Geohopping across Nevada

Burners at incipient plate boundary in western Nevada. Are they waving California goodbye? (original unknown)

Many times I've crossed Nevada in the company of Frank DeCourten and Norma Biggar (hereafter called D & B). Actually I've never met either one, but I know their Roadside Geology of Nevada well. That's where I learned of the state's traumatic history—torn apart, reassembled, buried in ash and welded rock, and now being torn apart again. These stories can be hard to grasp, but I've read and reread the lengthy introduction enough to be awestruck by landscapes that many travelers find dull.

Sturdily bound, with high quality paper—my copy has survived lots of use.

Maps, diagrams and photos are abundant!

In the eight years since D & B published their book, I've often parked off the highway at their suggestion to study and photograph a geologic feature. I think of this as geohopping to geostops, rather than my usual geotripping to geosights (and later blogging about it). Now it's time to give the geostops their due.

One of my favorite stretches of highway between Laramie, Wyoming (home) and the California Central Coast (home of relatives) is US 6 across Nevada. Traffic is light, towns are few, and the geology truly is dramatic!

Geo highlights along US Highway 6, May 2025.

For example, about thirty million years ago, widespread cataclysmic destruction associated with the Great Ignimbrite Flareup (GIF) created Hell right here on Earth. Supervolcanoes erupted repeatedly across today's Nevada depositing ash thousands of feet deep, much of it welded into rock by the searing heat ("ignimbrite" means "fire cloud rock"). Trying to recreate that terrifying Flareup in my mind is one of the joys of driving across Nevada.

But it's impossible to properly imagine the GIF, in part because "no volcanic eruptions ever witnessed by humans come close to rivaling these prehistoric paroxysms." And the geologic record suggests it may be one of the largest ever. Consider this: in Nevada at least 230 supervolcanoes ejected an estimated 17,000 cubic miles of lava! Here's another way to think about it: at least 30 of these eruptions each equaled 600 Mt. St. Helens eruptions!

Blue Jay Maintenance Station on left, remnants of cataclysmic destruction behind.

About 90 miles southwest of Ely, I stopped at Palisade Mesa in the southern Pancake Range. Parking is available at a small rest area next to the Blue Jay Maintenance Station. Volcanic rocks of the GIF are nicely exposed on the steep slope to the east.

Rock pancakes stacked oldest to youngest, from bottom to top.

Palisade Mesa is one of multiple gently-tilted stacks of volcanic rock that give the Pancake Range its name. The escarpment at Blue Jay shows at least four episodes of eruption, all from the immense Central Nevada caldera complex. The pale bottom (oldest) layer is a lightly-welded tuff from an ash flow c. 31 million years ago. Next is a thin black band of glassy vitrophyre—"a flow of glowing ash that became densely welded."

Vitrophyre—beautiful memento of incandescent destruction. James St. John.

The massive brown layer above the vitrophyre is a younger tuff, about 30 million years old. Being a fan of columnar jointing, it was my favorite. The summit is a 2.75 million-year-old tuff that's sufficiently welded to provide an erosion-resistant cap.

I 💖 columnar jointing—created by contraction with cooling.

The view south beckoned.

Palisade Mesa obviously deserved a longer visit, perhaps a hike along the base and up the valley to the south. But not this time. Instead I continued west.

Those who cross the middle of Nevada (e.g. east to west) soon become aware of its extensive deformation even if they have no idea what happened. For example: When I left the Pancake Range I crossed Hot Creek Valley, then the Hot Creek Range, then Stone Cabin Valley, then the Monitor Range, and then Ralston Valley before stopping in Tonopah near the crest of the San Antonio Mountains. This is typical Nevada topography—valleys and mountain ranges one after another, all trending roughly north–south. The great pioneering geologist Clarence Dutton called them “an army of caterpillars marching north from Mexico".

Left of center, caterpillars are marching across the Basin and Range Province (NPS).

The cause of this curious pattern is east-west continental stretching, which started about 30 to 40 million years ago and continues today. Some parts of Nevada and adjacent Utah and California have nearly doubled in width! In the process normal faulting has dropped basins, leaving adjacent land standing high, as mountain ranges.

In Tonopah, I stopped for gas and groceries as I often do. Here Hwy 6 merges with heavily-traveled Hwy 95, but at Coaldale Junction they diverge, and once again I had the highway mostly to myself. This is where I stumbled upon Radio Goldfield several years ago, broadcasting very local news and interesting country-ish, old-timey, new-to-me music. It's still going strong.

At the advice of D & B, I kept an eye out for a diatomite quarry on the left, near the junction with NV Hwy 264. The white patches were obvious. This diatomite is thought to be the same age as late eruptions of the GIF, but the setting was entirely different—a shallow freshwater lake where diatoms (microalgae) basked in the sun. Now they're diatomaceous earth, a soft crumbly rock that's 80–90% silica. Among its many uses are metal polish, toothpaste, cat litter, dynamite, thermal insulation, and bonsai soil amendments.

I would have enjoyed examining the diatomaceous earth, but wasn't clear on ownership.

Diatomaceous earth up close; scanning electron micrograph by Dawid Siodłak.

After continuing east across Montgomery Pass, I dropped into Queen Valley for the final geostop of the day, parking in a large pullout not far from California. Across the valley was the north end of the White Mountains; the snowy Sierra Nevada was visible in the far distance. It was a lovely peaceful place, or so it seemed that day. But nearby there were clear signs of geologic trauma.

White Mountains rise steeply above floor of Queen Valley.

Normal faulting evidenced by triangular facets (arrows).

Across the valley at the base of the White Mountains is a normal fault just 3 million years old. This is the Queen Valley fault—a tiny piece of the immense Walker Lane. I had entered a profound but vague tectonic boundary, where the Basin and Range Province meets the great Sierra Nevada.

At Walker Lane (yellow), very different tectonic regions meet. SAFZ is San Andreas Fault Zone, a critical part of this story (Carlson et al. 2013).

Walker Lane is young—just 10 million years old at the south end, and only a few million at the north. The combination of Basin and Range extension and transverse movement of the Sierra Nevada has created a complex zone of faults that's poorly understood. Even so, Walker Lane generates a great deal of excitement among geologists. Perhaps a new plate boundary is forming! Maybe California will drift away!

Like the better known San Andreas Fault to the west, Walker Lane is contributing to the slow, incessant, contrary motions of the Pacific and North American tectonic plates, which are pulling a large part of California northward. Currently the San Andreas is responsible for about 80% of this movement but Walker Lane appears to be catching up.

Fauds & Henry (2008) predict that in another 7 to 8 million years or so, the northern part of the San Andreas will join Walker Lane, extending the Gulf of California north by hundreds of miles and turning California into a peninsula along a new plate boundary. 

If this tectonic shifting continues, as the authors think it will, California will become the island that was regularly reported by explorers hundreds of years ago! This was the "famous cartographic error that appeared on many European maps from the 16th to the 18th centuries" (David Rumsey Map Collection).

"Novissima et accuratissima totius Ameriae" by Nicolaes Visscher, 1690. Large island off the west coast of North America is California. DRMC

Peering even further into the future, we may well find that California Island has become an exotic terrane (quit snickering!). As such, it could travel far and wide before being stopped at some convergent plate boundary, thousands of miles from its origin at Walker Lane.

But Emmie ... our ephemeral lives mislead us. The Earth is far from stable.

Sources

agimark 2018. Splitting North America – The Walker Lane; Part 1 – The Tectonics; Volcano Hotspot blog. Accessed Dec 2025.

Carlson, CW, et al. 2013. Kinematics of the west-central Walker Lane ...  Geosphere 9: 1530–1551.

David Rumsey Map Collection, an unbelievably wonderful resource for fans of old maps. WARNING: it's very easy to spend a lot of time here. https://www.davidrumsey.com/

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

Faulds, JE, and Henry, CD. 2008. Tectonic influences on the spatial and temporal evolution of the Walker Lane: An incipient transform fault along the evolving Pacific – North American plate boundary. Nevada Bureau of Mines and Geology, Arizona Geological Society Digest 22. The future of California is discussed on page 463. PDF

Wolterbeek, M. 2020 (Feb 18). How the burgeoning Walker Lane may split the American West; in Nevada Today, UNV Reno.

A Darkling Path through the Ferny Ferns

Be there dragons here?

It's November and The Monthly Fern series is winding down. Looking back, I realized that most of the ferns I chose are distinctive—they're aquatic or have dimorphic leaves or are primitive lycophytes or grow large enough to inveil a romantic tryst! So this month's post will feature the ferny ferns (my term)—the ones we immediately recognize as ferns. However, figuring out which specific kind isn't guaranteed. If only ferns had flowers—so showy and diverse! Instead we must rely on leaves (1).

As the days shorten it would seem that writing descriptions for our Guide to South Dakota Plants would be appealing, especially given my current subjects—ferns and their relatives. But they can be difficult, and at times inscrutable. Of course they aren't the ones to blame. We are—specifically we botanists who seek order in their labyrinthine world.

I try to make my plant descriptions user-friendly, as our intended audience is broad—professionals, academics, students, enthusiasts, and eager novices (2). Being online makes this much easier. There will be many photos so I can shorten the text and minimize technical terms. Even so, there remain features that must be explained, for example the lovely lacy leaves of the ferny ferns.

The much-divided leaves of ferny ferns are the basis for "fernlike"—for example, "Western Yarrow leaves are fernlike" (SAplants).

Fern descriptions typically start with the plant—height, form (erect, spreading, sprawling), behavior (colony-forming, clumped), and other fairly straight-forward things. Leaves are next. Position (basal, on the stem), color, dimensions, and overall shape are easy to describe. But then ... we're faced with the dreaded degree of dissection. How many times is the leaf divided? Are there true segments? Are the segments themselves divided and are these divided as well? Here the guides I've been using as examples diverge, perhaps out of confusion. Suddenly the way forward becomes unclear; the path darkens considerably.

Entering the darkling world of leaf division.

"Leaf Division" from Fern Structure (USDA Forest Service).

In my web wanderings, I found a figure showing degrees of leaf division (above). It seems clear, though one needs to know that "pinnate" means divided and "-fid" means "nearly". For example, "pinnatifid" means nearly once-divided—division doesn't quite reach the midrib of the leaf as it does in "pinnate".

I intend to use this figure, perhaps as a pop-up, but will replace "pinnate" with "division" thereby eliminating the need for translation. "fid" situations will be accommodated with "nearly", for example "nearly twice-divided".

My version—actually a common approach, not my invention. 

Declaration of degree of division is followed by description of the ultimate segments—their shape, size, hairiness, margins, and such. This can provide much-welcomed help with identification. 

Ready for a test? Using the photos below, describe leaf division in Male Fern, Dryopteris filix-mas, and characterize the ultimate segments.

Male Fern's clumped ascending leaves can be more than a meter long. Аимаина хикари

Once-, nearly twice-, or twice-divided? Note the toothed (but not spiny) margins of the ultimate segments (click on image to view). Nick Turland

The sources I use all say that leaves of Male Fern are nearly twice-divided (pinnate-pinnatifid). But you needn't feel bad if you chose a different answer—you are correct. Male Fern leaves are usually once-divided at the tip, often twice-divided near the base, and nearly twice-divided in between. But adjacent segments can differ as the photo shows. Some are true segments, with division reaching all the way to the midrib. Others don't quite make it.

With no obvious path through this shadowy world, let's change the subject.

It's not uncommon for fern identification to be difficult, as even experts acknowledge (e.g., Cobb et al. 2005):

"Many ferns are distinguished by the finer details of the blade and how it is divided, and descriptions of fern blades can seem difficult and frustrating to beginners" (italics mine; I too get frustrated, and take offense at being labeled a beginner).

This is where a truly user-friendly guide can help, with lookalikes and tips for identification.

Be discriminating in your choice of guides.

In South Dakota we have an especially fine (= difficult) example of lookalikes—Fragile Fern vs. Oregon Cliff Fern. They grow on the same types of sites and look oh-so-similar. Both have nearly twice-divided leaves (often twice-divided at the base) and their ultimate segments have rounded tips and toothed margins.

Fragile Fern, Cystopteris fragilis (MWI).

Oregon Cliff Fern, Woodsia oregana (MWI).

Several small but distinctive features can help with identification (10x magnification recommended). Ultimate segments of Fragile Fern are not glandular and usually hairless, and the margins are irregularly toothed. In contrast, Oregon Cliff Fern segments are glandular hairy (more so on the underside), and the margins are regularly toothed.

Fragile Fern, with irregularly toothed segments (MWI).

In Oregon Cliff Fern, segments are regularly toothed (MWI).

Those familiar with these ferns in the wild have another tip, and it's something that's easier to see. Our Cliff Ferns (Woodsia species) often have persistent dead leaf stalks. This isn't the case for Fragile Fern.

It's not unusual for a mature Cliff Fern to have more dead stalks than leaves (Andre Zharkikh).

You can relax now. No more tests. We're very close to the end, with reassuring light visible ahead. And if you found leaf division tedious and difficult, think how I must feel after attempting to explain it! Sometimes I have to remind myself that I love plants.

Notes

(1) Replacing fern terminology—frond, stipe, pinnae, e.g.—with the more familiar terms used for angiosperms—leaf, leaf stalk, leaflet—has become fairly common (for example, Flora of North America). Others adhere to tradition, explaining terms in a glossary or introduction (for example, Cobb et al. 2005).

(2) I'm not enough of an expert to write descriptions of South Dakota plants myself. Instead I rely on the knowledge of others, both in printed manuals and online. The majority of photos also are by others, available online through Creative Commons licenses.

(3) Some readers may be thinking, "Just find fertile leaves with sori!" (spore clusters). After all, we've been told repeatedly that sori are distinctive. But those of Fragile Fern and Oregon Cliff Fern are hard to distinguish at maturity. Fragile Fern does have distinctive pocket-like indusia, but only when young (see photo of leaf segments in post).

Sources

All fern art created with NightCafe AI Art Generator.

Cobb, B, et al. 2005. Peterson Field Guide to Ferns, 2nd Ed. Northeastern and Central North America. Provides excellent lookalike information and tips.

Minnesota Wildflowers, a guide to the flora of Minnesota. This was the first online guide I found, and remains the most user-friendly of those I've seen (there aren't all that many, online guides being relatively new). Fortunately South Dakota and Minnesota share many plant species, and this website will be our main source of photos.

USDA Forest Service. Ferns. Highly recommended.