[SampleMan]

If I were so inclined I would say it was designed upon seeing one, yet we see such arches created by natural processes.

I've never seen such an arch naturally created and neither have you. There are cut outs in rock, but that is hardly an arched structure of independent aligned stones dependent upon a perfectly place keystone. Manmade arches are supported during construction by other bracing, precisely because the parts won't stay in place until they are all in position. What would you conclude upon seeing the equivalent of the Arch de Triumph on Mars? That it was naturally occurring? Its statistically impossible to disprove.

[FormerRep]

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.

[Pistolshot]

I've never seen such an arch naturally created and neither have you.

Really?

[antiRepublicrat]

I've never seen such an arch naturally created and neither have you.

Oh don't give me that. I wasn't home for the rain yesterday, but I'm pretty sure that's what caused the erosion in the dirt portion of my back yard. Do you have a better explanation for the erosion? Or should I just say "God caused that erosion magically appear."?

Manmade arches are supported during construction by other bracing, precisely because the parts won't stay in place until they are all in position.

That is our method of construction. It's quick and efficient with materials. We tend to build up.

Nature doesn't have to be quick. Take a bunch of rocks and dirt, or some solid rock, run a stream under the middle. Make the stream bigger, let it start dislodging some rocks, let it flood at times. Eventually there's a good chance you'll have formed an arch as the remaining rocks compress against each other as they try to fall down. It's not going to happen every time of course, but out of the many instances with these conditions you'll get a lot of arches. This is also known as a natural bridge. Arches also naturally occur on coastlines and in caves.

I used to let the waves create these natural arches on the beach when I was a kid. It's pretty easy.

What would you conclude upon seeing the equivalent of the Arch de Triumph on Mars

Ask me again when I actually see it.

[ElectricStrawberry]

So because it takes longer than I can observe to create an arch naturally......that's supposed to mean I cannot have the valid and highly probable scientifically based hypothesis that water and/or wind erosion created the arch?

I'd stay away from "arches have keystones" argument if I were you. I see no keystone: