From Little Treasures Come Big Stories: Travels Through Death Valley National Park

Photo by Mrs. Geotripper

I returned from Death Valley to a load of work back on campus, but over the next few days I'll be posting on some of our adventures. On Saturday morning, we awoke to sunrise on the Mesquite Dunes east of Stovepipe Wells. We grabbed our packs and notebooks and hit the road. We had a lot of ground to cover.

Oddly enough, for having spent two days getting to Death Valley National Park, one of our first stops was outside of Death Valley National Park. This was for the simple reason that we were looking to understand the nature of the rocks that make up the mountain ranges surrounding Death Valley. Because we didn't have enough time to climb most of the mountains, we would need to see what had rolled out of the mountains during the many flash floods and mudflows that had scoured their flanks over the countless centuries. I have honest students, but their conscience would have had a tough time dealing with all the little treasures they were about to find. So we made sure we were outside the park boundaries when we let them out onto the alluvial fans coming down from the Funeral Mountains. For many of my students it was their first experience in finding a fossil.

Fossil crinoid stems. These are rare in oceans today (they are known as sea lilies), but during the Paleozoic era, they covered the sea floor like fields of wheat, and entire rock layers are composed of their fragments.

To most normal people, 300-400 million years of nearly continuous mud deposition is perhaps not the most exciting process to consider. But if that 300-400 million years covers the latest Proterozoic eon and the all of the Paleozoic era, such activity is irresistible to a paleontologist. A rock sequence that covers that time period contains the evidence of the rise of multicelled life on Earth, as well as the first appearance of all of the extant phyla known (plus a few extinct ones). A phylum, as a biologist will tell you, is one of the broader divisions into which all life can be organized. One phyla, the chordates, contains all the familiar animals with a notochord or backbone (fish, amphibians, reptiles, birds, and mammals). There are dozens of others, including the arthropods (bugs and crustaceans) and the molluscs (snails, clams and squids) which make up most of the species known today. A more or less continuous record of deposition makes it possible to detect patterns and trends in the evolution of life on the planet through time.

Grand Canyon National Park has a similar range of rocks exposed in the depths of the gorge, but huge pieces of the story are missing because of episodes of erosion. Where the Grand Canyon has about 4,000 feet of Paleozoic sediments, Death Valley has more like 20,000 feet! How can 20,000 feet of sediment fit into a mountain range that rises no more than 5,000-6,000 feet above Death Valley and other grabens in the region? If you look at the photo of the Funeral Mountains below, the answer is apparent: the sediments in the mountain range have been tilted. To walk through 400 million years of Earth history, we need only to walk a few miles along the base of the mountains.

How is it that sediments could accumulate for such a long time in such stable conditions? Most parts of the crust of the Earth are wracked by extreme tectonic activity like volcanism, folding, and faulting. The Paleozoic rocks of Death Valley accumulated in one of the most geologically "gentle" environments on the planet: a passive continental margin. A billion or so years ago, most of the world's continents were combined in a supercontinent we now call Rodinia. The continent began to break up at the end of the Proterozoic, which is a process that involves severe faulting and rifting, along with vigorous volcanic activity, but as the continents moved further and further apart, the processes became less active and finally stopped. The edges of the continents became a site of more or less continuous shallow marine deposition, and as more sediments were laid down, the crust slowly sank beneath the weight, allowing even more sediments to accumulate.

So, from a bit of wandering across a stony desert surface picking up random fossils, a story is told of massive supercontinents breaking apart and forming huge wedges of sedimentary rock that tell the story of 400 million years of evolution of life on planet Earth. In short, this is why I love teaching geology.