Slate is a metamorphic rock, not a sedimentary rock. It forms from the metamorphism of shale, which is a sedimentary rock.

Here's the process:

1. Sedimentation: Shale forms from the accumulation and compaction of mud and clay.

2. Diagenesis: Over time, the sediments in shale are cemented together, turning it into a solid rock.

3. Metamorphism: When shale is subjected to heat and pressure (often due to tectonic activity), it undergoes a process called metamorphism. This transforms the shale into slate.

So, while slate itself is a metamorphic rock, it is ultimately derived from the sedimentary rock shale.