In the last few weeks, the main mass of rock has been moving toward a gravel quarry at the base of the ridge. The company that runs the quarry has stopped operations and has voluntarily offered to put up evacuated residents in nearby hotels for five weeks at its own expense, though the cause of the crack has yet to be formally determined.

Some things, though, are clear. The slide is occurring in a rumpled layer cake of volcanic rock, or basalt, laid down by eruptions that began about 17 million years ago, interspersed with thin deposits of silt and other sediments. Over time, the layers have folded upward, forming the ridge, and the topmost basalt layer is now sliding over the sedimentary layer beneath it.

Many landslides are caused by infiltration of rainwater or groundwater, but geologists think that water is not involved in this case at all, just gravity.

That makes Rattlesnake Ridge a sharp contrast to the recent mudslides in California, which were caused by rain and erosion on lands loosened by fires — at least 21 people dead, with many more still missing — or the devastating Oso landslide in northern Washington State nearly four years ago, which killed 43 people, after weeks of heavy rain.

Still, all these events reflect the restlessness of the earth and nature that is part of Pacific Rim life. Even the name of the town, Union Gap, is a testament to natural violence: It refers to the huge gash in the ridgeline carved thousands of years ago by a colossal flood from ancient Lake Missoula. That lake once covered much of Montana, and when the Ice Age dam that constrained it broke apart, its waters left scars across the northwest as they roared toward the Pacific Ocean.