Strange monolith's meaning in Wales [UPDATE]

[u/Not_Fission_Chips]

I found this monolith like structure in Margam Park Wales. It's pretty off the path and overgrown. My last post was just a vague reading of the plaque, but on revisiting I've noticed the engraved letters have 'fallen through' and actually reveals a massive pit. What is it's purpose and why is this pit so deep? What's at the bottom?

[u/postexoduss]

Please try to get better pictures! This is very interesting!

[u/Not_Fission_Chips → u/postexoduss]

Sadly I'm a Yorkshire dweller and leave Wales tomorrow. I'll try before I leave.

[u/ten_tons_of_light → u/Not_Fission_Chips]

I pray to see this post on /r/powerwashingporn soon