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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?

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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.

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Thank you so much! I see the subtle difference between the two more clearly now. It threw me off a little, and I wasn't sure if I had rationalised it correctly. This is a relief! – santo Dec 10 '12 at 7:12
This answer rocks. +1. – J.R. Dec 10 '12 at 11:03
In actual fact, I believe a lot of sedimentary rocks do have microscopic fossils in them. – Peter Shor Dec 10 '12 at 15:47

Rocks is used when you have a countable group of rocks. For instance:

I found this coin under the rocks over there.

Rock can also be used in an uncountable sense when it refers to a big mass of rock, or "rock" as a general concept.

Rock is a common building material.

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:

Fossils are found in sedimentary rock.

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.

I found these fossils in those rocks over there.

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In general, the other answers are correct in that rocks usually refers to individual discrete rocks. However,

Fossils are found in sedimentary rocks.

could also refer to multiple types of sedimentary rock. For example,

Fossils are found in sedimentary rocks, such as chalk and limestone.

sounds correct. It might be less ambiguous to say ... sedimentary rock types ... explicitly, though.

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