Well, *some* may believe it's the 100th Meridian, but I have always maintained that it's at 97.4° W, along I-35 in Oklahoma.
I've lived on each side of it and it's observable by even an untrained eye - except I've always called it the "tree line" (not the mountainous kind at elevation). To the east, trees, greenery and dark brown soil. To the west, tan shades of earth colors, scrub brush, and red soil.
That's a very glittering generality, but I'm just a 97.4° rebel contra, what can I say?