Originally Posted by Jodie
It would depend on how far back you went in time looking for evidence.
I've seen debotage from 15,000 years ago.
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Water has oxygen and carbon dioxide in it that would facilitate the formation of rust on iron, I would think, if it's sea water I imagine the salt would increase the rate of decay. After 10,000 years, for example, I don't think anything iron would be left regardless of where it was.
The degradation of metals in salt water is rather well-known. Finding high concentrations of iron-rich minerals (hemetite, geotite, etc), copper-rich minerals (azurite, malechite, etc), and other materials in areas where they should not be according to the geologic data would be a strong indication that something weird is going on. Add foundation exacations, which are extremely obvious (I've seen wells in cross-section, and can attest to the fact that they stick out like a lighthouse), and the conclusion becomes obvious.
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If you are talking pre-iceage wouldn't the receding glaciers destroy any evidence of foundations or anything to do with a civilization for that matter?
Glaciers don't work that way. They scour some portions of the area down pretty far, but they also fill some in. So it really depends on where the buildings were, and what the glaciers did in those areas. Glaciers are huge, complex systems, and it's far, far more complex than just "Glaciers go through, scrape everything off". You also have to deal with mines, which will be even deeper than foundations (if it's not grown, it's mined).
Also, if they scrape out the material it has to go somewhere--morains, till, outwashes, etc. It's not that hard to identify where the materials in those formations came from, and what they were previously.
This illustrates another reason why ancient aliens are popular: people simply don't understand geology, and don't know what we can know and what we can't about the past. No insult to Jodie; this stuff is HARD, far harder than people give it credit for. The problem is, it LOOKS easy--so people look for an easy explanation. When they can't find it, they make up stories, because figuring out the actual explanation frequently requires technology well beyond your average person on the street (mass spectrometers, for example).