Holes in the Cliff

Holes in the Cliff

Observation - Holes in the Cliff. Description: I've puzzled about these small holes in the sandstone walls of the old quarry at Sandy Lodge RSPB for a long time now. I've never seen anything coming or going from them. Anyone got any ideas?

I've puzzled about these small holes in the sandstone walls of the old quarry at Sandy Lodge RSPB for a long time now. I've never seen anything coming or going from them.
Anyone got any ideas?