I have to tell you that I'm not a big fan of using arbitrary and capricious, omnipotent forces to explain why things don't make sense. Players don't tend to buy it either, but every group is different and YMMV.

Deserts are not a function of temperature, but rather of percipitation. I spent a year of my life living in a place that had almost no annual percipitation but rarely broke above freezing during the summer. When the snow that blew down off the ice cap melted off during the ninety days of sunlight, the ground below was as dry and dusty as the mohave.

I think the important thing is that, a desert way up north doesn't look like a desert down south. Glacial action, along with heavy freezing/thawing cycles have a much different impact on the land than wind and the occasional flood.

I don't want to discourage you from expressing yourself in a way that you find enjoyable and entertaining. I just want to say that geography by fiat is probably best done judiciously, and understanding the way things are and why is a better approach.

I don't pretend the understand geotechtonics in any meaningful way. If you spend an hour researching the subject on the internet you'll know more than me. I do know that the most significant fault line in the United States is in the west Tennessee area, if that revs your motor.