“ROCK & ROLLING WAVES”: ROCK EROSION ON ABACO

On 26th October 2012, the ‘roof’ of the geological feature at the southern tip of Abaco known for centuries as Hole-in-the-Wall was blasted apart by Hurricane Sandy. The combination of huge waves and powerful winds proved too much for a structure that had existed since the last ice age. I posted in detail about this event last year HERE so now I will concentrate on some geological aspects.

The photo below shows the Hole a year or so before its destruction. At the left-hand end of the bridge there is a paler area underneath where limestone rock had recently fallen into the sea, suggesting an underlying instability. At close quarters there were plenty of cracks in the apparently solid structure. (I’ve no idea why I didn’t straighten the image. Let’s say… Artistic Perspective)

This photo was taken a few days after the collapse, the first to be published of the new view. The clean white rock at either end of the ex-bridge is clearly visible. The section on the left became a new islet at the southern extremity Abaco, separated from the mainland for the first time in history. I proposed various names for it (not all serious) – Sandy Cay (or Isle) was the clear, indeed obvious, winner…

This close-up of the south side of the bridge shows the detail of the rock fragmentation. Tara’s photographs are (as far as I know) the very first of the new gap and of Abaco’s new islet, predating John Haestad’s by a day.

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We recently went to a beach near Crossing Rocks that we had never explored before (plenty of interest there for later posts). We were struck by the fragmented and pitted nature of the limestone rocks on the shoreline. We noticed some pale exposed rock surfaces, similar to those at Hole-in-the-Wall. We watched the incoming waves scouring and undercutting the rocks, leaving overhangs (see video below). Erosion is increased by the double action of the waves. The immediate area hinted that there may even have been a very small Hole at one time. On the day we were there, the sea was gentle. It’s easy to imagine how the hugely destructive force of the wind and crashing waves resulting from hurricanes such as Sandy can eventually shatter solid rock that has been gradually eroded and destabilised over centuries by wave action.

Photo credits as shown / RH. You have Mrs RH to thank for a shorter (50 secs) video than the one I originally made (75 secs). Now I’ve redone the thing, I agree – one wave does look very like the next… 

ADDENDUM Just remembered that I took an aerial photo of HitW from the plane the other day. It’s a phone photo through BahamasAir window glass, and I’ve had to tweak it a bit. It shows the relevant area…all the blurry dark stuff is part the pine forest of the National Park 

ADDENDUM 2 In response to a comment about the power of hurricanes, here is an image I have posted previously of a large rock some way out from the Delphi beach. Before Hurricane Irene in 2011, it was a solid rectangular slab. This is what it was like after Irene had moved on northwards