Here is a nice view of the left chimney from the right formation. You can see the trail to the petroglyph. There appears to be an encampment of some sort beneath the rock on the right: there is a stone wall and some interesting artifacts.

Pictured here are a wide variety of plants: the ocotillo, just above it is a prickly pear, and immediately below it is a purple prickly pear (you can tell there has not been much rain, because it gets more purple after long periods without rain), and a strawberry hedgehog cactus (Echinocereus enneacanthus) at the bottom.

Thanks to http://www.americansouthwest.net/texas/big_bend/cactus_photographs.html for a collection of pictures to help identify this and other cacti.