You Need to Know about the Hundredth Meridian

The Hundredth Meridian was identified by John Wesley Powell (a fascinating character in his own right) as a geographic tipping point separating the wet Eastern US from the dry West.  This has all sorts of implications for ecology and social structure.  Here is a fascinating YouTube video on the subject.

Basically, the dry West is due to the rain shadow from the Rocky Mountains (very broadly defined).  East of the 100th meridian, the land was wet and dominated by forests that were not dissimilar to those with which most European immigrants were familiar.  To the west of this line the land was dominated by grasslands and desert, to which the settlement patterns adopted in the East were not well suited.

"At the Hundredth Meridian" is also a great song by The Tragically Hip.