Another Whisper from the Past...and Another Catastrophe

I am always kind of haunted (in a good way) by ancient petroglyphs and pictographs. Most of my teaching in a classroom involves delivering information using one kind of rock (chalk) to form images on another kind of rock (slate, or an artificial derivative of slate). My images are far more transitory (they last right up to the moment the custodian erases them), but I feel an affinity for those who left images in stone hundreds or thousands of years ago. They were telling a story, sometimes with an obvious message (bighorn sheep here!), and sometimes with symbolism and codes that are long lost to history.

I'm back from our field studies journey in Death Valley, but I'm still finding a bit of material to post. We were out on the last day, making our way south out of the valley over Emigrant Pass and Wildrose Canyon. In a drier year we would have paid a visit to Aguereberry Point or the Wildrose Charcoal Kilns but snow and ice prevented us from doing so. With the additional time we paid a visit to some striking petroglyphs on a rhyolite ash flow of the Nova Formation. They depict animals, as seen above, and quite a few indecipherable symbols. Although one should NEVER touch the rock art, nor should one do rubbings, it is certainly a temptation! I wanted to reach up and touch this surface that was another's message. It's a moment of communicating with a fellow human being across the centuries (don't worry, neither I nor any of my students were touching the petroglyphs!).

While looking at the rock art, I took a step back and looked at the canvas the artist had chosen: an outcrop of tan colored rhyolite ash perhaps 30 feet thick. It lies interbedded with the conglomerates of the Nova Formation, which formed in a series of chaotic basins that developed over an extending terrane in the Panamint Mountains between 5.4 and 3.1 million years ago (the Emigrant Detachment). The rhyolite ash was formed in a violent explosion that inundated the region with the white powdery material. I looked closer at the rock and was struck by the absolute starkness of the basal contact. I was looking at a single moment in time, a matter of a few seconds from a momentous event several million years ago. It was a castastrophe.

By catastrophe, I don't mean a worldwide conflagration that eliminated whole species across the the planet. It was more local, perhaps affecting only a few hundred or few thousand square miles, but any living thing living within the region of the eruption was instantaneously buried in hot ash. Look at a close-up of the contact below...

This was an alluvial fan surface. A rugged mountain close by was shedding debris via streams, floods and mudflows, leaving a rocky landscape. It could have been a sunny day, maybe cloudy. Vegetation was growing among the rocks, and a variety of large animals were grazing nearby: numerous species of horses and camels, antelopes, mastodons, and the carnivorous animals, ancestors to today's canids and felines. There was a vast rumbling, a huge explosion ripped into the sky, and darkness quickly engulfed the area. Animals surely tried to escape, but there was nowhere to go. They were buried. Their last moment is memorialized by a thin line in a cliff that divides a conglomerate from a white ash layer.

Picture of the Day - A Great Outcrop, Part 3

Apologies to Ron and Silver Fox for the delay revisiting last weekend's outcrop; all that death defying stuff distracted me. This is a well-known stop for many Death Valley field trippers at the south end of the Resting Springs Mountains just west of Shoshone. It is a marvelous site for teaching about faulting and aspects of volcanism. The prominent fault appears at first glance to be a classic reverse fault because of a tendency in one's mind to link the orange layers. The layer in the footwall (right) includes a dark lavender layer that is not present in the headwall, and thus the layers do not correlate. The smaller fault shows the correct offset.


The rocks in the outcrop are primarily air fall tuffs, although a megabreccia is exposed nearby. The rocks have clearly been tilted a bit.


Once my students finish looking at and sketching the faults, their attention is drawn to the dark layer to the left. It is a vitrophyre, a rhyolite ash flow that was so hot when it was erupted that it remelted to obsidian in the middle. A close inspection of the outcrop shows pumice pyroclasts that are progressively flattened towards the dark obsidian in the center of the flow. The upper part of the flow shows vesicularity as gases moved upwards during the cooling phase. Apparently the flow is in the original orientation; it was not tilted, but was instead plastered against a sloping hillside. The vugs contain interesting minerals, including zeolites and other silicates.


I wasn't so smart with this outcrop the first time I saw it. My instructor convinced us first that it was a reverse fault next to a coal seam, and then pointed out the inconsistency of the fault interpretation. Then it was hmmm? maybe not a coal seam, could it be a lava flow? Was it a dike? Oh dear, maybe we would have to figure out this whole thing ourselves.


Researchers much more talented then myself have published a tract about the outcrop that has been reprinted and is on sale at the Shoshone museum, which is also worth a stop if you are in the region. They have an excellent exhibit on the mammoths and other creatures that have been excavated in the region. Check it out!

• Troxel, B.W., and Heydari, E., 1982, Basin and Range geology in a roadcut, in Cooper, John D., Troxel, Bennie W., and Wright, Lauren A., Editors, Geology of selected areas in the San Bernardino Mountains, Western Mojave Desert, and southern Great Basin, California: Volume and guidebook for field trip no. 9, 78th Anniversary Meeting of Cordilleran Section, Geological Society of America: Shoshone CA 92384, Death Valley Publishing Company, p. 91-96, 202 p.