3 May 2017
Storm beaches have I loved
Posted by Callan Bentley
This morning, I riddled you this:
I got a few guesses on Twitter, none entered as comments on the blog, but a slew of hearty, enthusiastic conversation on Facebook. That’s where the people are, I guess. Many of them came up with great ideas to explain this odd scene of big angular rocks lying on top of what appears to be a “lawn” of grass…. Time to reveal what’s really going on!
That scene is on the Eshaness coast of mainland Shetland, just north of Dore Holm, and just south of the Eshaness lighthouse:
It’s a marine terrace with a thick mattress of turf developed atop it.
…And as any four year old will notice, there are rocks lying all over the grass there!
This is a striking fact, because the grassy marine terrace is somewhere around 20 meters above sea level. As you walk toward the cliff from the grass (don’t get too close!), you see the source for these boulders and cobbles: bedrock of Eshaness volcanics.
Some force is removing big chunks of that rock and relocating them 10-20 meters inland from the grass’s edge, a bit inland itself from the rocky precipice.
What force could that be? Well, if you’ve visited the Grind of the Navir, you might be predisposed to think about storm waves – immense, ungodly powerful storm waves, crashing into the cliffs and climbing up, reaching up 60 feet above their base, and smashing out rocks, tumbling them inland as the wave energy dissipates.
Hard to see a grassy plateau dotted with rocks as a “beach,” but I think that’s what this sedimentary deposit qualifies as: a storm beach. The blocks of rock here aren’t as big as those at the Grind, nor as concentrated, but I think it’s reasonable to infer they are more recently deposited than the grass they lay atop. And therefore I think the origin is roughly the same.
Here’s another example we saw a few days later, from the Yesnaby region of Orkney (perspective is looking to the south):
Click either one to enlarge
It may not be a coincidence that all three of these locations are on west-facing coasts of their islands. Things must get pretty hairy here come wintertime.