When we dug the first one, we hit what we thought was cobblestone about a foot down.
It turned out there was an underground schist wall that ran down the client’s property, directly under where the fence would be built.
The wall went deeper than the 3 feet that we had to dig to avoid frost heave, and the client’s 2-stroke honda auger might as well have been a dollar store corkscrew against it.
We manually dug 8-10 holes, then rented a jackhammer for the rest, until the wall eventually ended.