There are numerous natural arches formed by the subsurface weathering of the strata, collapsed caves or sinkholes which leave the more solid ridge intact. Google up some natural arches and you’ll find some great photos.
True....but all the same, I think there's a bit more complexity involved in a manufactured archway than there is in a naturally-occurring one. Complexity involves more than just surface appearance.
See my other posts. You are making an apples and oranges comparison. The fact that soft material can erode and cause a keyhole doesn’t prove that mother nature can reproduce the Arch de Triumph.
The question is, if you were to find an arch such as the Arch de Triumph on Mars, would you explain it away as a natural occurance of hideously minute statistical probablity OR would you instantly conclude that intelligence was involved?
It seems clear to me that the former requires more faith than the latter.
I leave it to you to determine which is more complex, a stone arch or a man.