The island of Anguilla contains some of the most amazing stones in the Caribbean.
In this article, you’ll learn more about the geology of Anguilla.
Types of Rocks found in Anguilla
The information provided in this article by YesDirt.com is for informational purposes and is subject to change. Laws are updated. Accessibility guidelines and restrictions change. Be sure to confirm the land status and collection rules before you travel to an unfamiliar location or collect any material.
The island of Anguilla is a volcanic island that emerged from the ocean off the coast of South America.
The rocks that are commonly found there are limestone, basalt, tuff and quartz. You may find samples of brushite and lignite.
The island of Anguilla was formed as a part of the Lesser Antilles island arc.
These islands have a volcanic origin and all contain limestone as a result of being formed in warm salt water.
The island of Anguilla is less than 40 square miles. The island is between 2 and 3 miles wide and 15 miles long.
This unique geography makes it especially arid and vegetation is very sparse on the island.
Sombrero Island is part of Anguilla and is the northern edge of the Lesser Antilles.
Limestone formed from the remains of coral reef are often light in color and somewhat light for their size.
Limestone can vary in color from very light tan to dark gray, with lighter colors being more common on Anguilla.
Much of the island is covered in slabs of limestone.
Limestone is not a very hard stone with a hardness score of between 3 and 4 and can be scratched with the point of a knife.
As a result the stone weathers relatively quickly in the wind and rain.
You may see the weathered limestone around the many beautiful beaches on Anguilla.
It can be identified with the geological acid test in which a scientist uses a weak acid to see if the stone bubbles. LImestone will bubble.
You may also encounter fossils embedded into limestone.
These fossils may be shells or other plant or animal remains that have become embedded into the limestone while it was forming.
Fossils may look like holes in the limestone or places of discoloration or smoothness.
Limestone that constants multiple fossils can be referred to as fossiliferous limestone.
Most of the island is covered in limestone. There are two areas on the island where the volcanic foundation of the island are visible.
Crocus Bay and Road Bay are the only areas on the island where you can find exposed basalt.
Basalt is formed when low viscosity lava is cooled rapidly after being pushed to the surface.
These igneous rocks are less than 20% quarts and are very fine grained. They should have an even color and even texture, although you may find basalt with other minerals embedded into the stone.
Basalt is usually dark gray to black in color and may have a porous texture depending on how the basalt was formed.
In Anguilla if you are visiting Road bay you may observe the dark black or even purplish basalt with veins of calcite.
While basalt is made up of rapidly cooled lava, tuff is the rocks created from the volcanic ash.
The stones range in color from light brown to dark gray and have a hardness score between 4 and 6 on the Moh’s scale.
While there is plenty of variation, you should notice that the stone is very fine grained as it contains more than 75% fine volcanic dust.
The ash that formed the tuff present on Anguilla was actually created underwater. The interaction between the sea water and the basaltic lava created volcanic ash under water.
Those small particles become cemented together in open water or groundwater and become tuff.
Lignite is sometimes called “brown coal” and it has been recorded on the island as early as 1918.
As the name indicated lignite is a combustible stone.
Lignite forms as a result of the decompositions of peat in the same way black coal is composed of the remains of plant matter.
It has been called the most harmful fuel to human health as it produces more carbon dioxide with less heat as it burns.
This soft brown stone has obvious layers that can often be picked up on the beach in Anguilla. It is said to be more plentiful on the beaches between April and September.
Lignite has a very high moisture content and on Anguilla is often pulled from the sea. That high moisture content makes Lignite susceptible to spontaneous combustion.
If you find lignite on the beach, do not pack it home. It is a dangerous combustible material.
Brushite is a colorless crystal that can form on other rocks. Brushite has been found on Sombrero Island or Hat Island, the northernmost island of the Lesser Antilles.
Brushite forms very small and square crystals.
Brushite is often found in areas where guano is present and guano mining has been going on on Sombrero Island since the 1800s.
Brushite is formed when guano interacts with clay and calcite and crystalizes on old bones or other stone.
If you attempt to find brushite, be aware that there are some biological risks associated with being in close proximity to guano. You should not attempt this without a guide.
The minerals in Brushite are also the minerals that begin the formation of kidney stones.
Rocks on Anguilla
Anguilla is a small island that contains mainly limestone and basalt.
Like many islands in the Lesser Antilles the heat produced by the underlying volcanos draws calcium producing marine life which form the perfect conditions for limestone formation. The slow growth of the island through volcanic action pushes the limestone to the surface of the island.
When you are searching for interesting stones in Anguilla, I recommend that you begin at Road Bay or Curacao Bay. In these locations you can see the volcanic foundation and the sedimentary limestone.
A visit to Sombrero Island will bring you to the top of the Lesser Antilles and give you a chance to find a brushite sample.
Be sure to collect rocks only where it is permitted on the island to preserve the natural beauty for future generations.
There are also some safety concerns when collecting specimens like brushite or lignite which may have other hazards in transport.
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