It is the eastern-most region of the territory and borders the region of Cholla Springs to the west. The south, east and most of the north border of the region are defined by the San Luis River.
The terrain in this area resembles the Juniper Oak covered mesas in Arizona, New Mexico, and western Texas and is mixed in with terrain from the Plains of eastern Colorado and the Gold Country of north-central California, particularly in the southern part, around Greenhollow. The terrain under Mason's Bridge (Stillwater Creek, Dixon Crossing, and Thieves Landing) resembles the deep south, East Texas, Louisiana, and the lower Mississippi in particular, with lusher greener lowlands/marshes and swampier terrain in contrast with the rest of the region.
According to the Armadillo bank teller, Baxter Deaton, there is a lion in Hennigan's Stead that is being held by the locals. Although this is probably false, he could be referring to other NPC rumors that outlaws in Mescalero caught Khan the Jaguar.
Hennigan's Stead is possibly one of the most biologicly diverse locations in the game because in the south having scrubland similar to Rio Bravo, swamps to the east, pine trees to the north, and oak savannah in the center.
In the far northern area there is a small area in the hills, just out above the railroad tunnel that is snowy, much like northern Tall Trees. This is a treasure location in Undead Nightmare.