Tephra
104
The tundra is likely also artificial.
The Witch Queen says, “It was a fertile land - rolling grasslands and temperate hills that gave way in the south to the dryness of a desert. In the north were mountains, but not the snow-covered mountains that we had seen on our journey, just jagged rocky peaks. And a swamp, thick and brooding, that lay over the eastern part of the region like a shadow.”
This seems to indicate that the tundra did not exist yet at the time the Lemurian Remnant arrived, however, the map-room does depict the tundra. So when did the tundra appear? Between the Lemurians arriving and the war breaking out? The tundra likely appeared when the Disjunction was opened, so that (relatively) short time period may be when it opened and the frost giants arrived in the Exiled Lands.