Obviously something as large as the Grand Canyon could not be made by man – and difficult to believe the work of visiting aliens. We are looking at a construction that conforms to an unnatural pattern yet is semi-natural?
You might like to check out, and comment, on my post "Mysterious geographical features of South Australia". My suggestion is that there has been transfers from, and too, other dimensions of space and time. We got the pyramids, stone balls of Costa Rica, ancient rice terraces, Chocolate Mountains in Bohol Philippines, underwater cities, Nazca Lines etc., etc. – and they got Atlantis!
The concept seems too far-fetched but if we can conceive our planet as a living system then it relates to what we already know about living biological systems. Genes, and wholes sections of DNA, will transfer from one place on the chromosome to another. Transfers happen between one physical body to another – and even between species.
If our planet is a living and reproducing system then surely it is reasonable to contemplate transfers of physical matter from this dimension of space and time – to dimensions of space and time we cannot see? Such transfers are likely a necessary part of a universal living system – as they are a necessary part of earthly living systems.