THE GUNSLINGER: MOHAINE by Michael Whelan
The Mohaine Desert is the desert that Roland chases the Man in Black across. At one end of the desert lies the town of Tull and the other side meets the Cyclopean Mountains.
The ground is described in the second paragraph of the Dark Tower Series, in the Coda of the final book, and in Insomnia as being the "apotheosis of all deserts". The ground is flat hardpan and light enough in hue to appear white at first glance. Vegetation is sparse and hardy, and often non-existent, making most of the desert a flat expanse of white from horizon to horizon. The ongoing breakdowns in reality have made the desert even more of an expanse than it was before the world "moved on". Yet there are still occasional remnants of a time gone by: a half-buried tombstone sign that marks where a road had once been and the Way Station, fuelled by an "atomic slug", where Roland first encounters Jake Chambers. Once, there was also a train track known as the Western Line, which ran a thousand wheels or more, connecting Gilead with the Mohaine Desert. In the years before Gilead fell, however, this line ended at Debaria.