It is a stratovolcano with slopes mantled by five glaciers - Whitewater, Waldo, Milk Creek, Russell, and Jefferson Park.

Three Fingered Jack is a heavily eroded and deeply glaciated shield volcano and consists mainly of basaltic andesite lava.

Between Three Fingered Jack and the North Sister volcano and among recently active basalt lava fields, there is an array of cinder cones encompassing an area of 85 square miles (220 km2).

[7] Vegetation in Mount Jefferson Wilderness is predominantly Douglas-fir, Pacific silver fir, sub-alpine fir, mountain hemlock, lodgepole pine, ponderosa pine, several species of false cedars including, Alaska-cedar, incense cedar, and western redcedar as well as species such as vine maple, huckleberry and rhododendron.

[4] Common wildlife found in Mount Jefferson Wilderness include deer, black bear, coyotes, and bald eagles.