Recognize this famous outcrop?

This is another very short geology quiz, just one question.  But not to worry, it’s entirely optional and the answer follows shortly.

Another view -- Cambrian strata lying unconformably on Precambrian basement rocks, Cretaceous coal below.

These rocks are viewed by many tens of thousands of travelers daily, mostly via unconscious glances.  I myself have passed through this canyon many times but have rarely stopped, and aside from taking in the ambience of dark contorted walls viewed at 50 mph, I’ve never paid much attention to the geology of the area.

Colorado River in foreground, railroad on far bank, interstate highway inside forested slope above.

But this time I decided to check it out, and pulled off at the Hanging Lake Rest Area on Interstate 70 in Glenwood Canyon.

Looking downstream.  In addition to a railroad and an interstate highway, there is a paved biking/hiking trail in the narrow canyon bottom, the 16-mile Glenwood Canyon Recreational  Trail.

Glenwood Canyon is the Colorado River’s passageway through the late-Laramide (early to middle Eocene) White River Uplift.  From ca 20 to 7.8 Ma (million years ago), the river carved a wide valley about 420 m (1400 ft) deep.  Excavation of the inner gorge (850 m or 2800 ft) started ca 7.8 Ma, with the majority taking place during the past 3 Ma (Kirkham et al. 2000a).

The Hanging Lake Rest Area features wonderful views of the Great Unconformity, a huge gap in the rock record.  The Cambrian Sawatch quartzite rests unconformably on late Precambrian gneiss and granite, representing a missing interval of almost a billion years (the estimated age of the Earth is 4.54 billion years).

The Cambrian Sawatch, "white and buff to gray-orange, brown-weathering vitreous metamorphosed quartz sandstone in beds 1-3 ft thick" (Kirkham et al. 2000b), on top of Precambrian basement rocks.

.



Left:  late Precambrian gneiss, posing with a boxelder tree.


Below:  Cambrian Sawatch quartzite.

Hanging Lake Rest Area is at Exit 125 on Interstate 70 east of Glenwood Springs, Colorado.  Note:  there is no west-bound exit so west-bound travelers must continue to Exit 121 (Grizzly Creek), get back on I-70 eastbound, and return to the Hanging Lake Exit.  Sound confusing?  It did to me, but turns out everything is well-signed and easy to follow.

West end of Hanging Lake tunnels on I-70, Glenwood Canyon.

Passing through Glenwood Canyon in 1903.  Source.

See Glenwood Canyon 12 Years Later for details of the history and amazing construction of Interstate 70 through Glenwood Canyon.

For More Information

For more on the Great Unconformity, and a global perspective on its origins, see Written in Stone’s The Great Unconformity and the Late Proterozoic-Cambrian Time Interval: Part II

Kirkham, R. et al. 1996. Field trip guidebook on the geology and geologic hazards of the Glenwood Springs area, Colorado. CO Geol Surv Open-File Report (0271-888X).

Kirkham, R. et al.  2000a.  Constraints on timing and rates of Late Cenozoic incision by the Colorado River in Glenwood Canyon, Colorado: in Young, R.A and Spamer, E.E., eds: Colorado River Origin and Evolution - Proceedings of Symposium, Grand Canyon National Park, June 2000, pp. 113-116.

Kirkham, R.  2000b.  Guide to the Geology of the Glenwood Springs Area, Garfield County, Colorado, Earth Science Week Field Trip.  October 13, 2000.

USDA Forest Service.  Hanging Lake Trail (guide).