November 8, 2008
On November 8th 2008 we ventured out of our dorms to spend the day exploring the geology of the Paint Branch creek area on campus and an outcrop in Frederick County. While visiting Paint Branch creek, we identified a variety of different rocks, including quartz, granite and clay, and observed the affect water velocity had on different grain sizes immersed in the water. As it was painfully early in the morning for a Saturday, we further amused ourselves during some free time by experimenting with different types of rocks, determining which were ideal for skipping across the creek and which would make the biggest splashes. The walk between different stops in Frederick County was also thoroughly amusing as it allowed a group of college students to return to their childhood and play balance-beam and the ground is lava on the train tracks, not to mention the infinite enjoyment everyone got when told they were allowed to hit things with geology hammers.
At our first stop, we observed the difference between the point-bar and the cutbank. It was evident that the water of the stream flows faster along the cutbank, resulting in greater erosion, than along the point-bar, resulting in more deposition. At this particular site you could tell from the deposition of debris that the water level does not remain constant but rises and falls in relation to the weather (i.e. lots of rain floods the river). Further evidence of the effect the river has on the environment is seen in the young trees that have started to grow on the point-bar side. These trees have only just begun to grow as the stream pushes further towards the cutbank, revealing new land for vegetation to take root. Meanwhile, the fallen trees on the cutbank side show that the erosion has caused some instability as it wears away at the current bank. The shape of the point-bar is also evidence of the changing shape of the stream. As more deposition occurs along the point-bar, and as more erosion occurs along the cutbank, the curve of the stream becomes more pronounced. By observing these changes, we can assume that the stream has acted in a similar way since it formed.
The stream also acts as a mode of transport for rocks from further upstream. The high number of smooth, rounded rocks found along the bank of the stream indicate that they were transported from a far distance and, while the stream carried them to our observation point, were knocked against rocks of varying sizes until they were smooth and polished. For transport of the larger rocks to have occurred the stream must have been moving at a higher velocity, while smaller pebbles and sand-sized grains required a much lower velocity as they are lighter and more easily suspended in the water. Along with identifying where the rocks came from upstream, we can also identify which types of rocks are upstream. The presence of mica crystals in the sand indicates that mica-rich schist has been transported from upstream. We can also tell from the abundance of quartz that it is a much harder mineral than schist, explaining the lower amount of schist rocks. While this information is very useful in examining stream deposits, including ancient stream deposits in which you can identify rock type and transport method, it can only offer insight into the type of bedrock located upstream, which can be transported downstream, and not into the environment downstream.
When looking across at the cutbank, we can see two primary layers. The top layer, which is composed mainly of pebble-sized grains, is similar to the top layer of the point-bar side and was likely composed at roughly the same time (with some additions on the point-bar due to the aforementioned transport and deposition of rocks from upstream). Some mud has been deposited in the stream, but because of its small grain size it is more easily transported and stays suspended longer than the larger pebbles and rocks.
Moving on to the first stop in Frederick County, we found sedimentary rocks with coarse grains. This would imply that the depositional environment was in relatively slow moving water. The absence of large rocks indicates that the water was moving too slowly to be able to carry the large rocks. However, the coarser sand grains indicate that the depositional environment was likely a stream with a water velocity that could carry mud and clay but would allow sand to settle out. The sediment is mostly uniform, composed of sandstone, with some other minerals incorporated but not making up the majority of the sediment. The layers of sediment have been disturbed over the years and no longer lay horizontally but rather at a dip of roughly 28 degrees and a strike of 31 degrees East.
The second stop in Frederick County had rocks with sand-sized grains, suggesting that the speed of the stream in the ancient environment was very slow moving, allowing the small particles to settle. The sediment furthermore contained particles of mica, suggesting that mica-rich rocks existed upstream. Compared to the orientation of the beds at our first Frederick County stop, we could observe cross-beds instead of the single layers at the first stop.
The diabase does not show stratification like the other sedimentary rocks showed. However, its orientation relative to the sedimentary strata proves that it is younger than the sedimentary strata. Because the sedimentary stratum is tilted while the diabase intrusion is vertical we can tell that the sedimentary layer was deposited, tilted, and then the diabase intrusion was formed. Because there is no volcanic activity in Maryland, the diabase was probably formed as the tectonic plates separated and magma could rise from the rift basins.