NPR''s Alex Chadwick concludes a two-part National Geographic Radio Expedition to Yellowstone Park. Today, he reports on the park''s geology and talks to visitors about the fire that burned a large part of Yellowstone ten years ago. The National Park Service allowed the fire to burn, on the basis that fire is a natural part of the ecosystem. But regeneration takes a long time, and not all visitors agree that the damage to the park''s beauty was necessary. (8:53) For more information about the Yellowstone Fires, visit the Radio Expeditions web site at www.Nationalgeographic.com .