Hehe, well in the text I was considering a fictional "great lowlands" that were the great middle of a continent.. not, for the sake of argument, the real Great Plains.. in fact I was picturing a great salt marsh, almost =)
Thanks to you all a LOT for help with this question. Declensions are my achilles' heel, and this helps a lot to see where I've been going wrong, and how sometimes the mistake is a small one, like "mountain range" being singular/plural in our different culture's vernaculars.. Thank you!!!