Archeological resources are the primary, and often the only, source of information about humans in Yellowstone for most of the time people have been in the area. Archeological evidence indicates that people began traveling through and using the area that was to become Yellowstone National Park more than 12,000 years ago. Because the intensity of use varies through time as environmental conditions become more or less favorable for humans, archeological resources provide a means for interdisciplinary investigations of past climate and biotic change. Many thermal areas contain evidence that early people camped there. At Obsidian Cliff, a National Historic Landmark, volcanic glass was quarried for the manufacture of tools and ceremonial artifacts that entered a trading network extending from western Canada to the Midwest. These remnants of past cultures must be preserved as they are invaluable in our understand of early people in the area. More modern archeological sites in Yellowstone include the remains of early tourist hotels and Army soldier stations.