Geology 349 Colorado 144 Hiking 94 Automobile travel 61 Geology, Stratigraphic 60 Trails 55 Historic sites 48 West (U.S.) 46 National parks and reserves 42 New Mexico 38 United States 38 Outdoor recreation 36 Mines and mineral resources 25 Utah 25 Mountaineering 22 Natural history 20 Gold mines and mining 16 Arizona 15 Buildings 14 California 14 Mountain biking 14 Architecture 13 Historic buildings 12 Wilderness areas 12 Airplanes Environmental aspects 11 Airplanes Noise 11 Nevada 11 Noise Measurement 11 Parks 11 Canyonlands National Park (Utah) 10 Colorado History, Local 10 Ghost towns 10 Mountains 10 Rock climbing 10 Rocky Mountains 10 Southwest, New 10 Yellowstone National Park 10 Colorado Plateau 9 Europe 9 Mines and mineral resources History 9 Rafting (Sports) 9 Recreation areas Public use 9 Scenic byways 9 Washington (D.C.) 9 Wyoming 9 Arches National Park (Utah) 8 Bicycle touring 8 Colorado Description and travel 8 Cross-country skiing 8 Forest reserves Recreational use 8 Geology, Structural 8 Grand Canyon (Ariz.) 8 Idaho 8 Kansas 8 Overland journeys to the Pacific 8 Rocky Mountain National Park (Colo.) 8 San Juan Mountains (Colo. and N.M.) 8 Staff rides 8 Texas 8 Wildlife refuges 8 Alaska 7 Art 7 Bicycle trails 7 Bird watching 7 Canada 7 Denver (Colo.) 7 Extinct cities 7 Grand Canyon National Park (Ariz.) 7 Great Britain 7 History 7 Nebraska 7 Pacific States 7 Pioneers History 7 Roads 7 Travel 7 United States History Campaigns 7 Backpacking 6 Boats and boating 6 Canyons 6 Indians of North America Antiquities 6 Mining camps 6 Mountain passes 6 National monuments 6 Paleontology 6 Recreation areas 6 Volcanism 6 West (U.S.) Description and travel 6 Aspen (Colo.) 5 Birds 5 Business travel 5 Cities and towns 5 Grand Junction Region (Colo.) 5 Monuments 5 Noise Evaluation Statistical methods 5 Oklahoma 5 Petroleum Geology 5 Rivers 5 Rocky Mountains Region 5 Rome (Italy) 5 Sound Measurement 5