Brewster County is the state's largest with an area larger than that of Connecticut. Delaware, or Rhode Island individually, and larger than the combined area of Delaware and Rhode Island. It is one of the nine counties that comprise the Trans Pecos region of West Texas, and it shares a border with Mexico. Big Bend National Park is located in Brewster County. The temperatures are coolest and rainfall is most abundant in the county's northern highlands, as well as in the upper elevations of the Chisos Mountains in the south.
Presidio County is bordered on its east side by Brewster County, on the north by Jeff Davis County, and on the south and west by the Rio Grande and Mexico. Presidio County has a varied topography, geology, and vegetation. In the north and west, there are rolling plains known as the Marfa Plateau and the Highland Country. In the central, far western, and southeastern areas of the county, some of the highest mountain ranges in Texas are found. Temperatures are coolest and rainfall most abundant in the higher elevations of the Davis and Chinati Mountains. By contrast, the lowlands along the Rio Grande along the southern and western areas of the county are dry.