Here's a breakdown of what that means:

* Shale: A fine-grained sedimentary rock formed from the compression of mud and silt.

* Phylloid: Describes a rock with a layered, flaky texture like a leaf (phyllo means "leaf" in Greek). This is due to the layering of very thin, parallel layers of sediment.

The Burgess Shale specifically consists of black shale, which is rich in organic matter and has a dark, almost black color.

This unique rock type is responsible for the exceptional preservation of the fossils found in the Burgess Shale. The fine-grained mud helped to quickly entomb the organisms, preventing decay and allowing for the delicate details of their soft tissues to be preserved.