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(1) I don't know why you call the Grand Canyon "fast". Did you mean to write that the other way round?
Yes.
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(3) The rocks of the Grand Canyon are formed from different materials. Geologist can tell the difference between volcanic ash and tuff and (e.g.) marine limestone, you know. Where they find layers of volcanic ash, naturally they attribute them to the action of volcanoes.
From what I understand after Mt. St. Erupted, the volcanic ashes and particles did formed sedimentary layers, so both things are true. However, they are different I guess? But regarding the Grand Canyon they may have been unconsolidated as there are slump at the very bottom, right? Does Mt. St. Helens show that sedimentary layer can form rapidly, because they're volcanic lava and particles, but not rocks? So rocks takes longer?