I live in the Sulphur Springs Valley, a broad valley in southeast Arizona. I’m in an area which has received erosion debris from the Mule Mountains for millions of years. The surface of the ground here is paved with sand, gravel, and numerous chunks of limestone along with a little granite and sandstone. It’s a great walking surface; immediately after a monsoon rain I can walk without mud clogging my shoes.
The limestone appears in various shades ranging from pale yellow to purple. The fragments and cobbles are evidence that this area was submerged over and over. Phytoplankton, diatoms and such, lived and died, shedding their calcareous skeletons. This debris slowly drifted down to the temporary sea floor and formed sedimentary layers.
Every now and then, though, I’ll come across an anomalous chunk of rock. Dark gray to almost black, these small rocks display a grain structure which I surmise is cellulose and lignin transformed to a mineral form; i.e, petrified wood.
To me, this seems to be evidence that millions of years ago, long before the local mountains were uplifted, the climate was humid and forests carpeted the area.
Here is a photo of one of these fragments.