Thursday, September 27, 2012

To the Edge of the Entrada Erg

The eponymous stone sculptures of Arches National Park in southeast Utah were once sand dunes -- part of the vast Entrada erg of middle Jurassic time (Delicate Arch, National Park Service photo).
The Entrada erg (dune field), now preserved as the Entrada Formation, was huge.  It covered much of Colorado, eastern Utah, northern New Mexico and northeast Arizona.  After millions of years of deposition, followed by burial, lithification, uplift and erosion, the ancient erg now is part of the spectacular sandstone scenery of the Colorado Plateau.
In the McInnis Canyons National Conservation Area near Grand Junction, Colorado, trails wind among domes and alcoves of the Entrada Formation -- remains of ca 175-million-year-old sand dunes.
Entrada sandstone in the McInnis Canyons area is especially amenable to tafoni development (cavities).
Weathering and erosion have brought out the beautiful cross-bedding, as well as tafoni, in this exposure of Entrada sandstone; Twenty-mile Wash, Grand Staircase - Escalante National Monument, southern Utah.
One of the many cross-bedded hoodoo inhabitants of Devils Garden, carved out of Entrada rocks; off the Hole-in-the-Rock Road east of Escalante, Utah.
Even the most extensive ergs do not go on forever, of course.  The Entrada erg was bordered on the north and west by ocean, and was periodically flooded and exposed with changes in sea level.  Instead of massive deposits of sand, there were sometimes low dunes, sometimes tidal flats.  The result is a very different Entrada, like the exposures below at Capitol Reef National Park, located about a half mile west of the east Park entrance.  Cross-bedded sandstone strata -- old dunes -- are present, but are only about a foot thick, and alternate with layers of siltstone, the remains of tidal flats.
Thin veins of gypsum run parallel to and cut across strata.
The Entrada (left center), capped by the greenish-brown Curtis Formation -- deposited during a later marine advance from the north.

Sources and Additional Information

Doelling, H. H. et al.  2000.  Geology of Grand Staircase-Escalante National Monument, Utah.  Utah Geological Association Publication 28.

Fillmore, Robert. 2010. Geological evolution of the Colorado Plateau of eastern Utah and western Colorado, including the San Juan River, Natural Bridges, Canyonlands, Arches, and the Book Cliffs.  Salt Lake City: University of Utah Press.

Orndorff, R.L., Wieder, R.W. and Futey, D.G.  2006.  Geology underfoot in southern Utah.  Missoula, MT:  Mountain Press Publ. Co.

Monday, September 24, 2012

Super easy tree id ... while passing through Hell

Last winter, I wrote a post about using dichotomous keys to identify trees in the Rocky Mountains.  A dichotomous key presents the user with a series of choices, each with two alternatives.  At each step, a subset of candidate species is eliminated.  Eventually, the user arrives at a single species -- the tree at hand!
If we were to take this specimen though the key in the Rocky Mountain Tree Finder, we would decide first that it’s a conifer (without “regular leaves”), then a pine (with clustered needles, five or fewer per cluster).  Then we would choose five needles to a cluster, followed by needle-length of 1.5 inches or less ... voilà!  This is a bristlecone pine.

But there is an easier way to identify this tree, as I realized when it almost knocked me over while I was hiking the Cascade Falls trail east of Cedar City, Utah (map at end of post).  Just answer a single question:  “What kind of bottlebrush would it make?”

Left:  a branch of limber pine, Pinus flexilis, might make a usable bottlebrush in a pinch.

Below:  ponderosa pine, Pinus ponderosa, with its longer needles (4-9 inches), could be handy for cleaning jars.
This bonzai ponderosa pine is less than four feet “tall” -- hardly ponderous, but quite beautifully-formed in response to the harsh badlands environment.
I guess common juniper, Juniperus communis, might work if nothing else were available.
However, if there were a bristlecone pine tree nearby, you would toss your limber pine, ponderosa pine or juniper branches, for bristlecone branches look to be perfect bottlebrushes.
Botanists think bristlecone branches resemble foxtails, and in fact, three closely-related species are grouped into subsection Balfourianae, the Foxtail pines.  These include the Rocky Mountain bristlecone (P. aristata), the foxtail pine of the eastern Sierra Nevada in California (Pinus balfouriana) and the Great Basin bristlecone (P. longaeva) -- which is the one that I found along the Cascade Falls trail.  (The famous ancient bristlecones of the White Mountains in eastern California are Great Basin bristlecones as well.)

I didn’t expect to see bristlecone pines here, at only 8900 feet elevation, but the branches were a dead give-away -- lovely green bottlebrushes!

A bristlecone pine with the graceful symmetry of youth.

With the death of a main stem, this small tree already has developed the pitchfork growth form characteristic of aging bristlecones.
Bristlecone pines are named for the recurved bristles on the tips of the cone scales.
The Cascade Falls trail is located east of Cedar City, Utah, on Dixie National Forest in the vicinity of Navajo Lake.  Take the Navajo Lake turnoff south from Utah Highway 14.  Go about 0.3 mi on this gravel road, then turn left at the sign to Cascade Falls (small sign, only visible going west).  Follow this dirt road down to a gravel road in a drainage bottom.  Turn right (southwest) and continue to the road’s end and the trailhead.  Map from Google Earth.
There is much to see along the short trail to Cascade Falls, which passes through hellish badlands before arriving at a fount of clear, cold, gushing water emerging from a rock crevice!
Passing through hellish badlands ...
... to arrive at a blessed fountain,
... with smokey views of a distant Zion (National Park) en route.

Thursday, September 20, 2012

And the answer is ...

A few days ago, I posted some mystery photos, which Silver Fox of Looking for Detachment identified correctly, in a general kind of way.  And yes, the location is south of Denver, as Ben Nevis pointed out, and a bit west as well.  The complete answer is ... Mt. Hillers -- one of the five Henry Mountains, south of Hanksville, Utah.
“beyond are red and white sands -- inferior rocks tilted almost to the vertical and interspersed with dikes.  Moreover these sandstone hogbacks seem to trend in a curve around the mountain as far as they extend."  Grove Karl Gilbert describing the south side of Mt Hillers, 1875.
South side of Mt. Hillers, from G. K. Gilbert's journal (1875).
Gilbert’s work in 1875 and 1876 was revolutionary.  He concluded that the Henry’s were not volcanic, but rather intrusive igneous features, which he called “laccolites” (today’s laccoliths).  These intrusions pushed up overlying sedimentary strata, forming the nearly vertical hogbacks at the base.
Mt. Hillers, viewed from south.
The Henry Mountains in southern Utah, south of Hanksville.  Waterpocket Fold on left, Lake Powell lower right.  Note sandstone “hogbacks” along south base of Mt. Hillers (click to view).  From ArcGIS online.

Stay tuned for more about the intriguing Henry Mountains, as well as the work of G. K. Gilbert -- perhaps one of the greatest field geologists ever.

Tuesday, September 18, 2012

More famous rock outcrops ...

Yes, another geo-quiz.  Recognize this location?  Note the steeply upturned white, red and buff sandstones, mingling with upturned brown igneous rocks (probably sills).  This mountain is quite famous ... historic even.  In fact, "we are standing on Hallowed Ground" (words of Dr. Brainerd Mears, Jr).  Hint:  Grove Karl Gilbert slept here!
Above -- view from the south.  Igneous rocks continue up slope out of view to right.
Igneous rocks up close; below, with snake weed (Gutierrezia) for scale.
View from west.  Outcrops in first photos are at base of slope on right (click to view).

Note:  answer will be posted at next opportunity for internet access.

Monday, September 10, 2012

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).

Friday, September 7, 2012

Escape Imminent, Destination ... ??

"May your trails be dim, lonesome, stony, narrow, winding, and only slightly uphill."  Edward Abbey
On the road soon, leaving Civilization behind, headed southwest to the Henry Mountains and Grand Staircase - Escalante National Monument.  This is unknown country, a big blank spot on my personal map of the Colorado Plateau.  Any suggestions, recommendations?