Geology question ....

...(I think🤷‍♀️).

So, today we went to the beach. Socially distancing of course. The tide was out but we could see the shore line so I decided to walk over to it. Turns out it was further away than I thought and ended up knee deep in bloody quicksand!

I escaped obviously, after a bit of panicking, but my question is, why was my lower leg and foot black? It looked just Like it had been painted with black paint?

As I was walking out on the damp sand the was residue of what looked like black glitter. What is this ?


.....EDIT.....

This was at Brean Sands in north Somerset, England.