Stacks on stacks on (sea)stacks

Back in May I was invited to the BBC Cymru Wales Broadcasting House in Llandaff, Cardiff, for a quick spot on Good Evening Wales (BBC Radio Wales FM), the “drivetime round-up of the day’s news, sport, weather and travel.” I was asked to comment on a story that had appeared earlier that day in the Western Mail (WalesOnline) about a newly formed sea stack near Southerndown, in the Vale of Glamorgan, about 25 miles west of Cardiff. (My  colleague José Constantine was quoted in the write-up.)

A local rambler (as intrepid trail hikers are known here) named Anthony Jones reported what he noticed was an unfamiliar rock pillar in an otherwise familiar section of the coastal path. (Yet another example of patient, repeated observation revealing something magical.) Interbedded limestone and shale/mudstone strata comprise the Southerndown cliffs, lending them a striking, striped appearance. (For their geological importance, they’re also an official Site of Special Scientific Interest.) In terms of texture, however, climbers might describe long stretches of these cliffs as “chossy” – likely to come apart in your hands, under your feet, or on your head. But what’s bad for climbing can be good for stack-making.

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Reporter Peter Collins’ UAV (unmanned aerial vehicle, or drone) video is worth watching a few times – the third, fifth, and sixth images in the photo gallery offer a useful, if vertiginous, perspective, too. Something I didn’t say on the radio: I’m not convinced that waves are the primary agent of weathering in this case. The footage shows that the base of the stack sits above the uppermost shingle shelf, suggesting that the feature is only rarely within reach of wave action (albeit when wave forcing is heavy). You can see where rock and dirt appears to form a saddle between what’s now the stack and the cliff edge behind it. Cliff rocks on the west side of the feature (the right side, looking out to sea) and their grizzle of vegetation look largely intact. Unlike in the case of a collapsed arch, waves don’t seem to have undercut the structure at sea level. What seems likely to me is that pore pressure from groundwater in the cliffs themselves, forced by heavy rains (not just from last winter, but cumulatively), effectively pushed these rocks apart. The stack is essentially a landslide feature – at least for now.

Wave-driven cliff erosion is still a process in play here. Supplying sediment to the cliff toe is like giving the waves stone tools. Imagine you’re erosion: if battering the cliff with water feels good, battering it with water and rocks feels even better. However, this physical-process feedback doesn’t pass the Mae West Test (in which “too much of a good thing can be wonderful”) – no beach means no tools, but too wide a beach and the cliff gets insulated from wave erosion. (Colleagues Patrick Limber, Brad Murray, Evan Goldstein, and others recently published a detailed pair of papers in JGR Earth Surface on rocky coastline evolution; I also applied this “tools” idea to seawalls in an Earth Surface Dynamics paper this year.) Cliff failure along rocky coasts is a critical means by which sediment is supplied to the shoreline. The cumulative action of alongshore currents, driven by waves, redistributes that sediment up and down the coast, feeding sand into pocket beaches and dune systems and creating littoral zone habitats. Waves may yet do their work on this new Southerndown stack (check out this clangorous Daily Mail dispatch from last winter), but groundwater seems to have made the first move.

A final aside – the radio exercise was interesting for a couple of reasons. First, I was there to comment on a phenomenon I could only interpret from imagery. (Anyone working with reconnaissance data from other planets – anywhere you can’t pop by for a field visit – does this all the time. Military intelligence branches do it, too.) Second, my role on the show was to explain what I saw in a way that made sense to a general audience. I’ve had media training, but I haven’t had a lot of real-time practice. Complete sentences become surprisingly hard to formulate. I stammered, despite telling myself I wouldn’t stammer. I had notes in front of me, but I hadn’t worked aloud through the lines I’d drafted for myself. Being in the studio was fun – I’d love to go again – but cogent articulation on-air is a real skill.

A final, final aside – look here to learn a little more about The Old Man of Hoy (pictured in the gallery above).

A final, final, final aside – I gotta get me a UAV.

 

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