Ok, but how exactly were the Shadow Lands separated?

The DLC doesn't make it clear

My understanding is that in the Marika's Order, gold and shadow were born and in the Scadutree Avatar is told that Scadutree and the Shadow Lands represents exactly that shadow, then Erdtree and The Lands Between is the gold

So the Shadow Lands were separated when the Erdtree appeared, but when exactly did the Erdtree appear? And how did Marika choose which part would be the gold and which part would be the shadow?