On November 8th, 2008 I went on a adventurous field trip to learn about the information that different outcrops in local and western areas of Maryland have to offer about ancient Maryland.
Stop 1: Paint Branch
The first site that we visited was Paint Branch, it is a part of the Anacostia River that flows into the University of Maryland College Park campus. At the site I noticed that the stream is filled of different size rocks and grains. I learned that the deposition of the different sized rocks and grains were due to floods and during that time debris, rocks, and grains would be carried into the water bed. The cut bank, is the outside of the stream curve, it is where erosion occurs and the point bar, the inside of the stream curve, is where deposition occurs. As a kid I remember always throwing rocks into lakes and watching them sink. So at Paint Branch I threw some rocks in and they sank but when I put some clay into the water it did not sink instead it was suspended in the water and moved according to the flow of the water. The clay did not sink because its particles are very small and the low kinetic energy of the stream kept it suspended. As I continued to observe the site, I noticed that the vegetation around the point bar was different than that on the cut bank. The vegetation on the point bar included very young sycamore saplings with leaves from trees curved around them. There were also bricks, cement, and parts of the nearby parking lot on the point bar. While the vegetation on the cut bank included full grown trees with some of the trunks visible. This is evidence that the water level changes over time because vegetation cannot grow where the stream is but over time the point bar moves and as it moves new vegetation grows. Also the visible trunks of trees and the fallen trees shows that the stream on the cut bank side has been undergoing erosion and seriously affecting the ecosystem. Because of this I can infer that the stream bed will not be in the same location in the future and that the point bar will continue to move east as the cut bank continues to erode more sediment in that same direction. Most of the loose rocks(sediments) present on the site are a result of erosion from upstream bedrock and then the loose sediments are then transported downstream. As the rocks travel downstream they will lose their sharp edges and become smooth. One thing to keep in mind is that the most amount of rocks present at Paint Branch does not tell me what kind of rocks upstream because only harder resilient rock material is dominate at Paint Branch because it can withstand transport. This simply means that even if clay is dominant upstream it will not be dominant downstream because during transportation the clay particles will be broken down and separate. Instead rocks such as sandstone, granite, and quartz, and schist will be dominant because they can withstand transport. These rocks do not tell us about what is present downstream because the rocks are deposited from upstream. The cut bank is layered with clay from the Early Cretaceous, about 115 million years ago, and pebble-rich layer on the top and after that there is vegetation. Similarly, the point bar has a lot of sand and little rock mixtures that resemble the pebble-rich area. One of the most fascinating things about Paint Branch was that some of the rocks there were between 350-400 million years old.
Stop 2: Newark Supergroup, Part I
Our second stop was a railroad cut in western Maryland. The Newark Supergroup is a part of sedimentary and igneous rocks that originate in North Carolina and continue on to Maritime Provinces of Canada. The Newark Supergroup contains rocks that were formed around 225 million years ago in the Late Triassic and Early Jurassic. The size of the sedimentary grains were small and similar to beach sand. Because the grain size was small I can infer that they were deposited in a lake environment where the water speed was very slow. This is because in order for sedimentary rocks to form sediment deposits need to build up and then lithified due to immense pressure. This could not occur in a body of water that was moving at a very fast speed because it could not hole the particles in suspension but will instead transport them downstream which will lead to no build up of sediments. The only type of sediment present at this site was mudstone. The sedimentary rocks at this site were first deposited horizontally then tilted to their current position. In order to know the exact position of the tilt we used the Brunton compass which gave helped us determine the strike of 32E and Dip 28®. Strike is the line intersection of an inclined layer of rock and a horizontal plane. Dip is defined as the angle of inclination of the rock layer. This site was very beautiful and calming to be in. I felt as if I was on vacation and went site seeing.
Stop 3: Newark Supergroup, Part II
This site had sandstone, a type of rock that is made in a stream where the kinetic energy is high. The sandstone grains were course like beach sand. Because of the settings that are necessary for the formation of sandstone one can infer that in that region in the Late Triassic period there was a lot of streams. Also looking at these sediments one can tell that the upland regions were abundant with mica. The orientation of these beds differ from stop 2 because of the presence of cross-beds.
Stop 4: Newark Supergroup, Part III
This site was a perfect example of intrusive igneous rock. For intrusion to occur there should already be strata’s of igneous rock present and then magma from the depths of the Earth intrudes the present rocks vertically and crystallizes to form intrusive rocks characterized by their large crystal sizes. There are no internal structures present in the diabase. The mass of the diabase is oriented at an angle compared to the sedimentary strata. Also the mass of the diabase is oriented vertically(north to south) when compared to present day geography. Observing the rocks I can tell that the sedimentary rocks were present before the diabase because the diabase intruded the sedimentary strata. One main factor about diabase’s is that they result from volcanic activity and because there is not much volcanic activity in Maryland today, the diabases are sign that millions of years ago there was volcanic activity in Maryland. The Newark Supergroup deposits formed as great basins opened up along the middle of the super continent Pangaea, which rifted apart during the Late Triassic and Early Jurassic. The sedimentary rocks in the Newark Supergroup were formed as sediment was transported from the cliffs around the rift basins were deposited inside them. The intrusive diabase was a result of magma that flowed up and through pre-existing rock and cooled over time. The Supergroup deposits and the diabase are all the proof of the birth of the Atlantic and that is how they are all related.
