In the late nineteenth century there developed a line of longitude in the United States that represented the boundary between the moist east and the arid west. The lines was the 100th Meridian, one hundred degrees of longitude west of Greenwich. In 1879 U.S. Geological Survey head John Wesley Powell established the boundary in a report of the west that has carried to this day.

The line was not solely selected for its neat round number - it actually approximates the twenty inch isohyet (a line of equal precipitation). To the east of the 100th Meridian, average annual precipitation is in excess of twenty inches. When an area receives more than twenty inches of precipitation, irrigation is often not necessary. Thus, this line of longitude represented the boundary between the non-irrigated east and irrigation-necessary west.

The 100° West matches the western boundary of Oklahoma, excluding the panhandle. In addition to Oklahoma, it splits North Dakota, South Dakota, Nebraska, Kansas, and Texas. The 100° line also approximates the 2000 foot elevation line as the Great Plains rise and one approaches the Rockies.

On October 5, 1868, the Union Pacific Railroad reached the 100th Meridian and placed a sign marking the accomplishment of reaching the symbolic west by stating "100th MERIDIAN. 247 MILES FROM OMAHA."

When we look at modern maps, we can see that soybeans, wheat, and corn are most common to the east of the line but not to the west. Additionally, population density drops at the 100th Meridian to less than 18 people per square mile.