Burgess ___ (rock bed noted for its fossils) (5)

I believe the answer is:

shale

(Other definitions for shale that I've seen before include "Type of soft rock" , "Oil-rich rock" , "Easily split layered rock" , "Sedimentary clay rock, which may contain oil" , "Oil-bearing rock" .)

I've seen this clue in The New York Times.

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