Shale

Raindrops gently splattered pits into fine-grained sediment some 215 million years ago. Before the next storm erased the impressions, more sediment covered and preserved them.

Shale with graptolites

Tetragraptus approximatis, Ordovician Period. What time is it? If you found one this index fossil in a rock, you'd know. Each animal was once widespread but quickly became extinct. Only a small percentage of fossils meet these criteria.