When Faith talks about erosion leaving a flat surface, I wonder if she might be thinking about uneven rates of erosion due to different hardnesses of the material being eroded. For example, here's a photo from Monument Park in Arizona:
Now the picture is coming through. Yes I do think about rates of erosion having to do with different hardnesses. The stepped shape of the walls of the Grand Canyon show this kind of differential erosion, probably also the pattern of runoff from the top of the canyon, taking more from the top layers than the lower layers.
The erosion within the layers of the canyon is supposed to have occurred upon an already pretty flat surface, as these surfaces were supposedly built up over great periods of time, most of them underwater where they would have been quite horizontal. I've been thinking of erosion UPON a flat table-like surface over the millions of years supposed for any given layer -- or whatever relatively shorter period (half a million being conservative?) that layer is thought to have been out of water. Seems to me rivulets would have starting cutting into the surface and eventually spread out taking a lot of material with it over huge periods of time, making short order of the flat table-like appearance. It would have dumped sediments moved from it somewhere else in no particular configuration, the whole thing being an incoherent mess at the end of the supposed millions of years.
She might be wondering why the layers of the Grand Canyon have no buried structures like this that were first eroded into this uneven shape, then covered over again by deposition.
I'm sure it's an interesting story. What the monuments look like to me is islands of some kind of layering, left after water removed everything in the surrounding area. The tops look like something that was pushed upward, then the whole thing eroded first by more water than that which cut the canyon, as it left the surrounding area denuded and flat for miles in all directions, then eroded by normal weathering since then.
So what's the real story?