Fossil Tracks.

When animals walk on muddy ground, they leave tracks, and when conditions are right, the tracks may become covered and then converted to stone. These are called "Trace Fossils" to distinguish them from true fossils (e.g., fossilized dinosaur bones). Trace fossils are broad category that includes any mark left by an ancient creature. Footprints are the obvious "trace," but animal burrows are considered trace fossils.

During ancient times, animals crawled around and burrowed into the bottom of ancient seas. In some places, their burrows filled with minerals that became stone. Near Sandy Valley, two low hills are covered with the casts of these burrows. It turns out that the material surrounding the burrows eroded faster than the burrow casts, leaving the casts scattered across the landscape (incidentally, looking much like fossilized dog poop).

During the last several million years, mammals and birds wandered the landscape around Las Vegas. The tracks of extinct camels, wolves, and birds can be found in mudstones of Lake Mead National Recreation Area.