Here's why:

* Clastic sedimentary rocks are formed from fragments of other rocks (clasts).

* Grain size is a key way to classify these rocks. Claystone is made up of particles smaller than 0.0039 mm in diameter, which is considered very fine.

* Other fine-grained clastic sedimentary rocks include siltstone, which has slightly larger particles (0.0039-0.0625 mm), and shale, which is often made up of claystone and siltstone layers.

Let me know if you'd like to know more about sedimentary rocks!