If you had a volcano that was predictable, so you knew where the lava was going to flow, you put your forms in its path and wait. Maybe the forms are made from something like plywood sheets that end up getting burnt completely, but leave blocks that fit snugly along that space where the plywood was before. Maybe the dimples on so many of the blocks are where there was a vent in the form allowing air to escape so the form fills completely.

A lot of these stones are basalt, which is volcanic.