The landscape of the Chesapeake Bay Watershed changes drastically throughout its 64,000 square mile expanse. To use my home state as a prime example of these variations, In Maryland there are mountains, forests, beaches, rapid rivers, slow meandering waterways, and hundreds of tributaries and streams in the 42nd largest state in the US. Maryland is made up of 9,775 square miles of landmass, while 2,633 square miles of Maryland is covered by water. The highest point in Maryland is Hoye-Crest on Backbone Mountain at 3,360 feet above sea level, and the lowest point is sea level at the Atlantic Ocean. It is important to note that Maryland is only one of the six states in the watershed, and each state has its own unique geological characteristics.
The Chesapeake Semester group and I sought to find the changes in geology in the watershed through sediment sampling and data collecting as we travelled from the mountains of West Virginia to the Atlantic Ocean. In this regard, I stalked the geology of the Watershed through our 10-day journey, and what I found gave me a better understanding of how landscapes change over time. In this writing, I will describe some of the details of sediments that were collected to show the significance of visible geological changes in the landscape of the Chesapeake.
The processes that create sediment are weathering and erosion. Sediment is first created when water seeps into the cracks in mountain rocks, freezes, and causes the rock to break apart. This process makes boulders, and through subsequent weathering, the rocks are made smaller. Boulders are worn down to cobble, cobble to gravel, gravel to sand, sand to mud, mud to clay, and clay to granule. Rocks are always being weathered and broken down by wind and water. Wind carries sand particles that wear against rock like sandpaper and rivers weather the angular edges of rocks and make them smooth over time.
There are several measures used to describe the sediment samples we collected. The first is smell, whether the sediment smells like Hydrogen Sulfide, Petroleum, sewage, or other (usually a dirt smell). The measure of grain size is used to show if the grains are gravel, sand, silt, or clay and the predominant grain size can be classified coarse, medium, fine, very fine, or muddy. Then we determined if the texture of the sediment is considered cohesive, friable (breaks apart easily), Soft, compact, gritty, soupy, lumpy, sticky, dry, watery, firm, or smooth. Next came the classification of sorting. If sediment is well sorted, all the grain sizes are essentially the same. If sediment is moderately sorted or bimodal, the grains vary in size. Then we identified the grain shape as very angular, angular, sub angular, and well rounded. The further from the mountains sediment travels, the more rounded sediment becomes.
In the mountains of West Virginia and the river bottom of the Susquehanna near Harrisburg, Pennsylvania, the rocks were darker in color. This is because they had more mafic material and heavy minerals, and had relatively recently been weathered down from the mountainside. In the areas of the Susquehanna with faster currents, the sediment collected here was made up of medium sized gravel and sands, was moderately well sorted, and had a gritty and watery texture. In the calmer, wider sections of the river, the sediment was silty, lumpy, and had more organic matter like algae. This change happened because more rapid currents carry sediment downstream and slow currents allow for sediment to settle to the river bottom and for algae to grow. This change occurred within a short span of the river where it became much wider and slower. Further down river, I hand grabbed sediment among the Sub Aquatic Vegetation (fresh water grasses). This sediment was more sandy and coarse because the river had become narrower which made the current move fast. This faster current was slowed down where there were grasses, but not anywhere in between. Sediment was less likely to settle because the passageways between grasses made water move more swiftly.
As we moved further East to Gifford Pinchot State Park, the sediment we found was compact and gritty sandy gravel with a predominantly medium grain size. The grains here were far more spherical than the angular grains we saw in the Susquehanna. Further, the sediment collected was a lighter color, but held traces of an orange-red color from the oxidization of the iron still remaining in the sediment. This means that the heavy minerals coming from the mountains have not been fully separated from the sediment.
Smith Island was next on the sedimentary agenda, and it was here that we collected a sample very different from what we had seen so far. This sample smelled like hydrogen sulfide and had a clay and silt grain size. The sediment was cohesive, compact, firm, and lumpy. The grains were very well sorted and well rounded. The color was a dark brown, but mainly because the sediment retained so much water. When dried out, the color became a significantly lighter brown. What I gathered as being the biggest change was the fine texture of the spherical grains.
Upon our arrival in Assateague Island, I saw the stark contrast from the dark sediment of the Susquehanna river and the lightly colored Assateague beach sands. The sediment collected here dry, friable, and well-sorted sandy fine grains. In this sand there were still some heavy minerals present but only if you looked very closely and even then they were sparse. This means that the vast majority of heavy minerals and mafic material had been removed from the sediment through weathering. This sand had once been dark in color but as it moved further from the mountains and matured over time it became lighter in color.
Stalking the change in sediment and the geological changes of the Chesapeake region was fascinating because it explained how beaches are formed, why rocks are all different sizes varying with location, and the tremendous influence that mountains have over the shaping of the Earth’s landscape.