Shale splits easily into thin layers, so tradition reads it as something that seems solid but comes apart under pressure. It can also stand for old layers of memory peeling back.
Where in your waking life have you met something like this lately?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.