Since I am a mere home inspector, I would leave it up to the engineer.

Having said that, it seems to me that if this rock/pebble/boulder goes as deep or deeper than the footing, then it is probably OK. I have seen perimeter foundations constructed over large rocks/boulders with the blessing of an engineer. However, it might make a difference what kind of rock (say, sandstone or granite). And, as I am not a soils or structural engineer, I would defer to one in my report if I saw something like this on an inspection.

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The apostrophe troll.