As a first thing today we visited the ghost town of Terlingua, just next to Big Bend National park and very close to the Mexican border.
Big Bend National park is covering part of the Chihuahuan desert - one of the four deserts in the US (Sonora desert, Mojave and Great Bassin are the other three). Like in Sonora desert you can find many cacti in Chihuahuan desert. However, the species you can find differ. We saw Ocatillo cacti, prickly pear and several smaller cacti, but no saguaros, organ pipes or chollas.
In Big Bend we could see many - also very colourful birds - and many bird nests :-).
In Big Bend you can see the traces of volcanic activity everywhere in the park. Many of the stone formations are the result of erosion (especially loose parts like ashes eroded over the years).
Big Bend is crossed by Rio Grande (which is not really a big river), but one of the few rivers in this area. For many miles Rio Grande builds the border between New Mexico and Texas and also between US and Mexico.
We did a short hike into Elena Canyon which was carved millions of years ago by water forces.
In the heart of Big Bend are Chiso Mountains which are also home to mountain lions and black bears.