A relic of the ice age
In the high peaks of the Tateyama Mountains, what appears to be simple fields of rock and debris are "rock glaciers"—a mix of rock, soil, and interstitial ice behaving like a very slow-moving river. This is relict permafrost, a frozen legacy from the colder climates of the Pleistocene epoch. Permafrost is ground that remains at or below 0°C for at least two consecutive years. The features in Tateyama exist at the southernmost limit for permafrost in Japan, making them an indicator of climate change.
These formations are not glaciers, which are masses of flowing ice. Instead, the ice acts as a cement between boulders and soil, allowing the entire landform to deform and creep downslope under its own weight. Scientific monitoring has confirmed their movement. GPS measurements of similar ice masses in the Tateyama range, such as in the Gozenzawa ravine, have recorded movement of 6 to 30 centimeters per month. The ice thickness within these features can exceed 30 meters. This permafrost is found at elevations above 2,600 meters, often on shaded, north-facing slopes where it is insulated from summer heat.