Rescue workers are drilling a hole through hard rock. Why not a hard rock?

Question

Rescue workers are drilling a hole through hard rock. Why not a hard rock or hard rocks?

Answer 1

A hard rock would mean a single hard rock - an individual rock.

Hard rocks would indicate a number of such individual rocks.

Hard rock indicates a layer of rock, possibly bedrock or a solid rock (mountain) core.

Work to rescue trapped miners in different parts of the world has often involved drilling down through solid rock or layers or rock, sometimes just to supply the victims with food and water until rescuers can reach them.