To address the time issue: Let's go with fifty years after the eruption of Yellowstone.

Also, maps:

[img]http://orig02.deviantart.net/689d/f/2016/349/8/3/fourth_horseman_map_by_aaronmk-daroniu.png[/img]

Map claim:

[img]http://orig12.deviantart.net/5073/f/2016/349/8/5/fourth_horseman_map_territories_by_aaronmk-daronir.png[/img]