Recently, I came across this situation where I was asked which one was correct:
Fossils are found in sedimentary rock.
Fossils are found in sedimentary rocks.
Is there is distinction between these statements?
Both sentences are grammatically correct, but in general the first is true and the second false. “Sedimentary rock” refers to sedimentary rock structures or sedimentary rocks in general; “sedimentary rocks” refers to pieces of sedimentary rock. The first statement suggests that fossils may be found in sedimentary rock, which typically is so. The second statement suggests that if I have a sedimentary rock it will have a fossil in it. But most sedimentary rocks – for example, most pieces of limestone gravel – do not have fossils in them.
Rocks is used when you have a countable group of rocks. For instance:
Rock can also be used in an uncountable sense when it refers to a big mass of rock, or "rock" as a general concept.
Your example sounds like it's a general rule rather than a specific finding, which indicates that it's talking about rock as a concept. Thus the correct usage is:
If you were talking about fossils that were actually in individual rocks, it might be possible to turn the sentence around, but that does not fit your specific example.
In general, the other answers are correct in that rocks usually refers to individual discrete rocks. However,
could also refer to multiple types of sedimentary rock. For example,
sounds correct. It might be less ambiguous to say ... sedimentary rock types ... explicitly, though.