wall of rock/rocks

In Harry Potter and the Chamber of Secrets by JK Rowling, Lockhart casting a curse has caused the explosion of the wand and they were inside a tunnel. After that a rock wall formed between Harry and Ron.

Next moment, he was standing alone, gazing at a solid wall of broken rock.
"I'm here!" came Ron's muffled voice from behind the rockfall.
"I'll try and shift some of this rock."

Why 'rock' has been used instead of rocks as they are great chunks of ceiling rocks? I can't see any difference.

It’s like a :

a wall of glass

or

a wall of steel

or

a wall of sound.

It’s referring to the type of material therein.