San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Paleontology]

Paleontology

Introduction
Imperial Formation
Palm Spring Formation
The Ethics of Fossil Collecting

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Paleontology and Our Local Desert

Introduction

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.

Read about the Imperial Formation

Thomas A. Deméré, Ph.D.
Curator, Department of Paleontology