|Paleontology and Our Local Desert
I will never forget an experience I had during a field geology class in my undergraduate days. We were mapping the distribution of different rock layers in the Coyote Mountains out near Ocotillo. The geology in this area is quite complex and I had been working my way up a ridge-line composed of hard, crystalline metamorphic rocks when I noticed that the ground changed from hard rock to a soft, fossil-bearing sandstone. (I figured out later that a fault line separated the two rock types.)
From this vantage point, as far as the eye could see was desert, yet at my feet were the remains of ancient sea creatures -- scallops, and oysters. At that moment I felt a real sense of the history of this place. I could envision the ancient sea in which these prehistoric animals lived and imagine the quantities of sand and mud brought down to the sea by ancient streams to bury and entomb them. My imagination then turned to the tectonic forces which altered the position of land and sea, tilting and shearing the rocks to form the complex terrain around me. I have since felt these sensations in other areas, but the desert still provides me with the most dramatic sense of earth history.
Similar experiences have no doubt been shared by countless generations and I imagine that fossils must have sparked the interest of early people, who during their wanderings in the natural world found sea shells high on the sides of mountains far from any ocean waters. The "great flood" mentioned in many of the world's creation mythologies was probably an early attempt to explain such occurrences; for it is fundamental to our makeup to be curious and to seek explanations of natural wonders.
Today we realize that the earth is a dynamic sphere (each earthquake reminds us of this) and that sea floors can become mountain tops through the action of lithospheric plate movements. Although the processes of mountain building, and the corollary of erosion and peneplanation, have been a continuous theme in earth history, at no one time was the entire earth covered by ocean water. Rather, local regions of the earth have experienced mountain building (and erosion) at different times.
For example, the now deeply eroded Appalachian Mountains of the eastern United States were once a youthful mountain range with rugged peaks. Today, the West Coast of the United States is a geologically active region. In fact, our own local desert is one of the most active areas in the world.
Here the San Andreas Fault Zone joins the northern limit of the East Pacific Rise, the consequence being large-scale fault activity, widening of the entire desert basin, and high heat flow. On a more human scale, the effects of this geologic activity can be seen in the recent fault scarps along the western front of the Coyote Mountains adjacent to Route S-2. The scarps here are the result of horizontal and vertical tectonic movements along the Elsinore Fault; a smaller, subsidiary fault of the San Andreas Fault Zone to the east. It is estimated that the Coyote Mountains have been uplifted approximately 2 meters during just the past 10,000 years and that their present elevation is entirely due to movement along the Elsinore Fault.
One of the consequences of this uplift is the occurrence of tilted, fossiliferous, marine sandstone and siltstone layers on some of the higher peaks in the Fish Creek and Coyote Mountains. These fossil deposits provide us with evidence from which we can unravel the history of these mountains and the surrounding desert region.
I often use the analogy of a book when describing stratigraphy and the fossil record to others. The pages of a book contain information which we read and interpret. A sequence of layered sedimentary rocks is much like a book (laying on its back cover) with individual layers (the pages) containing information in the form of sedimentary particles (sand, pebbles, etc.) and fossils. The geologic "book" preserved in the rocks of our local desert has many incomplete or missing "chapters". I will focus on only two of these "chapters" for the remainder of this article.
In the Fish Creek and Coyote Mountains are preserved a thick sequence of sandstone and siltstone strata which geologists have named the Imperial Formation. These rocks are thought to have been deposited approximately 5-7 million years ago in a northern extension of the proto-Gulf of California.
The Imperial Formation locally contains abundant remains of marine invertebrate fossils. Most distinctive are remains of scallops, pen shells, oysters, whelks, sea urchins, and corals. In all, over 200 different fossil marine invertebrate species are known from the Imperial Formation.
Recent field work by staff of the Natural History Museum have recovered rare remains of marine vertebrates from these rocks including remains of bat ray, white shark, tiger shark, giant barracuda, pufferfish, sea cow, and baleen whale. Remains of an early walrus have also been collected from the Imperial Formation.
One of the more fascinating aspects of the Imperial Formation fossils has been the recognition of certain molluscan species having modern counterparts living today in the Caribbean Sea. In fact, some workers estimate that as many as 16% of the Imperial Formation fossil molluscs represent Caribbean species. The remainder of the fossil molluscan species are related to eastern Pacific or Gulf of California forms.
What does this mean? It means that 5-7 million years ago when the Imperial Formation was being deposited, the proto-Gulf of California had a direct connection with the Caribbean Sea. This connection was through the area of modern day Panama, prior to elevation of the Isthmus and the formation of a land connection between North and South America. Additional paleontologic and geologic evidence confirms this hypothesis of an open connection between the two oceans.
From this example we can see how our local fossil deposits lend support to general ideas of earth history and help to define the changes in geography which have occurred -- another reminder of the dynamic nature of the world we inhabit.
Palm Spring Formation
In the badlands region of Anza Borrego Desert State Park are exposed an approximately two mile thick sequence of sedimentary rock layers, called by geologists the Palm Spring Formation. These sedimentary rocks are thought to have been deposited in ancient piedmont, flood plain, and playa lake settings. Today they are high and dry and steeply tilted to the south. The Palm Spring Formation ranges in age from about 0.9 to 4.0 million years and contains a diverse assortment of fossil vertebrate remains.
The late George Miller of the Imperial Valley College and more recently George Jefferson and his crew of dedicated volunteers at the Anza Borrego Desert State Park have collected fossils in the badlands for many years and have documented a community of organisms quite different from that inhabiting the region today. Using these fossils as a guide we can travel back in time and view this ancient world.
Imagine lions, sabercats, and bears; herds of llamas, long-legged camels, horses and American zebras; family groups of mammoths; giant, solitary ground sloths; flocks of ice age vultures; innumerable small mammals including shrews, gophers, mice, kangaroo rats, and rabbits; and a variety of snakes and lizards. The vertebrate fossils from the Palm Spring Formation provide us with one of the most complete records in the southwestern United States of land life during the transition period leading into the Ice Age or Pleistocene Epoch.
From this brief discussion I hope that the reader can see that to experience earth history we don't have to travel to distant or exotic places like the Galapagos Islands. Instead, we can get the essence of this geologic experience right here in San Diego County, especially in our local desert.
The Ethics of Fossil Collecting
The fossil-bearing deposits of our local desert generally occur on public lands, either under Bureau of Land Management or State Park Service control. Collection of fossils on these lands is strictly regulated and permits are required.
In this age of explosive human population we need to realize that the pioneer mentality of our forebearers has to be tempered by thoughts of the needs of future generations. We can no longer lay claim to all we see and in the case of fossils should be content to observe their occurrence without requiring a souvenir.
I have seen areas where fossils were formerly abundant almost completely stripped of them by curious visitors. Unfortunately, in most cases the collected fossils were probably quickly forgotten or later thrown out when the collector grew tired of the clutter. Even museums need to be content in some cases to "only take pictures" and leave an area intact for future visitors. Please keep these ideas in mind during your next visit to the desert and enjoy the geologic experience
Thomas A. Deméré, Ph.D.; Curator, Department of Paleontology