Drifts are the annual round- ups, organised by the Agisters, where the commoners’ animals are herded into pounds and marked (see M). There are lots of reasons for rounding-up animals: to worm them, to brand them, to check their condition, to get them off the Forest if they are unwell, or to take them to a sale.
Drifts can be dangerous for spectators, and there can be problems when people get in the way of the ponies being rounded-up. So take care when you see the warning notices that are put out by the Agisters, and please keep clear of the drift if you’re not one of the official helpers.