrasion. Over time, this grinding deepens and widens the hole, producing the smooth, rounded cavities known as potholes.
Potholes are commonly found in environments with strong water movement. In riverbeds, fast-flowing currents spin trapped stones against the bedrock, slowly drilling into the rock surface. Along rocky coastlines, powerful wave action can create similar circular grinding motions when water and sediment become trapped in depressions within the rock platform. In turbulent water, small sediment becomes suspended in the water column. Wave action can transport this sediment and cause erosion, forming potholes. In places like Devil’s Bridge in Antigua, constant Atlantic wave energy moves sand and small stones across the limestone surface, gradually carving potholes into the coastal rock through repeated erosion.
References:
https://depositsmag.com/2017/07/06/the-geology-of-islands/
https://pages.mtu.edu/~raman/SilverI/The_Fault/Potholes.html
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