What is the difference between ‘a hard layer of rock’ and ‘a hard layer of rocks’ ?
Because rocks are something that can be counted, I think there should be rocks, but I see ‘a hard layer of rock’ in my book.
"Rocks" refers to "small" pieces of stone, the kind you can throw (eventually, if they are small enough). "A hard layer of rocks" means a layer of "small" pieces of stone.
"Rock" refers to the material. "A hard layer of rock" can mean that there is a piece of stone, large enough to be a layer in itself. Maybe in the past there were only sediments, but time joined them together in one piece. It can also mean "rocks", as they are made of the same material.