I was watching the show Britannia (set in 43 AD Britain) and saw a shot of a Celt working the land near the sea with a hoe-like tool, clearly making piles of a white substance. Here is screengrab of the shot: https://imgur.com/YezEzMs
What is he doing? Is it salt from the sea? Some other mineral? What is happening here?
Thank you for your time!
Yes, those are indeed salt pans. Several coastal areas in the UK have a long history of thriving salt-pan production, particularly Dorset and the Solway Coast area in Cumbria. The same technique is still used around the world, at sites like this one in Lanzarote.