good size piles of basalt rocks. Here you can see what the original color of the rocks was before the desert varnish turned them black.
Basalt is an igneous rock that in this area was formed by volcanic activity 2-5 million years ago. As the lava cooled, it fractured into the boulders you see covering most of the ground. Originally reddish-brown, the black coating, called desert varnish, is formed in arid climates over thousands of years on exposed rock by a combination of chemical processes and microbes, and is made primarily of manganese.