local selection = 1
turtle.select(1)

local function placeFence()
  turtle.digDown()
  turtle.placeDown()
  if turtle.getItemCount(selection) == 0 then
    selection = selection + 1
    turtle.select(selection)
  end
end

placeFence()
for i = 1, 4 do
  for i = 1, 63 do
    turtle.dig()
    turtle.forward()
    placeFence()
  end
  turtle.turnLeft()
end