Its chalk (though this one was concreted over,) , we have quite a few white horses in Wiltshire. This one is the oldest. It's a re-cut figure replacing an older one. It's to commemorate the Battle of Edington on the hill in 878. Alfred the Great's battle against the Vikings. There was 13 white horses in the county . Only 8 survive now .
https://en.m.wikipedia.org/wiki/Westbury_White_Horse
Thank you, Rob. That's cool. Always so much history in the UK!