The Arch de Treetrunk
There was some discussion from many readers about yesterday's blog about whether a jack arch is actually a true arch. The answer is yes it. This is because the line of compression between the stones travels in some sort of arc even though the voussoir stones look flat on the bottom.
As with this arch that I constructed between two tree trunks last summer, even though the 'arch' seems to be more of a droopy flattened inverted V, the 'stress' is still displaced through the stones in an arched shape.