Dunkeld bridge: thinking about arches

Having written about the flying buttress at Tewkesbury I was sat looking at Thomas Telford's bridge across the River Tay in Perthshire wondering how I'd describe the forces acting on an arch. Start with the central stone. Gravitational attraction is pulling it downwards but it doesn't fall. There must be an equal and opposite push back upwards. This comes from the stones on either side. These are angled so that they have a vertical component pushing up on the block. But being angled, the block has a horizontal component pushing sideways on the blocks trying to push them out of the way. This is repeated on stone after stone until the little bit of extra angle each time has meant that what started out as a horizontal push by the middle block on the two either side has become a vertical force. The vertical load in the middle was transferred horizontally and became a vertical force at the sides. I was also thinking about the frictional forces between the stones and how much that acts to keep them in place. I had long wondered how you built an arch but discovered some years ago that they used wooden frameworks to hold the stones as they put them in.