I am one of those weirdos who think the Grand Canyon was carved down in a much shorter time, and not so long ago.
From the air it looks so much like drainage from the huge ocean that was icebound in Canada and the Plains.
So, foot prints in mud at any level in the grand canyon is suspect in my mind.
Just my opinion. And I am a pretty hardcore traditional geologist guy from way back when Fossil Fred was teaching on the hill.
They cover that precise topic in this excellent documentary.
How The Earth Was Made: Grand Canyon:
https://www.youtube.com/watch?v=GMLPbRKdvSw
I was a geology major back in the mid 80s.
How do you explain discontinuities/unconformities, subaerial erosion and palaeo karst features within the Grand Canyon sequence, if it was laid down rapidly and then eroded in the way you suggest?
Fossil Fred?