Virginian Geological Record


 
 
~Introduction to Observation, Description, and Interpretation~  
 
Geology is fractal. That is, the detail and complexity of what we observe remains the same regardless of the scale of observation. Step back and look at Virginia from space, and we see a certain degree of complexity. Now, zoom in closer, and closer, and closer. Down even to a single outcrop, or a single rock. At all scales of observation, the closer we look the more we see that it is not simple. This fractal scaling is the way nature is organized, and to describe a part of nature, it is difficult to determine at which scale to observe and describe it. This is because even though fractal parts retain their complexity at all scales, it does not mean that they look the same at all scales. The big picture and the local outcrop are often different fractal images, and they must be understood and interpreted differently.
 
Curious about fractals? Click here
     
A daunting fact of the study of geology becomes apparent when one begins to examine the technical literature, which describes features at all conceivable scales. One could look at a field guide, for example, that spends pages describing and interpreting a single outcrop, while another paper discussing the same region may not ever refer to any of the details or features of the field guide as they are too small to be relevant at a larger scale. Inevitably, in this description, we must omit many things that seem crucial in order to generalize the geology of Virginia.
     
Another complicating factor in describing geology is that rocks are always the result of processes that have ceased long before the rock is exposed, and while a description is not an interpretation, one must appreciate the existence of the the processes that produced the rock being described. In summary, when describing a rock, one must try to understand the processes that created that rock, while not confusing a description for an interpretation. Geologists are trained to see rocks and realize the fractal scale they are in, or see rocks and imagine the processes active when they formed.
     
 
~Dividing the Rock Record~ 
 
Virginia has had a long, complex history extending as far back as 1.8 billion years. In that time there have been many events; events that have led to the movements that have folded, faulted, and rearranged the geology into complex patterns. Geologic records are organized in at least four ways:
 
1.   Tectonic History of the Rocks. Tectonics refers to earth movement and the structures that result. Here we divide the rock record into the rifting, mountain building, etc. events that have formed and shaped the rocks.
 
2.   Age of the Rocks This describes the rocks on a geologic time scale from oldest to youngest.
 
3.  Geologic Provinces. This describes the distribution of various kinds of rocks geographically.
 
4.   Physiographic Provinces These divide the major landforms (i.e., Piedmont vs coastal plain
 
Each of these gives a different geological perspective to the same region. The problem is that even though these divisions overlap, they do not exactly correspond with each other and each must be understood separately at first. Here we explore the last three divisions; first combining the discussion on geologic/physiographic provinces, and then describing the record by geologic age. The tectonic history is described in the summary of the 16 stages of geological evolution. 
 
~Geologic and Physiographic Provinces~
 
 
Click on any province name on the map to go to description. 
 
Virginia's land forms are strongly influenced by the geology under them, and they largely overlap. The map above shows the five northeast to southwest trending belts that make up the state. Arranged side by side, from west to east they are: Allegheny Plateau, Valley and Ridge, Blue Ridge, Piedmont, and coasta plain. Each of these provinces more or less extends up and down the east coast from Georgia to New York. 
     
Each province is distinct in a number of ways. They are physiographically distinct, that is, in driving from one province to another the land forms change and the countryside has a different appearance. Additionally, the types of rocks differ; they may be igneous, sedimentary, and/or metamorphic and the structure of the rocks differ; they may be flat lying, or folded and faulted. Finally, the geologic ages of the rocks differ. The combination of these features is unique for each province, and gives each its unique character.
 
Contributed by Lynn Fichter 
Thursday, July 24, 2014
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