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Geology 115/History 150
Geology Prep Assignment 5: Structural geology
Due Tuesday, May 3
Websites accessed:
Gettysburg NMP (1895):
• http://www.dcnr.state.pa.us/cs/groups/public/documents/document/dcnr_014596.pdf
• http://blogs.scientificamerican.com/history-of-geology/geology-and-generals-howgeology-influenced-the-gettysburg-campaign-part-i/
• http://blogs.scientificamerican.com/history-of-geology/geology-and-generals-howgeology-influenced-the-battle-of-gettysburg-part-ii/
Chickamauga and Chattanooga NMP (1890):
• https://www.nature.nps.gov/geology/inventory/publications/s_summaries/CHCH_gri_s
coping_summary_2009-0805.pdf
1. “Trafficability” is a military term meaning the ability of an army to move soldiers,
supplies and equipment safely and easily through an area. The town of Gettysburg has good
trafficability. Examine a map showing the Gettysburg area and its roads in 1863 (either the
Pennsylvania Department of Conservation and Natural Resources pamphlet or the
Scientific American blog will have it); how many roads lead into Gettysburg? Use Google
Maps or a similar site to examine Gettysburg today; how many numbered roads (federal,
state and county routes) lead into Gettysburg? How important was Gettysburg as a hub of
transportation in the 1860s?
2. The Great Valley, the Blue Ridge and the Piedmont (in fact, all of the Appalachians from
Georgia to Maine) were all tilted by the same massive tectonic event that happened about
400 million years ago. What was this event, and what type of plate tectonic boundary did it
occur along?
3. What type of rocks were found in the Great Valley (the Cumberland/Hagerstown/
Shenandoah valleys), and what do these rocks have to do with why the Great Valley is
there? (Hint: the geologic definition of the word “incompetent” may help)
4. Gettysburg is sited within the Gettysburg-Newark basin. What tectonic event (occurring
about 220 to 200 million years ago) occurred to create this and other similar basins (such
as the Culpeper basin to the south)? Along what type of plate tectonic boundary did this
event occur?
5. Cashtown Gap, through which the Confederate Army of Northern Virginia advanced on
Gettysburg, is a wind gap, similar to what you saw in the previous week. Yet this particular
gap is much more straight and level than other wind gaps through the Blue Ridge – what
other geological phenomenon influenced the formation of Cashtown Gap?
6. Diabase is the rock in the Gettysburg area that plays a major role in the ultimate Union
Army victory. What type of rock is diabase? How “competent” is this rock? How and where
in the Earth did the diabase form in the first place?
7. Both the pamphlet and the blog were written for the general public (that is, interested
people with no particular background in geology). Which do you think conveyed the
connection between the geology of the area and the historical events better? Why?
8. By contrast, the National Park Service itself publishes “Geologic Resources Inventory”
documents as part of their organization’s mission. In that document for the Chickamauga
and Chattanooga NMP (see link above, especially pages 12 and 13), how effectively are the
geology and Civil War history of the area tied together?
9. Did the same tectonic event in question 2 that affected the Gettysburg area also occur in
the Chickamauga-Chattanooga area? What is the evidence for this event in this area? Hint:
note the phrase “Alleghanian orogeny” – what’s an “orogeny”?
10. Four new terms appear in the text of the Chickamauga and Chattanooga NMP Geologic
Resources Inventory: anticline, syncline, décollement and thrust fault. Search all of
these terms at the same time, and find an image (or, more probably, two images) that
illustrate what these terms are. Sketch it (or them) as well as you can below, labeling each
of the terms to the proper part of the sketch. Why do all of these images have to be crosssections?