Earthquakes are the natural outcome of the fact that the earth, while seeming substantial and changeless, is constantly, if most gradually, in the process of change. Mountains are thrust up. Glaciers carve them down. Volcanoes pour out their molten rock. Rivers and floods scour their erosive paths. Sediments slide and settle. The enormous masses which great internal earth forces have raised up to mountain height, create counter stresses. These forces build up for years, sometimes for centuries or longer. Eventually something has to give. When this happens on a grand, or spectacular scale, we call it an earthquake. [Illustration: Stress, caused by the quake, resulting from slump of surficial material caused the attractive curving of this fence along Highway 287 east of Hebgen Dam. (U. S. Geological Survey)] Whether you’re a connoisseur, expert, or not, the spectacular 1959 Montana-Yellowstone Earthquake was a beaut. Geologists call it a “textbook earthquake,” because it included nearly all of the classic actions, or results which quakes are likely to cause. [Illustration: Damkeeper George Hungerford points out how much the tidal waves, or seiches, overtopped Hebgen Dam at the time of the quake. Hungerford is standing on earthfill washed down from dam-top level by the gigantic waves. The quake also cracked the dam’s concrete core as shown. (Montana Power Co.)] It ranked right along with San Francisco’s 1906 shakedown as among the severest earthquakes on the North American continent. In seismic measurements, it rated 7.8 on the Richter Scale, as compared with San Francisco’s 8.2. It set up so called tidal waves, or seiches, on Hebgen Lake. There were at least three of these huge waves—20-ft. high—which overtopped the entire 721-ft. length of the dam by four feet. Eyewitness statements relate that the velocity of the tidal wave was so great that it caused the water literally to leap over the top of the dam. It filled the small generating plant with a 2 to 4 ft. deep layer of rocks. Although the dam stood, the quake caused several fractures in the core wall, one of which showed a 3- to 4-inch separation, and shattered the dam’s concrete spillway. The earthquake created three major faults, with displacement on the Red Canyon Fault running as much as 20 ft., which stacks up impressively alongside the 26-ft. maximum displacement resulting from San Francisco’s quake. (These two earthquakes differed, however, in that the Montana displacement was vertical, while San Francisco’s was horizontal.) [Illustration: _BEFORE_] [Illustration: _AFTER_] According to the Society of Military Engineers, surveys from benchmarks outside the earthquake-affected areas show that the earth in the Hebgen Dam Quake area near Hebgen Dam has settled between eighteen and nineteen feet from its level before the quake.