If the sudden, vertical slip of a mass of soil from a hillside consists of dry material, both the event itself and the aftermath, or remnant, of the event are called landslides. If the material that slumps or shears away is saturated with water, it’s a mudslide. Like landslides, mudslides can be small- or large-scale events, though typically mudslides occur over a smaller area than landslides. (It was a landslide that created Earthquake Lake, Montana, on the Madison River.) Mudslides most often happen when vegetation that served to hold soils in place is removed, exposing the ground to sudden saturation and slippage. Ordinary earth tremblors might then trigger a full-scale slide—on the banks of a road cut, on overgrazed slopes, or on forest clearcuts. Mudslides occur naturally wherever erosion and earth-building are going on and vegetation is largely absent—in volcanic landscapes, on fire-ravaged slopes, along cutbanks, and in fault zones.

Barry Lopez