The action of wind and water can form cave openings like natural rooms within the rock.
VOA: special.2009.07.20
China is of course planning to employ other sources, like wind power, hydropower and nuclear, but the sooty black rock is expected to represent as much power generation as all those others combined.
Currently, Spirit is examining a series of specimens that were perhaps formed when wind-borne volcanic ash fell to the ground, mixed with water and eventually formed into layered rock.