Yes, I can imagine those gaps being plugged with very dry elm and then water being poured over. But the boulder is in association with a pretty much confirmed (by me) Neolithic monument. And that alters everything. There are other stones nearby that have been chiseled - but assuming which era it happened in isn't easy. It's the connection of the sizes of the holes with megalithic inches that persuades me that they are Neolithic. Thom's last book - often overlooked - is worth perusing as they (father and son) overlay idealised cup and ring diagrams over known rock art figures. They show pretty good agreement ...