Johann Gottlob Lehmann Advances the Understanding of Rock Formations
Johann Gottlob Lehmann Advances the Understanding of Rock Formations
Just as a book can be called the fundamental unit of a library, so can rock formations be thought of as the fundamental units of historical geology. However, until the eighteenth century, few people really noticed that rocks come in discrete "packages," and nobody thought to try to use these formations to reconstruct the geologic history of a region until Johann Lehmann (1719-1776). Although Lehmann's interpretation of the rocks has since fallen out of favor, his insights have proven fruitful and, over two centuries later, the concept of rock formations continues to be used in much the same manner and for much the same purpose as originally proposed.
The first geologists are lost in the depths of time. They were our most distant ancestors who noticed that flint and obsidian made better knives and scrapers than sandstone or shale. Combing their surroundings, they found that some locations were better suited for finding these rocks, and they returned there repeatedly for their raw materials. Lacking any idea as to why one place was better than another, they could not predict in advance which locations would prove rich or poor; they could simply look, find, and remember. Possessing only descriptive power, however, geology was not yet a science. It was simply a collection of observations about rocks.
One thing they undoubtedly noticed was that like stones tend to congregate, often in beds that form layers stacked one atop the other. Sometimes flat, sometimes folded, these rocks formed the foundation of the world, underlying mountains and plains, hills and valleys. As time passed and people began digging into the earth to bring up valuable rocks, they also must have noticed that similar rocks tend to congregate even underground. The same patterns and associations a person sees above ground continues in the subsurface. However, this remained an observation only (albeit a valuable one) for many more years, still lacking predictive or explanatory power.
One of the hallmarks of any science is its ability to explain what we see in the world around us in such a way that a scientist can accurately predict what he or she will find elsewhere, in a yet-unseen part of the world. The early "geologists" at this point still only observed. Using their observations to explain and predict properties of the Earth still lay far in the future.
One of the first steps towards making geology a science came with Nicolaus Steno's (1638-1686) recognition of fossils as the remains of once-living organisms and, more importantly, his use of fossils to help understand the relative positions of various beds of rock. These breakthroughs took place in the late seventeenth century. Less than a century later came another breakthrough: Lehmann's recognition that certain groups of rocks tended to be associated with each other, often over large distances. For example, a certain red sandstone might lie atop a distinctive limestone and beneath a particular green shale, and these rocks might also appear in another location a great distance away. Such rock formations are chapters in the history of the Earth, and reading them provides great insights into the geological history of particular regions or of the entire Earth.
In the middle of the eighteenth century, Lehmann published some interesting work. First, he identified three separate types of mountains—primary, secondary, and tertiary—each with a distinct set of geological characteristics and characteristic rock types. He then extended this by using these characteristics to try to determine the conditions under which the mountains formed, and he developed a set of drawings to try to show how these formations continued underground. Finally, he suggested that these rock formations and the rules he had developed to describe their characteristics could be extended throughout the world instead of being a purely local phenomenon. Although these observations and theories were designed as a way to help the mining and mineral industries, it is also significant that Lehmann regarded himself as a historian of the Earth, and was apparently the first person to try to use the rock record in this fashion. In spite of his attempts to place all that he saw in a biblical framework, his work helped introduce concepts that continue to be useful to this day.
Lehmann's findings resonated on a number of levels and were to influence geology and society for many years.
1. By introducing the concept of a rock formation, he helped give future geologists a fairly powerful tool with which to develop their science and their understanding of the Earth and its structures.
2. In addition, this was one of the first steps taken towards making geology a legitimate science because he used his rock formations to predict not only what the rocks looked like beneath the Earth's surface, but also to suggest how they might be arranged in other parts of the world. Even though many of his interpretations of the rock record were later shown to be wrong, many of the tools he developed are still in use.
3. Finally, although Lehmann felt strongly that the biblical account of the Earth's history was correct, his methods were later used by others to help show that the Earth is far more ancient.
As mentioned in the overview, the rock formation can be considered a fundamental unit of geological interpretation. By looking at a formation, and at the relationship between a series of rock formations, geologists can interpret the geological history of a region. Piecing together the histories of many regions, they can extend this understanding across a continent, between continents, and throughout the world. This process of understanding the Earth's history is still taking place. Geologists have roughed out the basics, but they are still mapping, interpreting, and puzzling out these histories. In this quest, they have developed any number of concepts ranging from mundane to revolutionary. As one example, the theory of plate tectonics was bolstered by noting the existence of nearly identical rock formations on both sides of the Atlantic Ocean and other identical formations shared by Africa and South America, Australia and Antarctica, and so forth. Without the basic concept of what a geological formation was, this process would have been much more difficult to follow. On a smaller scale, economic geologists use the relationships between various rock formations to help predict where to best drill for oil, dig for gold, or search for other economically valuable minerals that are necessary for our industrial society. In fact, the Industrial Revolution was powered in part by coal found in this manner, by tracing coal-bearing formations across England and Continental Europe.
In doing this, the early geologists were beginning to turn geology from a purely descriptive pursuit into a predictive and explanatory science. By predicting where coal could be found, for example, they helped save time, money, and lives; just as modern economic geologists help mining and petroleum companies better determine where to look for minerals and oil. This helps to save the companies money, but also helps reduce costs to manufacturers and consumers because raw materials can be sold for less. All of society benefits.
In addition, Lehmann consciously strove to use his rock formations to determine the history of the Earth and to describe why it looks the way it does. Using rock formations in this manner, he used geology to explain, in a relatively self-consistent manner, the observed physical features of the Earth. As mentioned above, his interpretations are now known to have been incorrect, but it was a decent first step. In fact, geology was to become one of the first recognized sciences in its own right towards the end of the eighteenth century, based partly on these newly developed abilities.
Lehmann's observations were later extended impressively to help unravel the geologic history of England during the nineteenth century. Most of the currently recognized geologic eras, based initially on the existence of similar rock formations with similar assemblages of fossils, was developed at this time, with radiological dating later used to determine the ages of the rocks. It is likely that Lehmann would have been pleased to discover that, although individual rock formations are usually not as wide-spread as he had envisioned, there is enough overlap of these formations to allow the tracing of geologic periods across the planet.
Finally, as knowledge of geology grew, it became obvious that the Earth had to be ancient. Finding the same rock formations on either side of the Atlantic Ocean, for example, implied that either the Atlantic Ocean had opened up, rifting Europe and the Americas apart, or that there was once a continent there that had eroded away, leaving identical rocks on either side of the ocean. Both of these scenarios required the Earth to be quite old because of the vast amounts of time required for either event. Similarly, finding formations of marine rocks high in the Alps suggested that, somehow, those rocks had either been covered with water that subsequently drained off the land or that they had been lifted into place at some time in the past. Again, either scenario required a lot of time. Ironically, a tool first used to help set rocks into a biblical context was used to strike some of the first blows in favor of an old, nonbiblical Earth forced into its current shape by huge, but decidedly, natural forces. This conception of the Earth's antiquity made its way back into the public consciousness, and most of the educated public gradually came to accept that the Old Testament could be interpreted as allegory as well as fact. This, in turn, sparked a debate that continues to this day in schools and courts.
P. ANDREW KARAM
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Oldroyd, David. Thinking about the Earth: A History of Ideas in Geology. Cambridge: Harvard University Press, 1996.
Rudwick, Martin. The Great Devonian Controversy. Chicago: University of Chicago Press, 1985.