Wallilabou Natural Arch EarthCache

Wallilabou Natural Arch

Wallilabou Bay

Wallilabou Bay is a bay on the west coast of the island of St. Vincent in the Caribbean. The name comes from the nearby village of Wallilabou. The bay was a filming location for the American film "Pirates of the Caribbean" (2003) and served as the fictional harbor "Port Royal". After filming, the sets were largely left indisturbed and are still there.

The location was chosen for an Earthcache because of the extraordinary sea arch at the northern end of the bay. Incidentally, the sea arch was also part of the filming of the aforementioned film. Getting closer to the sea arch would require a boat, but the Earthcache coordinates offer a good view of the arch, especially with binoculars, and good photos are also possible with a telephoto lens. Solving the Earthcache does not require a boat; just a visit to the bay.

Natural Arch - Natural Bridge - Sea Arch

A natural arch, natural bridge or rock arch is a landform where an arch has formed with an opening underneath. Natural arches commonly form where inland cliffs, coastal cliffs, fins or stacks are subject to erosion from the sea, rivers or weathering. Most natural arches are formed from narrow fins and sea sea stacks composed of sandstone or limestone (here in Wallilabou bay it is lava stone) with steep, often vertical cliff faces. The formations become narrower due to erosion over geologic time scales. The softer rock stratum erodes away creating rock shelters, or alcoves, on opposite sides of the formation beneath the relatively harder stratum, or caprock, above it. The alcoves erode further into the formation eventually meeting underneath the harder caprock layer, thus creating an arch. The erosional processes exploit weaknesses in the softer rock layers making cracks larger and removing material more quickly than the caprock. However, the caprock itself continues to erode after an arch has formed, which will ultimately lead to collapse.

The choice between natural bridge and natural arch is somewhat arbitrary - the Natural Arch and Bridge Society identifies a bridge as a subtype of arch that is primarily water-formed.

On coasts two different types of arches can form depending on the geology. On discordant coastlines rock types run at 90 degrees to the coast. Wave refraction concentrates the wave energy on the headland, and an arch forms when caves break through the headland. When these arches collapse, they form sea stacks and stumps.
On concordant coastlines rock types run parallel to the coastline, with weak rock protected by stronger rock. The wave action along concordant coastlines breaks through the strong rock and then erodes the weak rock very quickly. Sea coves are a reasult of this form of erosion.



When a cave forms in a headland it may erode through and erode on the other side. This forms an arch-shaped feature, called a sea arch. As the cave grows in size the arch becomes more impressive, often looking like a doorway through the headland. Eventually the arch won't be able to support itself, the top of the arch will collapse, and the seaward end will be seperated from the headland to form a sea stack. 

Your task to log the Earthcache:

Answer the following questions via message in English or German via my geocaching profile:

1.) Describe in your own words how you think the natural arch was formed!
2.) Will the arch change over time? Why? What shape might it take?
3.) Look to the sand to your feet and compare it with the rock of the arch - do you think, it is the same material? Justify your opinion! What rock might it be?
4.) Optional: Post a photo with your log, showing you and/or your GPS/or something identifying you near the location!


After you've sent me the message with your answers, feel free to log! If there's something wrong, I'll contact you!

The most exciting way to learn about the Earth and its processes is to get into the outdoors and experience it first-hand. Visiting an Earthcache is a great outdoor activity the whole family can enjoy. An Earthcache is a special place that people can visit to learn about a unique geoscience feature or aspect of our Earth. Earthcaches include a set of educational notes and the details about where to find the location (latitude and longitude). Visitors to Earthcaches can see how our planet has been shaped by geological processes, how we manage the resources and how scientists gather evidence to learn about the Earth. To find out more click HERE.