The term Great American Desert was used in the 19th century to describe the western part of the Great Plains east of the Rocky Mountains in North America to about the 100th meridian. It can be traced to the 1820 Stephen H. Long's scientific expedition which put the Great American Desert on the map.
Mark Twain's use of this term applies to an area known as the Forty Mile Desert, the Lahontan Valley, reportedly the most dangerous part of the trail to California. In August of 1861, the Clemens brothers completed their journey to Carson City calling it "forty memorable miles of of bottomless sand". The following December Twain recrossed this desert on his way to Humboldt. He describes the water as "like drinking lye, and not weak lye, either."