Excerpt from Valley 101: Great Big Book of Life, a collection of Clay Thompson’s columns for The Arizona Republic. (Originally published September 14, 2003.)

Q: Why does Arizona’s southern border slant off to the northwest instead of running straight east and west?

A: There is a very good reason for this, according to Marshall Trimble, state historian and swell guy.

After the Gadsden Purchase of 1853, surveyors were laying out the new boundary, working from east to west. When they came to Nogales, they discovered the closest bar was in Yuma, so they headed there directly, marking the border as they went.

This, of course, is not true. However, as Trimble pointed out, it makes for a better story than what really happened.

In 1848, at the end of the Mexican War, Mexico gave up a huge hunk of territory including parts of what are now New Mexico, Arizona, California, Colorado, Utah and Nevada, plus its claim to Texas.

The southern border of Arizona at the time was the Gila River. However, a few years later we decided we needed more, and James Gadsden, our ambassador to Mexico, negotiated the purchase of 45,535 square miles of what is now southern bits of Arizona and New Mexico for $10 million.

There were two reasons for this. One, some Americans believed we had snookered Mexico over that whole war business, and this was seen as a way of making it up to them. Secondly, and more important, we wanted the land for the route of a southern transcontinental railroad that would be usable year-round.

According to Trimble, Southern interests wanted us to buy a much larger piece of land from Mexico. Northern interests, worried about the South being too big, argued for buying just enough for the railroad.

And Mexico didn’t want to lose its land route to Baja California, fearing that if it did the Americans in California would just move in.

Eventually, the Northern interests prevailed and instead of buying up the head of the Gulf of California and all that other land, we got Yuma.