Why Do Cliffs Collapse?

This is a re-production of an article from the July 2017 issue of our Go Jurassic! Rangers newsletter.

It was written in response to the major rock fall at West Bay the previous month.


 

sam scriven avatarSome of you may have seen or heard about the massive rock fall at West Bay last month. Here, our Jurassic Coast Trust expert Sam Scriven explains why it happened!

The Jurassic Coast is comprised of natural cliffs, and change is a part of their very nature. Erosion and weathering of the rocks happens naturally, and over time those processes weaken the rocks and they collapse. I get asked a lot about rock falls and when the next one will happen, and my answer is always, “There WILL be one, but I don’t know when or where”.

The processes involved are so random and unpredictable that it’s impossible to predict the next big fall. All we can do is advise people to enjoy the coast safely, by staying away from the tops or bottoms of the cliffs.

The West Bay rock fall as seen from overhead. Not your average day’s golfing!

Thanks to James Loveridge  for the incredible snaps!

Why West Bay?

West Bay’s rocks are fairly hard sandstone, which tends to move all at once rather than in a slow, sludgy fashion like the softer mudstone rocks of Charmouth. The sea eats away at the bottom of the cliffs, and eventually gravity takes its course and the top of the cliff falls down.

This latest rock fall was partially caused by natural lines of weakness in the rocks, known as conjugate joints. These are long, thin cracks that run into the cliff face from the sea, and where two of these lines meet, it carves out a natural slice of the cliff. Just like cutting a cake!

These conjugate joints inside the rocks, coupled with erosion from the sea eating away at the cliff’s bottom, can create big rock falls like this one, which went so far inland that it took out a whole section of the coast path!

rock fall butterMake Your Own Conjugate Joints!

Take a stick of butter from the fridge, and cut two intersecting diagonal lines through its middle. These are your conjugate joints. Now press down on the top of the butter with your finger, replicating the natural tug of gravity on a cliff, but not so hard as to poke through the butter.

After a few seconds the whole top section of the butter will slide off, just like with a real cliff!

Leave a Reply

Your email address will not be published. Required fields are marked *