Geology Rocks!

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Unaweep Canyon, located 1 hour outside Grand Junction, CO, is a U-shaped canyon cutting across both the Uncompahgre Plateau and Colorado Plateau. This canyon is an incredibly unique area due to various factors, starting with the name! The name “Unaweep” means “canyon with 2 mouths” in Ute - two different creeks, West Creek and East Creek, flow out of opposite sides of the canyon. Acting as a miniature “continental divide”, with a high point (or divide) in the middle, West Creek flows towards the Dolores River near Gateway, CO while East Creek flows into the Gunnison River. This is the only canyon in the world known to do this.

 

The main rock found throughout the canyon is large, Precambrian-cored rock dating back to millions, even billions of years ago. Specifically, both igneous and metamorphic Proterozoic crystalline rock form the base with Mesozoic sedimentary rock above it. The huge granite cliffs found throughout the canyon previously was the bedrock of earth’s crust 1.4 to 1.7 billions of years ago, but rose after a geological uplifting event. A thick layer of “fossil fill” - ancient soil buried by other sediments and then hardened into rock - developed around the Pleistocene age as well. This change in rock formed the 70-km-long gap extending from Whitewater, Colorado to Gateway, Colorado. Unaweep Canyon is about 1 km deep (only 400 m deep in the inner gorge) and 6 km wide (3 km in inner gorge). The geologic origin of this canyon is still debated today. There are four main hypotheses: 1) The Uncompahgre Uplift 2) Volcanic Activity 3) Glacier Activity and 4) River and Lake Erosion.

 

The first hypothesis argues that during the Uncompahgre uplift, part of what is now the  Uncompahgre Plateau got stuck under the Grand Valley, resulting in a fracture of the Uncompahgre, forming Unaweep Canyon. The controversial aspect is not if the Uncompahgre uplift occurred - which it did - but whether the uplifting event caused Unaweep Canyon to form. 

 

In some areas of the canyon, geologists have found the presence of gravel volcanic rock and a possible history of volcanic activity in the area, dating back to around 1.46 million years ago. However, this could also signify a process called “fluvial erosion” where a cycle of river/lake formation and erosion occurred, alternating between carving and depositing the canyon. 

 

The third hypothesis involves glacier incision activity. Glaciers moved through this area and cut and carved the canyon. Evidence for this is found in rocks from the late Cenozoic or Pleistocene era found within the Precambrian inner gorge of Unaweep Canyon, inferring glacier activity carved the canyon during this timeframe. Specifically, rock structures throughout the canyon, such as spurs, cirques, hanging valleys, and block drainages, continue to provide more evidence for glacier activity as well. For some geologists, this is where this glacier hypothesis ends. Other geologists however argue the second part of this hypothesis exists: that after the glacier-carved though, it buried a water source, and eventually that water eroded the canyon once again. This water source was either the Colorado River, the Gunnison River or a combination of both those rivers. For geologists who include this aspect into their hypothesis, they also argue that the glacier activity occurred at an earlier time frame than the Late Cenozoic - they believe it occurred during the late Paleozoic era. 

 

Finally, the last (and most popular) hypothesis puts forth that during the late Cenozoic period, an ancient Colorado or Gunnison River attempted to move and cut through this area before the Uncompahgre uplift. This initially formed Unaweep Canyon. When the Uncompahgre uplift occurred though, the river continued to cut through it until the river hit a high point, the Unaweep Divide. Around 1.3 million years ago, a downward shift of soil, sand, rock from gravity  - also known as slope or mass movement - filled the canyon and cut off the ancient Colorado or Gunnison river, resulting in a sharp turn and the formation of  the U-shaped canyon. Several studies show that this is more likely to have happened to the Gunnison River than the Colorado River.. 

 

These four hypotheses exemplify the difficulty geologists face in determining the complex geological structures that resulted in the uniqueness of Unaweep Canyon. There is a strong possibility that a combination of these hypotheses could have occurred and that geologists need to continue discovering more information in order to determine the final cause(s).