Cooch's Bridge is the site of the only Revolutionary War battle that occured withini Delaware's borders. At the above coordinates, you will find a series of historical markers dicussing the events that led up to the battle. One sign, interestingly discusses the geology of the area. This earthcache will focus specificlaly on the fall line and other related geologic topics.
The Fall Line is a line that represents the divison between the oldest rocks and the youngest rocks of the east coast. On the side closest to the ocean, the costal plains and wetlands have relativaly flat topography will little variation in elevation. These lands are much younger, no more than 2.5 million years old. They formed during the Quaternary Period, which is the youngest period in earth's geologic history. We are technically still in the Quaternary Period today. In essence the Quaternary Period is a term used to describe extremly young deposits, most commonly associated with river and lake deposits (although other types of deposits such as glacial or volcanic could form as well). These quaternary deposits, becasue they are so young will form over much older, weathered bedrock.
Note: The red star indicates GZ for this earthcache.
In Northern Delaware, the Quaternary deposts are specifcially called the Columbia Formation which are ontop of the much older Potomac Formation. The Potomac Formation is of Cretaceous age, meaning it was actively depositing and forming during the early ages of the dinosaurs (145-66 million years ago).
On the otherside of the Fall Line are the much older and mature rocks. These rocks are harder, and sturdy than the sedemintary rocks of the costal plains. In Northern Delaware these rocks are known as gabbro, an ingenous intrusive rock. Intrusvie means the rock formed within the body of another rock (called the country rock). The specific name of this gabbro is the Iron Hill Gabbro, named after (you guessed it) nearby Iron Hill. Many incorrectly beleive that Iron Hill is the highest point in Delaware. In reality, the highest point in Delaware is the Ebright Azimuth within a neighboorhood of Wilmington, a few feet south of the Pennsylvanian border.
The Fall Line goes up and down the east coast, separating the costal plains from the more mountainous regions of the east coast. In some areas, the fall line can be abrubt like at Potomac Falls near Washignton DC. However, in most cases this can be gradual over several miles. This gradual change in topography is due to the Quaternary sediments that have been accumulating over the last 2.5 million years.
Note: The red star indicates GZ for this earthcache.
Using a cross section helps in determing where the fall line should be on a map. Imagine you have a 2-d geologic map and you draw a straight line. Notice the straight line (in red) in the map of Northern Delaware. If you cut open the earth along that straight line and view the layers from the side, what would you see? Well, you would see the map above this paragraph, a side view map of the earth and its layers. This side view map is called a geologic cross section map or simply just a "cross section". These are incredibly useful for geologist who want to undertsant the geology at depth. They can be used for oil and natural gas exploration or fault mapping, to name a few examples. The sign at GZ provides several great examples of cross sections in Northern Delaware.
Works Cited
https://www.dgs.udel.edu/delaware-geology/overview-piedmont
https://www.dgs.udel.edu/delaware-geology/summary-geologic-history-delaware
http://www.pencaderheritage.org/main/phgeo/phgeo.html
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER THE FOLLOWING QUESTIONS. RESPONSES CAN BE SENT THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER ON MY PROFILE PAGE. ANY INCORRECT ANSWERS WILL RESULT IN A FOLLOW UP MESSAGE FROM ME. ONCE YOU SEND YOUR EMAIL/MESSAGE, YOU ARE FREE TO LOG THIS CACHE.
1. "Cooch's Bridge - Fall Line" on the first line of your email/message AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails/messages in the spirt of earthcaching.
While at the above coordinates:
2. Take a look at the topograpy around Christiana River. (a) Are the banks of the river steep, gradual, or somewhere in between? (b) Describe the sediment build up (if any) that you see. (c) How big are rocks that line the banks? Is it mostly sand, boulders, cobbles, or bedrock? No need to walk to the river. You can get a good look at it from this vantage point
While at the Geology sign (see waypoint section for coordinates):
3. On the radar topography map (the one below the picture of fossils), take a close up photo of you pointing at where you believe the fall line is. Assume the fall line runs from left to right, so show where the fall line would be on the left side of the map. Assume the fall line runs directly to the right at the point from which you choose. For clarification, this photo must be uploaded to your found it log. I cannot accept photos through the message center (as images are compressed).
4. Due to the Fall Line, (a) how many feet does the nearby Christana River drop by and (b) in how far of a distance?
After you visited both sites (these can be answered at home):
5. Based your previous answers, would you say the fall line is more abrupt or more gradual in this part of Delaware?
6. Based on your own observations, are you north or south of the fall line? How can you tell? Use your answers to the questions above to provide evidence for your answer.
