Flaky rock

I think it could be either:

Which length description are you looking for?:

shale

'flaky rock' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for shale that I've seen before include "Soft rock; heals (anag.)" , "Layered sedimentary rock" , "Soft sedimentary rock" , "Stratified rock" , "Stone that splits easily into fine pieces" .)

I've seen this clue in the Universal.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.