The Trans-Pecos ecoregion occupies the westernmost portion of Texas. It includes the Chihuahuan Desert, the Guadalupe Mountains, the Davis Mountains, and Big Bend National Park. This ecoregion ranges in elevation from 2,500 ft. to 8,749 ft and receives on average less than 12 inches of rainfall a year, although this varies with elevation. Soils at the lowest elevations in this area are the result of ______ after the erosion of the Guadalupe and Davis Mountains.
(A) disposition
(B) weathering
(C) flash floods
(D) river action