Interpreting Earth History from the Geologic Record
Principles and Practices of Stratigraphy
The Meaning and Recording of History. The study of history in the high school setting is primarily restricted to the investigation of the chronology and sequence of events of human habitation. Such events are defined and interpreted based on tangible and compelling evidence left behind by individuals and their activities of the time of interest. The high school student derives his or her knowledge about such events primarily through the written word which can be preserved and handed-down to future generations of students. As long as the words are truthful, factual, and convey appropriate meaning, books are of limitless value in chronicling human activity. Books are not a spontaneous, naturally-occurring phenomenon, however. They are a product of the creativity and desire of humans to chart our future by preserving the events of our past. The age of oldest known preserved writing is measured in thousands of years, whereas graphic depictions on rock walls of historic events extend back tens of thousands of years. Prior to this time, very little is known about the history of humankind except from the few scraps of bone, stone, and other materials which have escaped the ravages of weathering, erosion, and time. The history of the vast majority of time that humans and their closest ancestors have been treading upon the Earth remains essentially an unknown. Since there are no books or writings which remain from those times, we must rely upon whatever repositories of data that have been left for us coupled with our contemporary understanding of human behavior and characteristics to arrive at a model of, for lack of a better phrase, our pre-history.
Geologic History. As humans we can relate to most types of human activity, whether modern or ancient. We have a close and personal understanding of what it means to be human, the human spirit, human goals, human abilities, and our interrelationship with the Earth. However, this relationship and understanding becomes hazier and less certain the farther back in time we try to extend our view. I we wish to better understand human events one million years before present, for example, our vision is almost completely blind. If we take this length of time and multiply itby a factor of 4,600 it would take us back to the birth of the Earth! Imagine the task that is then put before the historical geologist to develop a history of 4.6 billion years in duration in which there are no books to read, no art works to view, no tools to study, and no humans to leave records! The challenge is indeed mind-boggling. Nevertheless, we must endeavor to employ our intellect and inherent creativity and make the most of Earth's own bountiful historical records.
Objectives of Historical Geology. Since all humans dwell at (or very near) Earth's surface, our immediate knowledge of geologic processes and products via a direct analysis of rocks and observation of the processes that produce them is restricted to those occurring in our habitable environment. Although the ongoing investigation of the deep oceans has been accomplished through a variety of direct and remote-sensing methods, the direct observation of Earth's subsurface at depths of greater than 2 kilometers is extremely rare and the data equally sparse. Therefore, for the average historical geologist, the detailed study of surface rock exposures is still the main avenue of investigation. Since 85% of the Earth's surface is covered with sediments and sedimentary rock, it is logical begin a study of Earth history by appraising sedimentary processes and their products since they can be observed directly and there is a relative abundance of source material.
Principles of Stratigraphy. By definition, stratigraphy is that branch of geology that is concerned with the spatial and temporal (time) development and relationships of sediments and sedimentary rocks. There are six guiding principles (or rules), in addition to the Principle of Uniformitarianism, that guide the stratigrapher in the practice of his or her specialty. These are the principles of Original Horizontality, Superposition, Cross-Cutting Relationships, Index Fossils,Faunal Succession and Walther's Law.
Original Horizontality. Sediments (and the sedimentary rocks which represent their lithified equivalents) accumulate at the bottom of areas of depressed crustal relief in response to gravity in essentially horizontal layers called beds or strata. The bounding surface of a sedimentary rock bed, referred to as the bedding plane, is also created as an originally horizontal feature, perpendicular to the local force of gravity. The observation of inclined beds and bedding planes usually (although not entirely) suggests the mechanical rotation (or deformation) of the strata out of the horizontal by some dynamic geologic process or force.
Superposition. Sediments and sedimentary strata accumulate in a vertically-oriented, bottom-to-top, "layer-cake" succession, in which the first-formed (or oldest) layer appears at the bottom and progressively younger layers are encountered upwards, as long as the succession has not been overturned. Again, the discovery of older layers resting on top of younger layers strongly suggests some type of extreme deformation and rotation in response to dynamic geologic processes.
Cross-Cutting Relationships. Geologic features which "cut" (or interrupt) the bedding succession in an exposure of sediments or sedimentary rock are always younger than the sedimentary succession itself. Features such as faults, folds, fractures, veins, and dikes are always younger than the rocks in which they are found.
Index Fossils. If an organism occupied a broad geographic area of the Earth's surface but existed for only a (geologically) brief (but known) period of time, the rocks which possess fossils of this organism are the same age (coeval). These types of fossils are referred to as index fossils and are indicators the age of the rock in which its remains are entombed.
Faunal Succession. Fossil organisms of a particular type preserved in a vertical succession of undeformed sediments and sedimentary rock reveal morphological changes from more primitive in lowermost layers to more advanced in uppermost layers.
Walther's Law. The order in which the sediments or sedimentary rocks appears in an (undeformed) vertical succession must also have occurred laterally (horizontally) across the basin of deposition. Therefore, rock type alone is not an indicator of a rock's age.
James Hutton, Sir Charles Lyell, and William Smith were United Kingdom geologists working during the late 18th and early 19th centuries. Hutton (above left) is credited with having introduced and applied the concept of deep time to the interpretation of geologic history. Lyell (below) is credited with having introduced and applied the principle of uniformitarianism to the interpretation of rock sequences. Smith is credited with having introduced the concepts of original horizontality and superposition to determine the relative ages of undeformed sedimentary rock successions. [Photographs courtesy of the Geological Society of London.]
Frontispiece to Sir Charles Lyell's Principles of Geology [1830-1833]
1. Briefly define the term stratigraphy. Briefly describe the role of the stratigrapher, the types of rocks with which he or she works, and the places where such rocks are found.
2. Briefly define the following and their value to the work of the stratigrapher:
a. The Principle of Original Horizontality
b. The Principle of Superposition
c. The Principle of Cross-Cutting Relationships
d. The Principle of Index Fossils
e. The Principle of Faunal Succession
f. Walther's Law
3. Which other information must also be obtained from a sequence of sedimentary rock, in the absence of fossils, if an accurate geologic history and chronology of events is to be established? THINK!!!