POUDRE RIVER CANYON & LAKE AGNES

A few years ago I shared the stage at the University of South Dakota with one of my favorite authors, William Least Heat Moon.  I talked about undergraduate research while Heat Moon talked about his road and water travels.  You can imagine who had the more excited audience!  Heat Moon has authored a trilogy plus one of “on the road books”: Blue Highways, PrairyErth, River Horse and The Road to Quoz-An American Mosey; the first still being my favorite.

In 1978, after losing his teaching job and spouse, Heat Moon set out on a soul-searching, three month road trip to small-town America in an old van and covered ~13,000 miles. His major routes were on secondary roads, often printed in blue color on Rand McNally maps---hence the name Blue Highways. I have always been a Blue Highway sort of person, partially due to my geology interests and partially due to my general inquisitiveness of all things natural.  It has been a good life.

One of the great Blue Highways in Colorado is CO 14 trending from Teds Place, northwest of Fort Collins, westward to Walden. Teds Place is situated in a valley between the Cretaceous Dakota Formation (forming the prominent “Dakota Hogback”) and the ~1.7 Ga Precambrian metamorphic rocks of the Front Range.  The valley contains rocks of Pennsylvanian, Permian, Triassic and Jurassic age but they are not well exposed.


Google Earth© image of area near Teds Place.  CO 14 turns west off U.S. 287 crossing Quaternary sediments and poorly exposed Permian, Triassic and Jurassic rocks, hits the Precambrian schists and gneisses and heads west up the Cache la Poudre River Canyon.  Note how the canyon narrows as the highway and river meet the Precambrian rocks.

CO 14 generally follows the Poudre River, a Blue Ribbon trout stream, and travels through some of the most spectacular scenery in the state.  Immediately after leaving Teds Place the road plunges into the canyon of the River and its exposures of Precambrian rocks.  This lower section of the canyon is unglaciated and therefore has a characteristic “V-shape”. The upper reaches of the canyon have been glaciated and visitors will notice a distinct difference between this upper “U-shaped” section beginning about Home Moraine (~mm 85).  A glacial terminal moraine also backs up Chambers Lake near the summit of Cameron Pass.

The lower Cache la Poudre River, the “V-shape” part, preserves large meanders that are deeply entrenched into the hard crystalline Precambrian rocks.  These features indicate the drainage network has adjusted to renewed uplift of the Front Range (Kellogg and Klein, 2011).

Cameron Pass and vicinity have a wonderful section of Mesozoic rocks that are upturned and well-exposed along the Never Summer Thrust Fault.  These rocks reappear after so many miles of driving through the Precambrian.  One of Colorado’s rock units that is easily identifiable is the Pierre Shale of Cretaceous age, a dark- (black to gray) colored fissile shale originally deposited as “mud” in the vast Western Interior Seaway.  The Pierre, and its western correlatives like the Mancos Shale, is exposed at many localities in Colorado but most often in the basins, in the Book Cliffs, and on the flanks of mountain ranges (such as the Colorado Springs-Pueblo corridor).  Here at Cameron Pass not only is the Pierre exposed at a high elevation (10,245 feet), but across the highway the unit crops out as a hornfel (a low grade metamorphic rock) along the crest of a high mountain range: the Nokhu Crags-Mt. Richthofen area. 

As stated earlier, the Pierre was deposited as a marine layer in the WIS. During the Laramide Orogeny (uplift of the Colorado Rockies) the area now occupied by the Never Summer Range most likely was uplifted, and faulted, and then eroded.  At a major fault, the Never Summer Thrust Fault, Precambrian rocks glided westward over the sedimentary section and the Pierre Shale was tilted upward.  The entire area was then lifted again in a large block.  New volcanism in the area began around 32 Ma and produced a thick layer, perhaps a mile, of volcanic flows and ashes (Larson, 2004).  Then around 29 Ma the granodiorite and monzonite (intrusive igneous rocks rich in the feldspar, plagioclase) stock of Mt. Richthofen was emplaced (Larson, 2004).  This hot magma then provided the “heat” for cooking the shale of the Pierre and the hornfel was formed.  The entire area has been subjected to regional uplift in the last several million years and erosion and glaciation have produced the current dramatic landscape.  It is truly a place to visit and observe some spectacular geology.

The north end of the Never Summer Range taken from the summit of Diamond Peaks (11699’), Medicine Bow Range; looking south with Colorado 14 (unseen) in the valley between Diamond Peaks and the conifer forest. Nokhu Crags is an exposure of the Pierre Shale “cooked” to a low-grade metamorphic rock (hornfels) by igneous intrusions at Mt. Richthofen.

Also near Cameron Pass Larimer Co Rd 103 takes off north and follows the Laramie River to the WY-CO state line.  Since the road is graveled rather than paved it could not qualify as a Blue Highway but would be listed as what, a Blue Back Road?  The narrow river valley is bordered on the west by the Medicine Bow Mountains and on the east by the Front Range.  Both of these ranges have bounding thrust faults.


Digital satellite image looking north from Chambers Lake along the course of the Laramie River.

As the Front Range of Colorado trends northward from Denver, then passing Boulder and Fort Collins, it splits into two different prongs as it reaches Wyoming.  In addition, the Park Range (of Colorado west of North Park) extends into Wyoming as the Sierra Madre Range.

The eastern most range in Wyoming is the Laramie Mountains, extending almost to Casper, and bounded on the east by the Great Plains/Denver Basin and on the west by three intermontane basins: Laramie, Shirley and Hanna.  Laramie Peak, at 10,274 feet, is at the north end of the Range but can be seen for many miles across the flat plains and basins.  The Laramie Range is perhaps best known for lending its name to the defining mountain building event of the Rocky Mountains---the Laramide Orogeny.  West of these basins are the Medicine Bow Mountains with a subrange, the Snowy Range, dominating the scenery west of Laramie, Wyoming.  The westernmost prong is the Sierra Madre Mountains separated from the Medicine Bows by the Saratoga Valley. 

Map showing principal uplifts exposing Precambrian rocks in Colorado.  Note Front and Park ranges extending into Wyoming.  Map from Sims and others, 2001.

The Colorado Medicine Bow Mountains are cored by Precambrian rocks—mostly granites and gneisses (~1.7 Ga event).  They seem to rise directly up from the valley of the Laramie River and it is often tough to spot the high peaks (highest peak is Clark Peak at 12,951 feet).  No roads cross the Colorado Medicine Bows and much of the land is tied up in the Rawah Wilderness Area.

In climbing around on the Medicine Bows I noticed a few glory hours and the rocks seemed not very interesting, certainly not for collecting!  It is my understanding that a few early prospectors were out looking for copper.  However, when the state line is crossed there are several old gold/silver mines in the Wyoming section. 

The narrow valley of the Laramie River in Colorado exposes a section of Cretaceous rocks (and probably some of the basin filling Tertiary North Park Formation) that seems accidental and trapped between the two mountain ranges.  I prospected the Pierre Shale but was unsuccessful.  I was able to locate a few outcrops that contained numerous Cretaceous clams, mostly inoceramids (extinct clams quite common in the Mesozoic).  Without a detailed geologic map I was a little uncertain as to exact location in the stratigraphic column; however, I suspect they were from the Cretaceous Carlile Formation/Group.

REFERENCES CITED

Klein, T. and K. Kellogg, 2011, Central Colorado Assessment Project: http://esp.cr.usgs.gov/research/central_colorado/index.html 

Larson

Sims, P.K., V. Bankey, and C.A. Finn, 2001, Preliminary Precambrian basement Map of Colorado--A geologic Interpretation of the Aeromagnetic Anomaly Map: U.S. Geological Survey Open File Report 2001-0364.