This isn't a "fence" question per se, but I wasn't sure where else to ask it.

I'm having a problem putting in a post. Quick background: the local town did a street drainage project a few years ago, and as a part of it put in a few dry wells in my side yard. They look ugly, and to hide the closer one I'm making a faux wishing well--a circle of retaining wall bricks, with a roofed top.

I picked up a couple of 4x4s for the roof supports and started digging holes; because of the layout, I'm really limited in where I can put them. I got no more than 6" down when I hit rock. Or something. I have no idea what it is; it's pretty flat and smooth; I banged at it with a stone chisel and tiny bits flew off, though it's hard to tell and I can't really get to it easily, so I'm not really sure what it's made of.

I made the hole a little bigger to see if I could find an edge but I couldn't; I don't want to make it too big because I don't want to undermine the nearby stack of stones in the well, and because a big hole would undermine the point of a post-hole digger.

So I'm not sure what to do at this point. I can't relocate the hole; should I get a masonry bit to drill holes to bolt a metal post bracket in place? ... that's assuming this is something I can even drill into.

FWIW, this is the second hole; with the first hole, I dug down about a foot and hit water (really high water table). Not sure where to go with that, either.

 

Hmmm. Edit button gone, so I guess I waited to long to edit.

Found that I can't use a post bracket bolted to the concrete: they don't provide enough lateral strength, so without some sort of top support, it'll fall over easily.

 

If it's flat I would assume you've found concrete. Could it be part of the city's drainage project? If so I would not attempt drilling into it or chipping away at it. If you damage it you may be stuck with a hefty bill repairing it.

Since you can only dig about 6" I would abandon anchoring the posts in the ground. I would consider a new decorative design or build your wishing well as a complete, portable structure that can just sit on top of the ground.

 

As best as I can figure it’s the base of a gazebo that was torn down 45+ years ago. Definitely nothing to do with the town.

 

Well, OK....
Building this as a portable structure is absolutely unworkable. I'm on the coast and we get high winds here as well as coastal flooding, so it would have a VERY short life that way.
Here's what I went for:
I drilled a 6" deep hole in the bottom of the 4x4, parallel to its length. I used that to mark a matching hole in the concrete and drilled in 3"; a 1/2" stainless steel rod went in place. In the stacked stone (actually, formed concrete) of the well, I drilled a series of 4 holes through three and a half levels--down halfway through the bottom level--and put foot-long 1/2" SS rod through. Then I used two levels of copper strapping to attach the 4x4 to the stone wall. Finally, I built a form and put in about 8" deep concrete around the pole. Not perfect, but attached to the other post (which is properly anchored) with a 2x4 crosspiece, I'm hoping it's good enough.

 

So, let me get this straight- your local town is worried about drainage,
but they installed a dry well over top of an old concrete gazebo foundation.
Did they drill a hole through the foundation for the dry well?

IIWY, i'd uncover the gazebo foundation and use it for patio/a fire pit area, perhaps with a pergola - if there IS a dry well, you should have plenty of water for climbing vines etc...

 

I'm not 100% sure if they cut through it or if it's on an edge. There's definitely a well there--see the first picture below. Where I tried to put the post (but hit the concrete) is just to this side of the rod-filled well wall at bottom (the bottom stones all have large gaps so that water can drain to the grate).

I can't uncover the slab. Quite a bit of other stuff is over it (one reason it was a surprise) and, from what I hear, it was taken down in part because it was in bad shape, the concrete included. I also don't have any use for a patio/fire pit area... and with coastal flooding, not much does well there. Tons of water... but salt. See pic 2--that's where I put the well.