It was decided the underground chamber we are building should have an arch as well as lintels
This was partly for looks and partly as a kind of a nod to the French tradition of building relief arches in their 'cabanes en pierre sèche' in combination with lintel stones. The difference is these arches were usually built over the lintel openings and were filled in completely with stones. The arch directed the thrust of the weight of the dome roof away from the lintel.
I liked making this arch. I used local limestone and sandstone. I prefer to chisel the voussior shapes rather than saw them. The keystone was a challenge as I busted three before I was able to get this one into shape. It's made of a very difficult conglomerate stone which I found in the pile of bedrock extracted from the hole. I was expecting it to bust into a dozen jagged pieces like the other limestone but it never showed any signs of cracking.
The final keystone fitting is always slow going, sliding it in, taking it out, chipping away the sides carefully and sliding it in again. The arch stones should all tighten up just before the moment keystone touches the form.
While I built the arch Norman had gone round the chamber adding the final courses of stones up to the top of lintel height. All the higher rows will begin to be corbelled into the centre of the circle creating a dome shape. Hopefully. No form will be used to create the dome.
A big furry visitor came just as I finished the arch.
He agreed to sit in the arch opening for a spontaneous photo op.