10.
The dark colored splotches are lichens. A lichen is composed of two different organisms, an alga and a fungus, living together and supporting each other. The fungus furnishes the moisture for the alga, and the alga the food for the fungus. This coexistence is known as symbiosis.
As you walk from No. 10 to 11, you will pass several potholes that become pools of water after a rain. Some of these were artificially enlarged by the Indians who lived on the mesa top, to serve as supplementary sources of water.