What do you think this is?

[ErrolFlynn]

537500

Hint: It's not a rock structure you might find at the beach. Though, it could be.

https://www.youtube.com/watch?v=pmyIreIZItQ

[ForeverYoung]

reminds of flint that you see in those rubble wall constructions in the UK.

[ErrolFlynn]

Rubble is a good choice of word.

I hope the pebbles were picked up from a river rather than the beach. Using pebbles seems so wrong for concrete. Too smooth. I wonder if that's why it fell away.