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'rock' is a noun in form, and when it's used to modify another noun like rock wall, it function as an adjective, which in this case makes 'rock' a substantive (a substitute).
[1] The wall, the one made of rock (noun), crumbled and fell from old age.
[2] The rock (adjective) wall crumbled and fell form old age.
Note, every word has a form -what it looks like - and a function - what it does.
rock as a adjective 
Linear Mode
