Slumgullion Earthflow, Natural landslide formation in Hinsdale County, Colorado.

Slumgullion Earthflow is a slow-moving landslide in the San Juan Mountains of Hinsdale County, Colorado, a few miles southeast of Lake City. The 4-mile-long (6.4 km) and 2,000-foot-wide (610 m) mass covers 1,291 acres (522 ha) and is crossed by Colorado Highway 149. The photograph shows a steep slope of dark green conifer forest with patches of yellowish-green vegetation and exposed purplish-brown earth and rock.

About 700 years ago, volcanic rock from Mesa Seco collapsed and sent a massive flow of debris down the valley, blocking the Lake Fork of the Gunnison River and forming Lake San Cristobal. A second, younger flow began around 300 years ago and is the part of the site that remains in motion today.

The name Slumgullion refers to a thin, greasy stew eaten by miners during the gold rush, which the yellow, muddy soil resembled to early settlers. The flow lies a few miles southeast of Lake City and is crossed by Colorado Highway 149, the main road linking Lake City with Creede.

The Windy Point Overlook along Highway 149 gives a clear view of the flow and the lake it created, and it is easy to reach by car. Stopping at a few different pullouts along the highway helps you get a better sense of how far the flow extends and what the surface looks like up close.

The younger flow has kept moving for roughly 300 years over older, stable rock, advancing about 7 metres (23 ft) per year. That rate is far faster than the few centimetres per year the current text claims, and it makes the Slumgullion site one of the most closely watched slopes in the United States.