Why Does Your Pony Dig A Hole? Because He Is An Archaeologist...Or A Pig

Well there are lots of theories....mostly to do with vitamin needs. Maybe they are digging up the root of the grass.....maybe they need vitamins from the soil?
Charlie has dug small holes before.  I started giving him pink powder in his feeds, in case he was deficient in some important mineral. Not so in his case.
Sorry to dwell on the weather ( I Am British), but this week has been cold.  We have had a strong North Wind and showers. The ponies are all fat and have grass, but we succumbed to rugging Polly as she is old and as close to a TB as possible for a pony.  Then we rugged Daisy (she was tucked up and shivering pathetically).  Lu and Charlie have shown no signs of being cold.
And yet.....he has done something new to Jo and I in all our years of being pony girls.  Over the years I have looked after at least a 100 ponies, and I have never seen a pony dig like Charlie......
He has made a hole a foot deep and spread the dirt over about 6ft x 4ft. And he used it for a dust bath! Every inch of him is black....his feathers, his mane, tail, his face.  He looks like a coal-miner fresh from the pit!  And he seems to have dug up what looks like a stone age flint knife!  What he doesn't realise is when summer arrives he is having a bath!!!