The type of sedimentary rock composed mostly of clay-sized particles is shale.

Here's why:

* Clay: Clay is a very fine-grained sediment, smaller than sand or silt.

* Shale Formation: Shale forms when layers of clay particles accumulate and are compacted and cemented together.

* Other Characteristics: Shale is typically fine-grained, layered, and often breaks into thin, flat pieces. It can also contain other minerals like quartz, feldspar, and mica.