But how was it determined that things were overturned?
Remembering back to something I read two or three decades ago, that is determined by the orientation of fossils and ephemeral markings contained within. There are a number of things that can only be buried top-side-up, so when you find them top-side-down then that would be an indication. Examples would include ripples on stream and lake/sea bottoms, animal tracks, plant root systems, animal burrows on the sea bottom. I seem to recall that trilobites normally get buried right-side-up.
The rock layers themselves can also offer evidence. Just that the layers are in the reverse order that you would expect from everywhere else they occur would be a clue that something may have happened. And layers that were formed by rapid depositation (ie, in a single depositation event) would be expected to exhibit a "peanut jar" distribution of materials with the larger particles on the bottom of the layer, since they would have settled out first, and graduating up to the finer particles which would have settled out last -- ie, actual hydrodynamic sorting. If you find a layer where that order is reversed, then that would be another clue. Or the upper surface of a surface lava flow oriented downwards. Etc.
Of course, you should verify all of that. As I said, I remember having read about that several years ago.
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