There's also natron and potash which have been known since antiquity, but I can't find any reference to them being used as leavening agents before the early Industrial era.
Sodium carbonate (washing soda) is used in baking to encourage browning, but it doesn't produce carbon dioxide when heated.
You need sodium bicarbonate (baking soda) which is only a small constituent in trona, and without knowing how to concentrate it, or that you can, it's unlikely it would have been used as a leavening agent before the advent of modern chemistry. You'd have to add so much that it would ruin the batter or just turn it bitter.
Sodium bicarbonate (baking soda) turns into sodium carbonate when heated in an oven, which is used by amateur chemists sometimes to make the carbonate if they don't have it on hand.
Baking powder is a combination of baking soda and a food-safe dry acid, which react when water is added. This wouldn't have been invented before the chemistry of acids and bases was discovered.
Either that or Nature Valley crunchy granola bars.
(Dwarf bread is probably based on hard tack, a puck of dense biscuit eaten by sailors in the far off days of long ahead. A bread so dense it would make US Southerners weep, and explains the lack of teeth on sailing ships far better than scurvy)
It's a national symbol of Scotland stolen by England after putting down Scottish rebellion. There's another large stone very important to Ireland that was also stolen by the English.
They then had the Scottish stone put into the seat of a throne so the king or queen sits on Scotland and they had the Irish stone put into basically a footstool and kings and queens would rest their feet on Ireland.
It is highly likely this was the inspiration for the Scone of Stone, but I'm not aware of Sir Terry talking about it. The wiki editors certainly see a connection tho, and I think it's a fair assumption to make that this is the thing and the whole of the thing