The rock glacier originates in an empty cirque on Rainbow Mountain. The lower photograph shows particularly well the characteristic steep apron lying at the angle of repose above the toe of the glacier. The top of an active rock glacier flows faster than its base. Consequently, near the toe of a rock glacier, material from the top rolls or slides down the front of the rock glacier to form a slope lying at the angle of repose of the loose rock composing the glacier. By means of this characteristic slope at the toe, one can recognize the rock glacier and that it is active and moving.
Rock glaciers occur by the hundreds in the Alaska Range.
They occur in the Wrangells and many other mountain ranges around the world including the San Juan Mountains of Colorado and the Sierra Nevada of California. For a rock glacier to exist several conditions must be met. There must be a source of blocky rock such as a weathering cliff or mountain slope or perhaps even the terminal moraine of a dying ice glacier. Secondly, the mean annual temperature must be low enough for water percolating down through the blocky rock to freeze and form an ice matrix in the spaces between the rocks. In other words, the temperature must be low enough to support permafrost.