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> patches of black soil instead of the usual acidic
This is a well-known fact:
https://en.wikipedia.org/wiki/Terra_preta
> rectangular building remains
It's hard to find credible sources about building remains on the Internet. Some photographs of pyramid-shaped hills are popular but those are natural formations.
There are also claims that the concentration of human-usable trees (like fruit-bearing species) is higher than what one would expect for a natural forest and this concentration is highest around ancient points of denser settlement. This could be because ancient Amazonian populations molded their landscape into something more convenient to themselves.