– – – Exploring The Countryside and Lanes of Dorset – – –
My ninth quirky thing about Dorset is in fact a natural phenomenon that occurs in various places and in various conditions throughout the world, and we have one such place right here in Dorset. This phenomenon is known as Beach Cusps.
Beach cusps occur in places along the coast and are patterns on the beach consisting of regularly shaped small ‘bays’ separated by horns of higher sand or shingle which point out to sea. They are most noticeable as the tide washes in and out with the surf separating into tongues as it washes up into the ‘bays’. This gives the appearance of cog wheel teeth. On the Dorset coast, Man o’ War Bay is a good place to spot them.
The cause of Beach Cusps is something that has been debated for 50 years with no definite resolution. There are two main schools of thought. One suggests that they are caused by the action of two sets of waves coming together, the main waves coming into shore and secondary waves that are created and run across the shoreline. It is the meeting of these two opposing forces that creates the cusps. The second school of thought suggests that any beach has natural undulations and the effect of the waves on these exaggerates and evens out these undulations, making them more regular.
Whichever theory is right, the phenomenon tends to occur on steeper beaches of coarser material such as shingle and grit, and where the waves are reasonably sizeable. Usually the cusps are a few meters long as in these at Man o’ War Bay, but they can be much larger. And once they are there, they become self sustaining as the waves continue to drive the coarser material onto the horns and then erode the finer material of the ‘bays’ as they flow out again. I think the picture below gives a fairly clear illustration of this.
I find the effect of these Beach Cusps fascinating. It is not something that you see everywhere and even along this part of the Dorset coast they are not evident in many bays. It seems almost as if Man o’ War Bay has something unique about it which allows these to form. As you can see in the middle picture, even the next bay along, St Oswald’s Bay, doesn’t have them.
Now that’s quirky 🙂 !
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