Wednesday, December 14, 2022

Tree Following: tree in a tree & a geography challenge

Where was I?
The large trees on the shore of this small shallow lake, one of many such lakes in the area, are Plains cottonwoods. Note they have green leaves in October. And the wind isn't blowing, just a light breeze. Obviously I wasn't home in Laramie, Wyoming.

While trying for artistic shots of gnarled bark, I noticed a small green ash tree growing out of a cavity in the trunk of a cottonwood.

I thought for a while about how this might have happened. Maybe the winged seed (samara) landed and germinated where enough debris had accumulated for the seedling's tiny root to keep it in place. Certainly enough rain (ann. av. 24") and snow (ann. av. 33") falls here to sustain it. But what does the future hold? Will the green ash live on when the cottonwood senesces, dies, falls? Will large recreational vehicles continue to fill the campground nearby? Will there still be a skiff or two on the lake at sunset?
Back to the challenge: Where was I? Specifically, in which US state and which quadrant (SW, SE etc.)? The next photos may be helpful. You can post your answer as a Comment, but sorry ... no prizes.
Clue on the horizon.
View from the top: trees galore, prairie in the distance.
This particular state quadrant was the destination of a recent trip. Blog posts to follow.

This is my contribution to the December gathering of tree followers, kindly hosted by The Squirrelbasket. More posts here. With no news about my tree (it's winter in Laramie), I'm reporting on trees elsewhere.

Thursday, December 8, 2022

Devil's Playground—another intrusion, this one with wildflowers

Late afternoon at the Devil's Playground in northwest Utah—my kind of #vanlife.
I ended my tour of Utah and Nevada last May with a stop at the Devil's Playground north of UT Highway 30 near ... well ... near not much of anything. It's at the south end of the Grouse Creek Mountains and about 40 air miles west of the north end of the Great Salt Lake. The Utah Geological Survey provides directions here.

Like several other stops on the trip, this one featured an igneous intrusion—the Emigrant Pass pluton, emplaced 41 to 34 million years ago in three phases. Rock in the Devil's Playground area is part of the youngest phase (Egger et al. 2003). This pluton is especially interesting to geologists studying metamorphic core complexes (MCCs), for it is in the southern part of the Albion–Raft River–Grouse Creek MCC (ARG in map below).

Black blobs are MCCs; arrow points to Emigrant Pass pluton. After Strickland et al. 2011.
Note multiple plutons in Albion, Raft River, Grouse Creek Mts. After Egger et al. 2003.
The diagram below shows a textbook metamorphic core complex (actually there may be too much variation to have a classic example). During continental extension, rock strata stretched, broke, and slid along low-angle faults, revealing older deeper rocks which domed upward. But continental extension has occurred across much of the Basin and Range Province without making MCCs ... hmmm, puzzling.
Based on Peterson & Buddington 2014, DeCourten & Biggar 2017.
As I learned during my trip to the Ruby Mountains, MCCs are controversial. Among topics debated is why they're clustered in this part of North America (thickened crust?). Another is the role of plutons in MCC formation. Some geologists argue that plutonism is the main driver, supplying heat that softens rocks and facilitates extension. After all, plutons roughly contemporaneous with extension "are ubiquitous in many of the core complexes" in this region. But other geologists disagree, arguing that plutonism plays a minor role at most (see Introduction in Egger et al. 2003 for more discussion).

I like metamorphic core complexes very much, largely for their mystery. But they're difficult. It's challenging just to spot one, even with a guidebook. These are giant structures, visible only as parts exposed here and there. Plutons are much easier to understand, fairly common, and yet still worth contemplating. I became a fan when I realized that if I can see a pluton, something dramatic must have happened.

It seems plutons are often sculpted into intriguing forms, like the Harrison Pass pluton in the Ruby Mountains.
Tors carved from one of our local plutons, at the crest of the southern Laramie Mountains.
Notch Peak intrusive at the base of the west face of the House Range; photo by Mike Nelson.
Emigrant Pass pluton—a tilted world. Did it tilt during uplift? 
Plutons, like the god Pluto, reside in the Underworld. They form when magma solidifies well below the surface, where it cools slowly and forms visible crystals. So unless we make a Dante-esque excursion miles underground, we can only see plutons if they've been exposed in some way. When the Grouse Creek Mountains rose about 13 million years ago, during Basin and Range extension and faulting (Ege 2006), erosion set in. That's probably when the "devils" of the Playground were born.

Rocks in this part of the Emigrant Pass pluton have been called granitic and granitoid. These are handy terms because the range of granitic rock types is broad and hard to subdivide neatly. Egger et al. (2003) are more specific: "a virtually homogeneous coarse-grained biotite granite". The granite is criss-crossed with aplite and pegmatite dikes, which formed when still-molten magma—hydrous and therefore last to crystallize—was injected into fractures in the solidifying pluton.

Aplite is more resistant to erosion so dikes stick out a bit from the granite.

My field assistant pointed out a dike in a different kind of granitic rock.
Weathering of the pluton may have started underground, with groundwater enlarging fractures. In any case, today's wonderfully enigmatic forms are largely products of physical and chemical weathering above ground (Ege 2006 explains this nicely). These processes will continue until eventually the alcoves, spires, arches, fins, and other devils disappear.
Fine example of spheroidal or onion-skin weathering at the Devil's Playground. Photo courtesy scienceteacherexplorer (click link for more great shots).
Geocacher enjoying spheroidal weathering (source).
Are these young devils, recently emerged? Or elderly ones, to dust returning?
In the company of plants and rocks :)
Among the devils were spring wildflowers, a nice touch. Perhaps the most common was Stenotus acaulis, the Stemless Mock-goldenweed. It grows on rocky soils in drier areas across the western US. This is a DYC—"damn yellow composite"—but only because we find yellow composites difficult to identify. Seems to me that's our damn problem.
Stenotus acaulis; those of us who have been around for awhile may know it as Haplopappus acaulis.
Another yellow composite (Compositae is the old name for the sunflower family) caught me by surprise—Balsamorhiza hookeri, Hooker's Balsamroot, a plant of the Great Basin. I know Arrowleaf Balsamroot well, but didn't recognize this plant as a balsamroot. Based on online specimens and discussions, the plants here might be hybrids; more research needed.
Hooker's Balsamroot seems so different from Arrowleaf Balsamroot. Most strikingly, it is short (these plants are 10 to 15 cm tall), and can thrive on very dry rocky sites.
With so much sagebrush in the area, it wasn't surprising to find its common parasite—paintbrush; this one is Castilleja angustifolia, the Northwest Paintbrush. The low gray-green shrub next to it in the photo is sagebrush. Like DYCs, paintbrushes are difficult to identify to species. But I've never heard anyone damn them. Thanks to markegger for the identification, via iNaturalist.

Paintbrushes are hemiparasitic. They can photosynthesize, but by tapping into sagebrush roots they grow more vigorously. This ability may vary among species, perhaps explaining conflicting reports online.
Such a lovely parasite!


DeCourten, F, and Biggar, N. 2017. Roadside Geology of Nevada. Mountain Press.

Ege, Carl. 2006. Geosights: Devil's Playground, Boxelder County, Utah. Utah Geological Survey Survey Notes 38 no. 1, January 2006.

Egger, AE, et al.  2003. Timing and nature of Tertiary plutonism and extension in the Grouse Creek Mountains, Utah. International Geology Review, 45:6, 497-532.

Peterson, J, and Buddington, A. 2014. A geological study of the McKenzie Conservation Area, Spokane County, Washington. Conference Paper.

Strickland, A, et al. 2011. Timing of Tertiary metamorphism and deformation in the Albion–Raft River–Grouse Creek metamorphic core complex, Utah and Idaho. J. Geol. 119:185–206.

Saturday, November 12, 2022

Tree-following Therapy

Quaking aspen near Pole Creek, Laramie Mountains, November.
It's November, and in the Laramie Mountains much of the color is gone. We're left with black, brown, gray, white, dark green (needles), and blue (sky). Somehow photos look better in black-and-white.

Off to see the poplar tree :)
Not pleased with short days, and especially not pleased with so much time needed indoors lately, I felt much better after visiting the old balsam poplar I've gotten to know this year. We're lucky to have the Happy Jack Trailhead just 15 minutes from town!

And of course we visited the Carboniferous Pond, for meditation and exploration. Along the margin I spotted some color—green buttercup leaves, still making carbohydrates before winter sets in.
 Ranunculus gmelinii, the lesser yellow water-buttercup; leaves are 0.5–1 inch across.

This is my monthly contribution to the gathering of tree-followers kindly hosted by The Squirrelbasket. More news here. For more about tree-following, see this post. Consider joining us—it can be very therapeutic!

Tuesday, October 25, 2022

House Range: more than a Tertiary fault block

Sun sets on the House Range, west central Utah. Thanks to Mike Nelson for photo and info.
Last month I wrote about my stay in the House Range, a large mass of rock that started as sediments in a Cambrian sea. Now 500 million years later, they stand nearly 10,000 ft above sea level. On the steep west face the layers are obvious. On the opposite side we see gentle slopes that disappear below the surface of the valley to the east.
Steep west face, with the "well-defined stratification" observed by Capt. JH Simpson in 1859. Because the crest looked like structures, he called these mountains the House Range.
Road leading to the east side of the House Range. Note the gentle slopes. The pale rock in the draw on the right is not sedimentary.
Pioneering geologist Grove Karl Gilbert visited the House Range in the early 1870s, as part of GM Wheeler's Surveys West of the One Hundredth Meridian. The asymmetry of the uplift blew his mind. "... the beds exhibit in cross-section but a single direction of dip" (italics mine). He had expected something like the Appalachian ranges, whose structure was considered typical of all mountains. "... it was only with the accumulation of difficulties that I reluctantly abandoned the idea." (Gilbert 1875)
Gilbert's cross-section through the House Range (1875).
Of course surprises were to be expected; geology was still a young science. For example consider orology—the science of mountains (now orogenesis). In Gilbert's time, geologists struggled to explain how mountains formed even in a very general way. Their theories ranged from crustal wrinkling as the Earth cooled to fiery forces underground.

Gilbert admitted he was far from understanding orology in the "Basin Range System" (now Basin and Range Province). But after studying so many ranges he could shed some light on the subject. He suspected the House Range was bounded on the west by a steep normal fault, which had tilted strata downward to the east. And whatever caused this uplift must have been operating on a grand scale, for he had seen similar structures over a huge area.

The Basin and Range Province overlaps the Great Basin but is larger, mainly to the south (sources differ on boundaries). In the BRP, ranges generally trend northerly, are bounded by normal or listric faults, and are separated by sediment-filled basins.
Gilbert also discovered "cross faults" in the House Range. He included one in his diagram of the west face (below), which emphasizes the southerly component of overall dip. In several places, strata have been disrupted by faulting. Gilbert shows this by labeling beds of quartzite ("q", looks like "g") and limestone ("l"). Near Notch Peak the quartzite and limestone are higher than they are at Dry Pass to the north, contrary to what we would expect based on dip. Gilbert had no explanation for these faults, except that they probably predated uplift of the House Range (Hintze & Davis 2003).
Click on image to view displaced quartzite and limestone (Gilbert 1928).
Today geologists generally agree that Basin and Range orogenesis is due to stretching of the continent, as the Pacific and North American plates grind past each other along transform faults (DeCourten & Biggar 2017; see Busby's Walker Lane diagram). Extension is thought to have started 30–40 million years ago, and so far has doubled the distance between Salt Lake City and Reno. It continues, as evidenced by earthquakes and precision GPS measurements. As Gilbert suspected, this is orogenesis on a grand scale—from eastern California to central Utah, and from southern Oregon and Idaho to northern Mexico.

Gilbert also was right about the relative age of the cross faults in the House Range (Hintze & Davis 2003). They were part of earlier mountain building (late Jurassic through Cretaceous), when western North America was being compressed as the Farallon plate dove under the west coast. The result was 200+ million years of orogenesis, producing the Sierra Nevada, Rocky Mountains, and the lesser known Sevier orogenic belt (DeCourten & Biggar 2017).

During the Sevier Orogeny strata were shoved eastward, sometimes great distances, along low angle thrust faults (detachments). Seismic exploration has shown that the central House Range is underlain by several shallow-dipping major faults formed by regional, easterly-directed thrusting, most likely during the Sevier Orogeny (Stoeser et al. 1990).

Non-sedimentary rocks near the base of the House Range's west face.
Oddly, Gilbert seems to have ignored a prominent geologic feature of the House Range, though he may have hinted at it in one sentence: "The rocks are almost wholly sedimentary" (italics mine) (Gilbert 1928, p. 74). The topographic map (Plate 31) provides another hint:
Note "Granite Canyon" northeast of Notch Peak. It's now called Miller Canyon.
Finally, Gilbert's diagram of the west face shows steeply tilted strata below and just north of Notch Peak, but with no explanation (below; red annotations mine).

Are the lower rocks in this photo Gilbert's steeply tilted strata?
In 1905, the eminent paleontologist Charles Doolittle Walcott came to the "great House Range" to study its fossil-rich rocks, and to establish "the interrelations of the strata and faunas in the North American Cordilleran area". In his 1908 report, he included photographs of exceptional sites so that "geologists and paleontologists who have not had an opportunity to see the sections may get an idea of the completeness of the exposures of the strata in the Cordilleran area." One such site was the House Range.
Notch Peak and the west face of the House Range. "... an intrusive mass of granite porphyry is intruded into the Cambrian beds on the north side of the peak (left side)." Aside from this caption, Walcott made no mention of the intrusion in his report.
Being a huge fan of GK Gilbert—such an observant open-minded adventurous geologist!—I have to wonder why he omitted this obvious granitic intrusion in his reports on the House Range. Maybe he just didn't want to struggle with yet another puzzling geologic feature.

But now much of the puzzle has been solved. "The Notch Peak intrusive presents a case study of the whole gamut of magma emplacement ... Seldom can one find in so neat an area the geologic record of such a variety of processes generated by a single intrusive body," wrote Arthur L. Crawford, Director of the Utah Geological and Mineralogical Survey in 1958. No longer do geologists ignore it (e.g. Gehman 1958, Stoeser 1990, DeCourten 2003).

Notch Peak intrusive viewed from east. Intrusions by definition form underground. If we surface creatures can see them, something must have happened—in this case, uplift of the House Range and  erosion.
The Notch Peak intrusive is composed of quartz monzonite, sometimes called granite (they differ slightly in composition). It's assumed to be Jurassic in age (dated at 193–143 Ma), is about 3 mi in diameter or 2.5 x 4.5 mi in area, and may be a laccolith. Other features include aplite dikes and sills intruding adjacent sedimentary rocks, zones of pegmatite, crystal-lined cavities, and mineral-rich skarn.

The beauty of skarn!—from Osgood Mountain intrusive (into carbonates), Nevada. Like Notch Peak skarn, it contains tungsten and molybdenum. James St. John via Flickr.
Intrusions have created great wealth in Nevada and Utah, in the form of ores. As molten magma ascends and crystallizes, hot fluids are expelled. These circulate and dissolve surrounding rock. If conditions are right, new minerals precipitate out in sufficient quantities to produce ore, where "one or more valuable substances can be mined at a profit." (Mineral Resources)

If magma intrudes carbonates—limestone or dolomite—it often creates skarn when saline metal-rich fluids alter the host rock to form new minerals. In the House Range, the Notch Peak intrusive had ample opportunity to alter limestone. The resulting skarn contains tungsten and molybdenum in moderate concentrations (Stoeser 1990).



Here's a simple timeline for the House Range. For dates, check this geologic time scale.

Paleozoic Era, Cambrian Period: Marine sediments accumulate to great thickness off the coast of Laurentia.

Later Mesozoic Era: Sevier Orogeny deforms sedimentary rocks in the area of the future House Range. Notch Peak quartz monzonite intruded into sedimentary rocks during Jurassic Period.

Cenozoic Era, Tertiary Period (continuing to today?): Continental extension with block faulting uplifts the House Range. Erosion sets in, eventually exposing the Notch Peak intrusive.


DeCourten, F. 2003. The Broken Land; adventures in Great Basin geology. U Utah Press.

DeCourten, F. 2022. The Great Basin Seafloor. University of Utah Press. Supplemental Field Guide (PDF) available online.

DeCourten, F, and Biggar, N. 2017. Roadside Geology of Nevada. Mountain Press Publishing Co.

Gehman, Jr, HM. 1958. Notch Peak intrusive, Millard County, Utah. Geology, petrogenesis, and economic deposits. UT Mineralogical & Geological Survey Bulletin 62. PDF

Gilbert, GK. 1875. Report upon the Geology of portions of Nevada, Utah, California, and Arizona, examined in the years 1871 and 1872 in Wheeler, GM. Report upon United States Geographical surveys west of the one hundredth meridian v. 3. Washington [D.C.], G.P.O. BHL.

Gilbert, GK. 1928. Studies of Basin Range structure. USGS Professional Paper 153. PDF

Hintze, LF, and Davis, FD. 2003. Geology of Millard County, Utah. UT Geo. Surv. Bull. 133. PDF

Simpson, JH (US Army). 1876. Report of explorations across the great basin of the territory of Utah for a direct wagon-route from Camp Floyd to Genoa, in Carson Valley, in 1859, by Captain J. H. Simpson ...: Making of America Books, U Michigan.

Stoeser, DB, et al. 1990. Mineral resources of the Notch Peak Wilderness Study Area. US Geological Survey Bulletin 1749. GPO. PDF

Walcott, CD. 1908. Cambrian sections of the Cordilleran area, in Cambrian Geology and Paleontology. Smithsonian Misc. Collections 1910, v. 53 no. 5:167–230. BHL