Sunday, April 8, 2007

Where the Mountains Meet the Plains

Where the Mountains Meet the Plains

Geology Field Trip

Sue Avre
GEY 121
April 2007

1. Owl Canyon Road #1 - 3
2. Poudre Canyon #4 - 7
3. Bellvue #8
4. Horsetooth Reservoir #9 - 12

1. Owl Canyon Road
2. Owl Canyon Road
3. Owl Canyon Road
4. Cache la Poudre Canyon
5. Cache la Poudre Canyon
6. Cache la Poudre Canyon
7. Cache la Poudre Canyon
8. Bellvue Dome
9. Horsetooth Reservoir
10. Horsetooth Reservoir
11. Horsetooth Reservoir
12. Horsetooth Reservoir

The geological history of Colorado begins about 2.5 billion years ago with the formation of cratons (a craton is a portion of a continent that has not undergone tectonic deformation since Precambrian or early Paleozoic time) known as the North American Platform. From about 1.8 to 1.4 billion years ago three separate plate collisions happened, producing the Precambrian rock (also known as basement rock) now found in Colorado. This includes the Precambrian granite and metamorphic rocks. For the past 480 million years, Colorado drifted around the earth due to tectonic plate movement, eventually became part of Pangea, split off, and slowly drifted northwest, (Chronic & Williams). Approximately 70 through 28 million years ago the Laramide Orogeny (orogeny is a term meaning mountain building) built the Ancestral Rocky (the first Rocky Mountains that eventually eroded away). After the Laramide Orogeny, erosion deposited gravel and debris over the eastern plains. About 5 million years ago was another uplift happened and built the Rocky Mountains we see today, (Swartz).

The field trip:
There are gorgeous views of the Rocky Mountains driving north on Hwy. 287. As you near the Owl Canyon Road turn-off, you will see on the west side of the highway is the Dakota Hogback and on the east side is the Lyons Sandstone Hogback, (Chronic).

STOP 1: Owl Canyon
The field trip begins at the intersection of Shields and US Hwy. 287 on the north side of Ft Collins, CO. Go 14.2 miles north on US Hwy. 287 to the Owl Canyon Rd, a.k.a. County Road 72 and turn east. At 0.5 mile is the first stop.

Photo 1:
This is a photograph looking north over at a small uplift of the Grayback Monocline. This area consists of Cretaceous Pierre shale overlaid with eroded sandstone that gives the soil its red color, (Matthews).

Photo 2:
This photograph shows an angular unconformity. An angular unconformity is one in which the upper layers overlie lower beds that have been folded by tectonic processes and then eroded to a more or less even plane. In an angular unconformity, the two sets of layers have bedding planes that are not parallel, (Grotzinger).

Photo 3:
On the south side of the road is Deadman Butte; seen in photograph. It consists of Triassic strata, (Matthews). Buttes are a hill with steep sides and a flat top formed by erosion. The top is usually a cap-rock of hard material covering layers of softer rock underneath, which are easily eroded away. They are usually found in flat lying sedimentary rock areas, (

Stop 2: Cache la Poudre Canyon
Turn around here and go back to Hwy. 287. Turn south and proceed 4.2 miles to the Cache la Poudre River exit (a.k.a. Hwy. 14) and travel west 5.8 miles. There will be a small turn-out area on the highway to park. Looking to the northwest is photograph #5. Carefully cross the highway and walk about 50 yards to the east for a close look at metamorphic rock.

Photo 4:
This is a photograph of Precambrian metamorphic rock. According to Chronic and Williams, “Rocks that wall Poudre Canyon, 1.8 billion-year-old sediments and volcanic flows of an island arc, were steeply folded and highly altered by the immense heat and pressure of the Early Proterozoic Orogeny 1.7 billion years ago.”

Photo 5:
Close up photograph of the metamorphic rock.

Turn around and go back down the canyon 1 mile. There will be a small pullover on the other side on the road to park at. Very carefully cross the highway and walk about 30 feet to the east.

Photo 6:
This is a photograph of light pegmatite veins within darker amphibolite. There are several areas in the Poudre Canyon where this can be seen. During the Early Proterozoic Orogeny fluids filled cracks in ancient rocks buried between 7 to 11 miles deep and at temperatures ranging from 500 to 700 degrees Celsius allowing minerals to crystallize, (Chronic).

Photo 7:
This is a close up of the metamorphic rock gneiss and intrusive igneous rock pegmatite. An intrusive rock is “a coarse-grained igneous rock that crystallized slowly when magma intruded into country rock deep in Earth’s crust.” (Grotzinger).

Stop 3:
Continue back down the canyon to Hwy. 287. Turn south and proceed 1.4 miles to the LaPorte Road (a.k.a. Co. Rd 54G) and turn south. Go 1.5 miles to the Rist Canyon road (a.k.a. Co. Rd 52E) and turn west. Go 0.8 miles to the Watson Lake turn-in area. There will be a parking lot.

Photo 8:
This is a photograph of Bellvue Dome. “A dome is an anticlinal structure with a broad circular or oval upward bulge of rock layers. The flanking beds of a dome encircle a central point and dip radially away from it”, (Grotzinger). The exposed strata show formations of the Ingleside, Satanka, and Lyons. Some geologists believe there is a fault running just to the west side of the dome, (Matthews).

Stop 4:
Return to LaPorte Road and turn right. Proceed 0.1 mile and turn left (south) on Co. Rd 23. Go 2.5 miles to the Horsetooth Dam and turn right, driving over the top of the dam. On the west side of the dam will be a small parking area. Do not stop on the dam, it is posted “No Parking”. Walk about 100 yards back across the dam for view of photograph #9. Then walk back to the boat ramp area and down near the shore line, proceed about 300 – 400 yards to the west where the remaining photos were taken.

Photo 9:
Horsetooth Reservoir is a manmade reservoir built between the foothills (right side of photo) and the Dakota Hogback (left side of photo). The hog back exposes both Precambrian rock and Phanerozioc strata. The Lykins Formation of the lower Triassic and Upper Permian periods form most of the area under Horsetooth Reservoir. The Lykins Formation consists mainly of reddish-brown siltstone and fine-grained sandstone. The dam from where this photo is taken built upon Forelle Limestone.

Photo 10:
This is a close-up photograph of sandstone. Note the symmetrical ripples on top. The ripples were causes by water movement (oscillating waves) when the sediment was laid down. This area had once been a shallow sea.

Photo 11:
This is a close-up photograph of mudstone or siltstone. Notice the classic cracking know as mudcracks. Mudcracks are formed from the evaporation of water leaving the minerals and sediment behind or the intermittent wet – dry of a inter-tidal area.

Photo 12:
This is a photograph of the layering of sandstone. The layers are basically horizontal with slight lifting to the east (left). The layers between the mudstone and sandstone can be seen on the right side of the photograph.

Photo 13:
This is a photograph of sandstone with “something” in the middle. It is a dark semi-circular ring with and indention in the middle. The “ring” is not of another substance. I am not sure if this is a fossil or a fossil imprint. What do you think this is?

  1. Chronic, Halka and Felicie Williams, “Roadside Geology of Colorado”.
  2., 2007,
  3. Grotzinger, Jordan, Press, and Siever “Understanding Earth”.
  4. Swartz, Dave, “The Horsetooth Quadrangle Virtual Geological Field Trip” 2003 - 2004