The type of rock that forms where the remains of plants and animals are deposited in thick layers is sedimentary rock.

Here's why:

* Sedimentary rocks are formed from the accumulation and cementation of sediments, which can include the remains of organisms (fossils).

* Organic sediments like plant matter and animal remains can form layers over time, getting compacted and cemented together.

* Examples: Coal (formed from compressed plant matter), limestone (formed from the shells of marine creatures), and oil shale (formed from organic-rich mud).

Let me know if you'd like to learn more about specific types of sedimentary rock!