Range 45 28. Radar image of lava flows in southwestern Yellowstone National Park 46 29. Obsidian Cliff 47 30. Thick rhyolite lava flow along Firehole River and closeup view of specimen 48 31. Brecciated lava flows 49 32. Outcrop and closeup view of glassy rhyolite lava 50 33. Basalt flows at Tower and closeup views of outcrop and specimen 52 34. Giant glacial boulder of Precambrian gneiss at Inspiration Point 54 35. Glacial terrain along the Northeast Entrance Road 56 36. Typical profiles of canyons cut by stream erosion and glaciation 57 37. Aerial oblique view of Electric Peak 58 38. Extent of ice in Yellowstone National Park during the maximum spreading of the Pinedale glaciers 60 39. Beds of sand, silt, and clay deposited in a glacially dammed lake in Hayden Valley 61 40. Waterfalls in Yellowstone National Park 62 41. Grand Canyon and Lower Falls of the Yellowstone River 65 42. Various stages in the development of the Grand Canyon of the Yellowstone 66 43. Common kinds of thermal features in Yellowstone National Park 70 44. Norris Geyser Basin, showing solid floor of hot spring deposits 72 45. Diagram of a thermal system 72 46. Infrared image of a portion of Upper Geyser Basin 73 47. Mound of geyserite (sinter) at Castle Geyser 75 48. Terraces of travertine at Opal Springs and closeup of specimen 76 49. Algal-colored terraces lining the west bank of the Firehole River 78 50. A geyser in action 80 51. Rock rubble surrounding Seismic Geyser in Upper Geyser Basin 83 52. Old Faithful in full eruption 84 53. Mud volcano near Pocket Basin in Lower Geyser Basin 86 54. Reactivation of a fault during the Hebgen Lake earthquake of August 17, 1959 88 Yellowstone Country