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.

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 3: LIFE!

Fossils! Do you remember that first moment that you found a fossil as a child? Or did you never have that experience? I vividly remember exploring a meadow on the Kaibab Plateau north of the Grand Canyon as a 10-year-old. I didn't know what they were at the time, but later learned that they were disks from crinoid stems and sponges from the Permian seas 300 million years ago. I was impressed; I was not even disappointed that they weren't dinosaurs. I can look back and see this moment as one of the seminal events that led to my choice of geology as a field of study.


All kids go through a fossil phase. I also know that most geologists and paleontologists simply never outgrew it. And so it is that today's post is about fossils. Particularly the earliest complex life that evolved on our planet.

600 million years ago (give or take, what's a few million years between friends?), the continents of the world were in the process of splitting and breaking up. Many laypeople have heard of Alfred Wegner's Pangea, but we are not talking about that particular supercontinent (although we will in a later post). The continent that existed around a billion years ago has been named Rodinia. Such continents seem to be unstable, perhaps due to the buildup of heat from underneath. The land is pushed upwards in vulnerable places, the continent splits, and new seas are formed. As the continents move further apart, they subside and the shorelines around the margins of the continent invade the land. All over the world, the seas were transgressing across continental surfaces, forming shallow epicontinental seas (I am guessing that Hudson Bay comes closest in today's world as an example).


The split in North America cut across the southeastern part of California, and through central Nevada. During the Cambrian Period, the transgressing seas covered most of the southwest, with the exception of a portion of Utah called the Transcontinental Arch. The region was located close to the equator, so the ingredients were there for life: nutrients, shallow water, warmth. Life was about to get a whole lot more complex than the single-celled organisms that had existed for billions of years. The details are for other experts to describe, but multi-celled life appeared, and thrived.


Look at the picture of the Grand Canyon. Down in the depths there is a prominent terrace called the Tonto Platform. The platform exists because the Bright Angel Shale cannot form cliffs for very long, being easily eroded. Just below lies a cliff of Tapeats Sandstone, and above are a few slopes and ledges of the Muav Limestone. These three formations, sand, shale and limestone, record the invasion of the shallow sea across the Grand Canyon region.


Pick up enough fragments of the Bright Angel Shale, and you may be lucky enough to find the signs of the explosion of life that swept across the planet around 540 to 500 million years ago. Worm trails and burrows are plentiful, but the prizes will be the discarded exoskeletons of trilobites.


Trilobites are arthropods in the same phylum as the insects, crustaceans, arachnids and other jointed-legged creatures. Because they were one of the first animals to possess a hard shell, their fossils tend to dominate the fossil record, even though a large number of soft-bodied animals were doubtlessly around (see one of my first posts). They walked and burrowed in the sediments of the ocean floor; some grazed on algae, some were mud processors, some may have been predators. They ranged in length from a quarter inch to one monster that exceeded three feet in length. Their closest living relatives are probably the horseshoe crabs (see this NOVA geoblog article). Some were capable of rolling up like pill-bugs to protect themselves. They grew by molting, so not every discovery is a dead creature, simply a cast-off shell.


Other early creatures included brachiopods, sponges, and strange cone-shaped archaeocyathids, which may have been distantly related to corals.