On-the-Rocks Field Trip: Tour of Fairy Caves at Glenwood Springs

Title: On-the-Rocks Field Trip: Tour of Fairy Caves at Glenwood Springs, and Geological History of Glenwood Canyon

Trip Leaders: Bob Kirkham and John White, Colorado State Geological Survey

Date: Saturday, July 15, 2000

Publication: The Outcrop, June 2000, p. 9-10

[The Fairy Cave portion of this abstract is excerpted from Mark Longman’s August, 1999 Outcrop article, “Colorado’s Newest Tourist Cave.”]

The standard tour is a 1/3 mi walk with 200 ft of elevation gain. It takes about an hour, and allows visitors to see historic Fairy Caves, spectacular views from Exclamation Point, and the new Glenwood Caverns, including magnificent King’s Row and ‘The Barn,” a soaring 5-story room decorated with various stalactites, soda straws, and great examples of “cave bacon.”

Solution of the 325 my old (Mississippian) Leadville Limestone created the caverns, mainly during the past 7 to 9 my as the Colorado River was eroding Glenwood Canyon. The caves are at about 7100 ft elevation. Passageways totaling 2-1/2 mi have been mapped to date, but there probably are many more yet to be discovered. Fairy Caves are now dry but speleothems are still forming in other parts of the cave system.

After lunch we will have two or three stops in the spectacular 3,000-ft deep Glenwood Canyon to discuss the erosional, depositional, and volcanic history of this gorge. The canyon cuts obliquely across the southern margin of the Laramide White River Uplift and provides some of the best exposures of Paleozoic and Proterozoic rocks found anywhere in the state. Faults and folds are locally prominent in the canyon, but it appears to be largely a superposed valley formed by climate change and regional uplift.

Colorado Geological Survey geologists have discovered several late Miocene and Pliocene basaltic lava flows that constrain the timing and rate of canyon cutting and shed light on paleogeography. Their investigation greatly benefited from cooperative work with the U.S. Geological Survey that involved numerous Ar40/Ar39 age determinations and geochemical correlation of late Cenozoic volcanic rocks, which we will discuss.

From about 25 to 20 Ma (early Miocene), thick basaltic lavas accumulated on the White River Uplift north of the present Colorado River, capping the Flat Tops. Some of these flows rest on river gravels. After about 20 Ma, the Colorado River began to carve a broad valley across the southern edge of the uplift. By about 7.8 Ma, the river had eroded to about 1,200 ft below the Flat Tops plateau. The possible rate of incision during this time ranges from 0.1 to 0.6 ft per thousand years. Near the end of the Miocene, the ancestral Glenwood Canyon was merely a broad, relatively flat valley with gently sloping walls, lying parallel to and several miles south of the present canyon.

Another series of basaltic lava flows erupted about 7.7 Ma; they also formed a broad volcanic plateau. A large remnant of this plateau now caps Dock Flats and Little Grand Mesa, which lie about 3,000 ft above the Colorado River and form the southern rim of Glenwood Canyon. At least one thin flow spilled into the paleochannel of the Colorado River, which was about 200 ft below the main volcanic plateau. The river channel was south of Cottonwood Pass prior to these eruptions, but north of Dock Flats and Little Grand Mesa afterward; the flows may have dammed the river, forcing it to flow around the northern margin of the volcanic field. The river has remained in this same general position for the past 7.7 my.

A small remnant of a 3.0 Ma basalt flow caps Gobbler Knob about 300 ft below the 7.7 Ma flow. This indicates the river cut down only a few hundred feet between 7.7 and 3.0 Ma, a relatively slow incision rate of about 0.11 ft per thousand years. The river rapidly cut an additional 2,500 ft during the past 3.0 my, at an average rate close to one ft per thousand years. During this time the steep inner walls of Glenwood Canyon formed. This period of rapid incision closely coincides with Pleistocene glaciation that started about 2.6 Ma.

We now know that the canyon has aggraded throughout the Holocene. One might expect bedrock to lie at shallow depth below the modern river channel. However, more than a thousand drill holes cut by the Colorado Department of Transportation during the design and construction of I-70 demonstrate 200 ft or more of Holocene sediment in the riverbed. This sediment has resulted from mass wasting of the canyon walls, primarily rockfall, but also debris-flow, alluvial, and probable deltaic deposits.

A large Holocene rockfall near the western end of the Hanging Lake tunnel partially dammed or slowed the river, causing fine-grained, gray lacustrine sediments to be deposited upstream of the rockfall dam. This notorious “gray layer” forced very expensive foundation designs for many highway structures. C14 dates on organic material in the “gray layer” range from 9,820 ± 130 to 3,890 ± 120 ybp. This large rockfall is also responsible for Cottonwood Falls, the most challenging stretch of white water in Glenwood Canyon.

If time permits and the field trip attendees are interested, we will conclude the day with a discussion of some of the geologic hazards that plague the canyon and point out some of the mitigation techniques utilized to reduce the hazards.

Trip participants should convene at the Park-n-Ride at 6th Ave. and Simms St. at 7:00 a.m. From there we will car pool to Glenwood Springs for a 10:30 tour. For those wishing to meet us at the cavern visitor’s center, you can meet us in Glenwood Springs NO LATER THAN 10:15 a.m. The visitor’s center is at the Colorado Hotel, 508 Pine St. There will be a $10 fee for the tour. Temperature in the caverns is a cool 52°F, so a light jacket and good walking shoes are recommended. After the cave tour we will have lunch, then proceed to the outcrop stops in Glenwood Canyon. Participants are responsible for their own lunches.

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