Monday, 15 October 2012

Fossilised giant that replays ancient sea-levels

This weekend I visited Cayman Brac, a small island a mere 90 miles northeast of Grand Cayman. One of the largest fossil structures that can exist on a continent is preserved here on these azure shores. This is no giant sauropod or bleached bones of an enormous whale, but the echo of an ancient shoreline. Yes, a beach can become a fossil, if you know where to look!

The island, like the rest of the Cayman Islands, is there due to the same submarine ridge that forms the carbonate backbone of these coral-aproned islands. The first thing that you notice about the 'Brac' (a gaelic word for bluff) is the relatively high and sheer limestone cliffs that hoists the main portion of the island ~100 feet above sea level.

As you stand on the road that follows the southern shore of the island, you cannot help but notice the land-shapes or geomorphology. Facing west along the line of the Brac, you stand on what is clearly an almost level surface that gently dips to the south by a few degrees into the beach area and tidal zone.  Facing north, you are met with an almost sheer cliff of the Brac, that runs from east to west, that meets the platform on which you are stood at a sharp 90 degree angle. The base of the cliffs are hidden by dense vegetation, that you have to scramble through to see a geomorphological feature that has a curious explanation. When you reach the cliff base, you see there is a concave notch, seemingly cut-out from the base of the cliff, that runs the whole length of the Brac...both north and south facing walls.

To understand what this feature represents, you only have to walk towards the present-day shoreline of Cayman Brac and watch the waves pound the limestone shoreline. The constant wave action slowly carves a neet concave ridge, just above the gentle slope of the wave-cut platform that allows the waves to continuously roll in from the Caribbean Sea. The feature that so defines this high wall of the Brac, is simply a function of wave action from an earlier time, when sea-level was higher than present day. Around 120,000 years ago the globe was a tad warmer and was basking in the sun of a interglacial phase...literally 'between ice'. The water that was not locked-up in ice-caps caused a global rise in sea-level of a few metres, raising the ancient wave-action to cut the notch at the base of the Brac....what we see today. The subsequent glacial phase, in which we still live, resulted in a global fall in sea-level...stranding the wave-cut platform and distinctive notch as a vast, fossil beach....even a beach can become a fossil...and you too can find them where ever you are around the world, you just have to know what you are looking for!

My next blog will explore the caves and wildlife of Cayman Brac.

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