Albert Barnes Commentary Ezekiel 24:7-8

"For her blood is in the midst of her; she set it upon the bare rock; she poured it not upon the ground, to cover it with dust. That it may cause wrath to come up to take vengeance, I have set her blood upon the bare rock, that it should not be covered." — Ezekiel 24:7-8 (ASV)

The top of a rock - The blood was poured upon a naked, dry rock where it could not be absorbed or unnoticed.