Short answers, pulled from the story.
The Hebgen Lake landslide on the 17th of August 1959 was caused by a magnitude 7.2 earthquake that triggered a catastrophic failure of schist bedrock. The schistosity planes within the rock created zones of weakness that allowed the mountain to slide with terrifying ease, resulting in the deaths of 26 campers.
Geologists today define schist as a medium-grained metamorphic rock that shows well-developed schistosity and splits into flakes or slabs less than 1 centimeter thick. Before the mid-19th century, the terms slate, shale, and schist were not sharply differentiated by those involved with mining.
Schist typically contains mineral grains ranging from 0.5 to 2 millimeters in size that are easily visible with a 10x hand lens. These grains often include platy minerals such as mica, chlorite, talc, or graphite, and over half of them show a preferred orientation that defines the rock's character.
Schist forms during regional metamorphism accompanying orogeny when rock is subjected to elevated temperature and nonhydrostatic stress. This process rotates or recrystallizes platy minerals into parallel layers perpendicular to the direction of greatest compression.
Schists make up one of the three divisions of metamorphic rock by texture, with gneiss having poorly developed schistosity and thicker layering, and granofels having no discernible schistosity. Schists are formally described as such only when the original type of the rock prior to metamorphism is unknown and its mineral content is not yet determined.
Schist bedrock poses a challenge for civil engineering because its pronounced planes of weakness create discontinuities that influence the strength and deformation of rock masses. These schistosity planes develop perpendicular to the direction of greatest compression, allowing the rock to slide with minimal resistance during tunnel, foundation, or slope construction.