I visited Barbados on a cruise in January 2019, and stopped at this location to find a traditional cache (https://coord.info/GC27WGT). As I looked up towards the top of the nearby hill and took in the sight of the amazing geology, I said to myself “this place definitely deserves an Earthcache!” And so, this is it! I hope you enjoy visiting this spot as much as I did!
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Barbados Geology
Barbados is the easternmost island in the Caribbean and is relatively flat in comparison to its more-mountainous island neighbours to the west. The reason Barbados differs from these neighbouring islands in the Lesser Antilles magmatic arc (the semicircular string of islands in the southeastern Caribbean), is because it is not a volcanic island. The only volcanic rocks on Barbados are actually some ash beds that came from eruptions on nearby islands.
The geology of the island of Barbados is pre-Pleistocene. The Pleistocene era is more commonly referred to as the Ice Age, and lasted from about 2.5 million to 11,700 years ago. So, essentially the “rocks” you see on Barbados predate the last ice age. The landscape of Barbados includes exposures of reef-related carbonate rocks spanning 85 percent of the island's surface, and in places this coral rock formation is 70 meters thick. The oldest rocks are in the “Scotland Formation” and include Eocene age (from 56 to 33.9 million years ago) turbidite (deep ocean sediment deposits resulting from gravity flow) and radiolarites (hard, fine-grained sedimentary rock that is composed predominantly of the microscopic remains of “radiolarians” – i.e. zooplankton)
The island of Barbados is part of what is called the Barbados Ridge Accretionary Prism, exposed as deep ocean sediments were "scraped" to the surface as the Atlantic oceanic crust subducted beneath the Caribbean Plate. The process of accretion involves the addition of material to a tectonic plate. When two tectonic plates collide, one of the plates may slide under the other, a process known as subduction. The plate which is being subducted is floating on the asthenosphere - the highly viscous and deforming region of the upper mantle of the Earth that lies at depths between approximately 80 and 200 km below the surface. This plate is pushed against the other, over-riding plate. Sediment on the ocean floor will often be “scraped” from the subducting plate. This causes the sediment to accumulate as a mass of material called an accretionary prism (or wedge), which attaches itself to the upper plate.
Although such accretionary wedges exist at subduction zones around the world, they are usually submerged. Barbados is one of a few places on the planet where enough material has piled up to push the accreted sediments above sea level. These unusual circumstances make Barbados one of the best places in the world to determine the history of sea-level rise and fall during the last 700,000 years.
Volcanic island arcs such as in the Caribbean may collide with the continent, and as they are of relatively light material (i.e. low density) they will often not be subducted, but are added to the side of the continent.
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Chalky Mount Geology
Under Barbados, the accretionary wedge’s waterlogged mud forms a diapir (not to be confused with a diaper – which is also full of nasty goo), a blob of sediment that is less dense than the surrounding rock and thus rises toward the surface like a bubble in a lava lamp. The island emerged from the waves about 700,000 years ago when this diapir began to force up the great sediment pile, which it continues to push ever higher. Although the uplift rate varies from place to place, on average the island is rising about 30 centimeters every thousand years.
In the western half of the island of Barbados the land rises gently from coastal lowlands into rolling hills. Beyond those hills in the central highland region stands the island's highest point, Mount Hillaby, at 340 meters above sea level. In the eastern third of Barbados, the landscape rises sharply into low hills that shadow the coastline. Perhaps the most striking and interesting of these hills is Chalky Mount – the beautiful place where you now find yourself. Imagine the immense forces it took to create and push this hill up from the sea floor!
Your task for this Earthcache is to make some observations of the amazing geology of the area and report your answers for the following questions:
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As you stand at the posted coordinates by the road (there is no need to enter the fragile and sensitive area up the hill), take a look towards the hilltop - forming a jagged profile against the horizon. Describe the layers of rock that you see: how thick are they, are they straight or folded, what colours are the layers?
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Why do you suppose this hill is called “Chalky Mount”? How high do you think the hilltop is?
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Closer to the road are exposed areas of very different-looking rock/material. See the reference point coordinates (N13° 14.572' W059° 32.899'). Please describe these also (colour, texture, size…). What type of rock/material do these appear to be made of? This area of Barbados is historically known for its pottery – could this be related?
Photos are very welcome, although not mandatory. Thanks!
Please send your answers via the “Message this owner” link, and do not include them in your online log.
For anyone interested in hiking up to the top of Chalky Mount, the parking for the trailhead is not far away. N13° 14.400' W059° 32.782'
Credits:
Earth Magazine, Travels in Geology: Barbados: Ascending the sea-level staircase
Barbados Pocket Guide
Geology of Barbados -- a Brief Account of the Island's Origin and Its Major Geological Features
Wikipedia
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