Through the eons, the geologic processes of erosion, deposition, uplifting, and subsidence have formed the landscape of the Lake Fork of the Gunnison River watershed.
Within the watershed’s 277,000 acres (432 square miles), the landforms vary greatly and include rugged mountain terrain in the upper reaches, the relatively broad flood plains of Capitol City, Sherman, Lake City, and Gateview, and the steep canyons of lower Henson Creek and the Lake Fork. Elevations range from a high of 14,306 feet at Uncompahgre Peak to Blue Mesa Reservoir at 7,519 feet.
Volcanism and Calderas
Today, the surface geology of the Lake Fork watershed is dominated by volcanic rocks that are part of the San Juan volcanic field. Between 23 and 34 million years ago, numerous erupting volcanoes threw thousands of feet of andesitic and rhyolitic rocks across the region.
Our watershed had two dominant volcanoes: the Uncompahgre and the Lake City. The Uncompahgre volcano collapsed after its last major eruption 28.4 million years ago, forming a major caldera 14.4 miles long and 11.8 miles wide. Over the next 5.4 million years, the Lake City volcano grew inside the Uncompahgre caldera until it erupted 23 million years ago. After this eruption, the Lake City volcano collapsed and made the 8.8- by 7.2-mile Lake City caldera. A dome of volcanic rocks then grew the area between the Lake Fork and Henson Creek to produce two area fourteeners, Sunshine and Red Cloud peaks. By this point, however, the energy driving the region’s volcanic upheaval was waning, and no more major eruptions occurred in the San Juan Mountains.
The Uncompahgre and Lake City calderas are essentially collapsed shells of the volcanoes for which they are named. During eruption, the Uncompahgre and Lake City volcanoes emptied out, spewing hundreds of cubic miles of ash into the atmosphere. Collapse of those volcanoes occurred along a circular set of faults that nearly line up with the valleys of the Lake Fork River and Henson Creek. Associated with these circular faults are radial faults that also formed during caldera collapse phases. Between 25- and 20-million years ago, many of these faults were filled with ore deposits containing lead, zinc, silver, gold, and other metals. These deposits were exploited by miners in the late 19th into the 20th centuries and are the primary reason that the town of Lake City was established.
During the last phase of ore emplacement, about 21 million years ago, acidic water and gasses invaded the area surrounding Red Mountain and severely altered older volcanic rocks. Massive amounts of alunite (potassium sulfate) associated with low grade deposits of molybdenum and copper were emplaced along the northeastern margin of the Lake City caldera.
Along the highway between Lake San Cristobal and Slumgullion Pass, rock that was weakened by the acidic water and gases disintegrated, creating the Slumgullion Earthflow (landslide). The slide—designated a National Natural Landmark—began its creep down the hillside about 1,200 years ago. It moved a full 4.2 miles to dam the Lake Fork and create Lake San Cristobal, Colorado’s second largest natural lake and Lake City’s namesake. The active part of the slide still moves approximately 20 feet annually.
Beneath the volcanic deposits lie Precambrian granite and associated metamorphic rocks. In the lower part of the watershed below Red Bridge, Precambrian gneiss and schist are exposed.
Sculpting of Mountains
Following the volcanic period, erosion and glaciation created today’s rugged mountains. High mountain glaciers carved U-shaped hanging valleys and cirques, which are amphitheatre-like areas at the heads of valleys, and left piles of debris known as moraines. Bare rocks on the floor of American basin and elsewhere are scarred with grooves carved by the movement of those glaciers.
Valley bottoms typically contain alluvial deposits washed from the surrounding high country. Streams flowing from high valleys create alluvial fans at their downstream ends. In our watershed, these fans are visible adjacent to Henson Creek and the upper Lake Fork. Other alluvial deposits are located between Lake City and Gateview in the Lake Fork valley south of Blue Mesa Reservoir.
The Slumgullion Earth Flow
A more contemporary geologic phenomenon is the Slumgullion Earth Flow. Approximately 700 years ago, a massive volume of hydrothermally altered volcanic rock slumped from a glacial cirque at 11,500 feet on Mesa Seco and quickly flowed 4.5 miles downhill, damming the Lake Fork of the Gunnison River and creating Lake San Cristobal.
A second earth flow began some 300 years ago, on top of the older slide deposits. The younger slide is now 2.5 miles long, with an impressive 100 foot high leading edge, and parts of it continue to flow as much as 20 feet per year.. This active movement can be seen in the strange angles of the trees, predominantly Engelmann spruce, that struggle to survive in the orange-yellow clay soils.
Because the Earth Flow is so sparsely vegetated, there is considerable sediment erosion, especially from the younger active flow. Most of this material is carried by Slumgullion Creek, which empties into the northeast corner of Lake San Cristobal, creating an expanding delta near the lake outlet. This creek was diverted from the main stem of the Lake Fork below the slide into the lake in the mid 20th century. In wet years large amounts of yellow sediment discolor the lake near the outlet and contribute to the ever growing quantity of sediment in the lake, much of which is high in metals such as arsenic.
The Slumgullion Earth Flow is home to a rare plant, the reflected moonwort (Botrychium echo).
Because of the size of the earth flow, ease of access, and its continued active movement, it is regularly measured and studied by the United States Geological Survey and academic researchers from around the world.
The National Park Service has designated the Slumgullion Earth Flow as a National Natural Landmark. The Earth Flow is also a designated State Natural Area by the Colorado Park Service’s Natural Area Program and managed as a United States Forest Service Special Interest Area and Bureau of Land Management Area of Critical Environmental Concern.
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