great question! i had to look that one up myself.
shale is a sedimentary rock. see "shale" in wikipedia.
a chute is a sloping channel. could be quite wide - like the side of a mountain - or could be quite narrow.
i found a youtube video of a mountain biker on a steep shale chute.
steepest shale chute seems like trying to do a solo traverse with a heavy pack caused our woman to rethink things. smart.