DeKarra's Field Trip Report: Local and Western Maryland Geology
On November 8, 2008, I participated in the Local and Western Maryland field trip. My classmates and I visited two locations, the first location was near a permanent stream of the Northeast Branch of the Anacostia River that flows around the eastern edge of campus in Paint Branch, Maryland; the second site was located next to some railroad tracks in Frederick County, western Maryland. In both of these locations, my classmates and I observed environmental factors that shaped the landscapes to their present geological states.
Paint Branch, College Park, MD
When my classmates and I arrived at the site of the permanent stream, the water was shallow and had receded to the opposite side of the bank. The side of the bank on which we were standing on was covered with various types of loose rocks. When I picked up some of these rocks and threw them into the stream, I noticed that the water did not interact with each rock equally. For example, heavy rocks and sand sunk immediately to the bottom of the stream. On the other hand, substances such as clay remained suspended in the water for a period of time. When I observed the water closely, I noticed that rocks and other sediments that could not be easily moved by the current of the stream were located in areas where the water was flowing swiftly. Clay and other lighter particles gathered in areas of the stream that were flowing slowly or were standing still. It seems that the lighter material can be carried farther distances than heavy material such as rocks.
Moreover, I saw differences in how the two sides of the stream banks were formed. On the bank side my classmates and I were standing on, the ground was relatively leveled with vegetation and a buildup up of rocks and manmade materials washed downstream; this area is also known as the point-bar. The water flows slowly making it easier for rock and other sediments to build up on that side of the stream. On the opposite side of the point-bar the bank is known as the cutbank because of the constant erosion of sediments, rocks, and vegetation. The water flows more swiftly near the cutbank, obvious signs of erosion included fallen trees and lack of rocks and loose sediments on the cutbank. One tree that suffered from the erosion was probably over twenty feet tall and laid on its side in the water. Despite this undesirable fate of the trees on the cutbank, I noticed they were older than the saplings on the point-bar. I deduce that when the stream floods from rain the vegetation on the point-bar are uprooted or simply die from being submerged underwater for long periods of time. However, plants on the cutbank are spared due to their higher elevations. At this rate, the stream will most likely transform in the future, the cutbank will probably be further back while the point-bar on which my classmates and I were standing at the time will be submerged.
Also I would like to address two more observations at the Paint Branch location: layers within the cutbank and the type of rocks present on the point-bar. I noticed the cutbank had three major layers color-coded from top to bottom: brown, red, gray. The top layer consisted of recently deposited material that were cemented together and formed a brown coloration such as schist, a metamorphic rock. The red and gray portions were most likely formed millions of years ago in an ancient lake bed, I believe the sediments may be clay and mud. Furthermore, the majority of rocks on the point-bar were made out of durable materials. For example, rocks such as quartz were more abundant than darker rocks that seemed to crumble easily when smashed against another rock. Hence, it is fair to say that the durable rocks lasted longer and were simply eroded into smoother shapes in the water than their less durable counterparts which crumbled and changed into smaller particles.
Western Maryland
The rock formations my classmates and I saw on the second portion of our field trip in Western Maryland were unique in that they were older than the formations in Paint Branch (dating back to the Triassic and Jurassic era) and that they consisted of one basic material from location to location along the railroad tracks. For example, at Stop two it was difficult to see individual grains and the material of the rock formations were considered to be mudstone; Stop three consisted of small grain particles, characteristics of sandstone. In addition, the rock formations did not follow the Principle of Original Horizontality, which states sediments are deposited horizontally, instead they were tilted at an angle. By using Brunton compasses, my classmates were able to determine dip and strike which show how tilted a rock formation was to a horizontal plane. Dip expresses the angle off a horizontal line while strike illustrates the direction of the line of the rock in relation to the compass in terms of North, South, East and West.
Additionally, my classmates and I learned the area next to the train tracks were underwater in an ancient stream at some point because of the cross-bed formations throughout all of the stops we made in Western Maryland. Streams naturally change direction, as a result all the stops had tilted formations with similar dip and strike measurements. This also explains the cross-bed formations that occur across some of the diabases - small, intrusive igneous rocks - at our last stop. Sedimentary rocks must have settled in the area, then magma arrived at some point and fused with the sedimentary rocks while it was cooling. The only way magma could have occurred in Maryland is when Pangea split causing volcanic activity.
Before I went on this field trip, I had no clue as to how rich the history of Maryland was in terms of it geology. I glad I had this experience to explore the geological history of the state of Maryland and the University of Maryland College Park campus. The fact that I can touch something from over a million years ago is pretty cool and all I have to do is walk outside to simple areas such as my backyard, a stream, or some railroad tracks.
Links: