By
G. Alex Sill
ESU Earth Science
Emporia, Kansas
April 3, 2001
  Figure 1:  View looking south southwest along upper Cucharas Creek valley towards Trinchera Peak (4,096 m 13,517 ft.) from a vantage point near the crest of the Culebra Range in south central Colorado.  Persistent snow and hardy wildflowers are visible in this mid July photograph.  Note the tiny blue tents in the lower left of the image.
 

Regional Setting

Trinchera Peak is located in Huerfano County Colorado near the village of Cuchara.  Here, the Culebra Range of the Sangre de Cristo Mountains forms a north striking ridge.  The ridge forming sandstone bedrock (Sangre de Cristo Fm.) is a detridal arkosic wackie derived from alluvial fan sediments of the ancestral rockies.  These sediments were laid out during the Pennsylvanian and Permian eras.  The formerly flat lying sandstone strata were thrust into a vertical orientation by uplift, faulting, and block rotation events dating back to the mid- to late Miocene (Lindsey, 2001).
 
 
 

Figure 2:  West of the Culebra Range lies the world's largest alpine valley.  The San Luis Valley has an area of 55,680 Km (8,000 square miles) and and average elevation of 2,212 m (7,500 feet) above sea level.  This space shuttle image (Sangres.com, 2001) shows the expansive San Luis Valley.   The mountain ranges to the right are the Sangre de Cristo mountains.  The ranges to the left are part of the San Juan Mountains.  Note the Fishhook shaped bend in the Sangre de Cristo range.  The snow capped peak at this location is Mount Blanca 4,219 m (14,345 ft.).  Just north of Mount Blanca, near the "eye" of the fishhook, Great Sand Dunes National Monument can be seen as a small bump against the left side of the range.  The Great Sand Dunes are a unique feature in that they are one of the most diverse and expansive dune complexes in the world.  Sediments derived from the San Juans have been piled up along the Sangre de Cristos by persistent westerly winds.  The bend in the range forms a cul de sac entrapping the sand.   These dunes reach a height of nearly 200 m (700 feet) above the surrounding terrain.
 
 
 
 

East of the Culebra Range lies some interesting features of the High Plains.  Most notable of these features are the Spanish Peaks.  These twin mountains tower far above the surrounding plain and are somewhat anomalous since they are quite removed from any other mountain ranges.  West Spanish Peak (4,007 m 13,625 ft.) and East Spanish Peak (3,730 m 12,683 ft.) are thought to be small batholiths called stocks that were uplifted and thus subjected to erosion during tectonic events 35 million years ago.  Despite popular belief that these structure are extinct volcanos they are in fact not of volcanic origin.
Figure 3:  West Spanish Peak seen from the west.  Note the rainbow in the picture.  Afternoon rain showers are extremely common and often times severe during the summer months.  The person to the right is Quaternary Geology instructor Dr. James Aber enjoying a bowl of camp supper.
 

 Figure 4:  A large dike radiating from West Spanish Peak.  There are many large dikes like this one radiating from West Spanish Peak.  These structures formed about the same time as Spanish Peaks formed.  Magma welled up through cracks in the sedimentary bedrock.  The cracks formed during crustal uplift and warping associated with the injection of the Spanish Peaks stock.
 
 
 

 Glacial Structures of Cucharas Creek Valley
Cucharas Creek Valley is a classic expression of an alpine glacial valley.  Glaciation events causing the erosion and deposition of glacial structures probably dates back to the last North American Glaciation during Pleistocene or even Holocene time.  This figure is uncertain due to lack of datable material in moraine structures.  In the upper valley, glacial erosional features include a large, well excavated cirque and a few smaller mini-cirques.

Figure 5:  This image of Trinchera Peak shows the location of the main cirque and a mini-cirque (semicircles with "X").  Shown by the dashed lines are thrust faults caused during mountain building episodes during the late Miocene.  The large, main cirque is the one on the thrust faults.
 
 

 Figure 6:  During times of extreme cold, when glaciers were retreating and exposing the ground, rock glaciers, patterned ground, and talus became features on the upper flanks of Trinchera Peak.  This image shows talus on upper Cucharas Creek Valley.  The brown blotchy coloration on these rocks are lichens.  The presence of lichens on the talus and rock glaciers in this region suggests that these formations are inactive.  Active rock glaciers need a persistent ice matrix to allow the rocks to flow somewhat like an ice glacier would flow.
 
 

While the upper valley is typified by glacial erosional features, the lower valley is a complexity of innertwining glacial sedimentary features.  End, lateral, and medial moraines begin to become evident in the landscape in the middle valley, about where the tents are located in figures 1 and 5.  Slightly further down, a series of vertical bedrock slabs influenced glacial erosion.  At these barriers, advancing glaciers dug in and scooped out large kettle holes.  Further down the valley, large and very complex moraine structures are dominant in the landscape.

Figure 7:  Cucharas Creek flowing over a bedrock barrier.  When the glacier over rode this barrier, the ice plunged downward scooping out sediment between this barrier and the next one.  A series of seven such barriers exist between the summit of Trinchera Peak and the campground in the lower valley.
 
 

References:

Lindsey, David A. Petrology of Arkosic Sandstones, Pennsylvanian Minturn Formation and
Pennsylvanian and Permian Sangre de Cristo Formation, Sangre de Cristo Range, Colorado--Data and Preliminary Interpretations. U.S. Geological Survey Open-File Report 00-0474 Version 1.0.
http://geology.cr.usgs.gov/pub/open-file-reports/ofr-00-0474/HTML/INDEX.HTM. 2001.

slvsatimage1.jpg. Sangre.com For you daily dose of the mountains. http://www.sangres.com/slv/. 2001.