David said God was his Rock, he is much more but lets look at a rock.  rock /räk/ noun the solid mineral material forming part of the surface of the earth and other similar planets, exposed on the surface or underlying the soil or oceans. It is a great thing that much can be done with it. You can through this surface depending how large. I am sure David was talking about a much larger Rock one that can't easily moved. I often here rock and sheild in the same setence