Gravel sheets in the suburbs of Washington, DC
Deborah Painter (USA)
If you live in western Prince George’s County, Maryland in the USA, in the towns of Oxon Hill and Suitland and you want to dig to place a water line, plant a garden or excavate to construct a foundation for any building, chances are you will encounter sandy soil with hundreds of cobbles and boulders. Some boulders encountered could be in the form of large flattened slabs. You might be wondering why these are present, since these towns are in a coastal plain, far south and east of the rocky outcrops of the Piedmont area of Virginia and Maryland. For someone like me, who was born and raised in the Coastal Plain area of Virginia, these ubiquitous cobbles and boulders seemed out of character for the region.
I discovered these odd boulders and cobbles when I joined a colleague from an office in a northern state to assist him in ecological studies for two small sites not too far from the United States Capital of Washington, in the District of Columbia (DC). Our goal was to help our client know if there were any threatened or endangered species, wetlands, hazardous materials or other site constraints, as this would assist the client to decide whether to purchase the properties. Our first Prince George’s County site for an ecological study was one of a few hectares in size in Suitland, a suburb of Washington, DC and approximately 8km southeast of the border of the capital city near the shore of the Potomac River.
It was on Pennsylvania Avenue, which is the same avenue where the White House is located. However, the White House is over 17km to the northwest. Included in our site work was a wetland determination/delineation and this involved not only a comprehensive study of the vegetation, but also a hydrologic assessment within the first 46cm of the soil column. It also involved the use of a hand auger to characterise the soil layers within this elevation of the soil column and determine the hue, chroma and value of the soils.
When Phil and I dug in the mowed and maintained area of the first site, we had difficulty getting through because of the many large cobbles in the sandy loam. Could this be a fill site? It was in a small wood lot in a suburban area, surrounded by apartments and lawns (Fig. 1).
The second site for an ecological study was a few kilometres to the southwest in Oxon Hill, in a wooded location near homes and a park. It is located approximately 15km east of Alexandria in Virginia on the shores of the Potomac and approximately 2.7km northeast of the National Harbor on the opposite shore of the Potomac (Fig. 2).
Both sites had very gently rolling terrain. The soil at the Oxon Hill site was identical to the soil at the first site and just as difficult to dig (Fig. 3). Along one side of the parcel was a drainage easement, a natural stream and tributary to Oxon Run, which in turn drains into the Potomac River. In the wooded lot of the Oxon Hill site, the cobbles tended to be larger and some were exposed on the surface among the vines and other vegetation. Clearly, the area was less disturbed than the first site.
The forest showed no sign of past thinning and was undisturbed long enough for the survival of oaks, American elms and tulip poplars approaching “champion tree” status in their crown spread and trunk diameter. The size of the tulip poplar and American elms, in particular, caused me to dismiss the hypothesis of the onsite cobbles having been brought in originally as fill. These species would require 80 to 100 years to reach their current size (Fig. 4).
It made less sense to me that the area had been filled with fill dirt and then left to remain undeveloped for 80 years or more. Past use for farming would have meant the removal, not addition, of cobbles and small boulders. Several large (3m-wide and 3m-long) slabs of sandstone lay on the forest floor and along the banks of the small stream (Fig. 5).
In one location, along the steep embankments along the deeply incised creek, slabs reposed horizontally across the creek and formed a small waterfall (Fig. 6). The stones at the Oxon Hill site showed no sign of any glacial striations. This strongly suggested that the cobbles and boulders had been worn by water.
At both the sites Phil and I studied, the cobbles and boulders were all sandstone, quartzite and chert; no other varieties were seen (Figs. 7, 8, 9 and 10).
Where had the cobbles and slabs come from?
Following our site surveys, I returned to my office to write my report that would merge with my colleague’s. The presence of cobbles, slabs of stone and sands was not a factor in our clients’ decision-making but it was interesting to me personally, so I sought literature on the county’s geology. I knew the area had not undergone continental glaciation during the most recent Wisconsinian glaciation. It was simply too far to the south of the southernmost extent of the glaciation. The two sites are mapped by the US Geological Survey as The Upland deposits of late Miocene and early Pliocene, fluvial deposits formerly known as the Brandywine Formation. They are 12.2m thick at their thickest areas further east and are approximately 4m in thickness near the Potomac River.
The Upland deposits consist of poorly sorted sand with gravel sheets, consisting of pebbles of quartz and cobbles of chert, quartzite and sandstone. Bedding is mostly lenticular and cross bedded to massively bedded. The basal beds of this deposit include boulders and slabs of up to 3.6m in diameter. The Upland deposits are the bed of the ancient Potomac River, which once flowed in this area – it has since been diverted to the north by heavy deposition as discussed in detail in JP Owens and CS Denny’s 1979 study (referenced below). During the Pleistocene and the Holocene the results of Wisconsinian glaciation contributed even more deposition to that which had been taking place during the Miocene and Pliocene, as discussed in the following paragraph.
To the north, sandwiched between the Upland deposits and the Quaternary alluvium of the Holocene epoch, poorly stratified alluvium that forms the shoreline of the Potomac River, are the Severn Formation Cretaceous deposits of silt-clay facies. They were deposited on the inner marine shelf during the Late Cretaceous period. Here, terrestrial plant fossils and, uncommonly, dinosaur bones and teeth have been found. Interestingly, older sediments are mapped between the newest sediments. A major regional unconformity is thus present, separating the Upland Formation from the earlier deposits. During the Miocene and Pliocene, an ancestral Potomac River system deposited aprons of sand and quartz gravel at various terraces across the coastal plain.
During the Pleistocene epoch, continental glaciation lowered the sea level, and the ancestral Potomac downcut into older sediments, shifting its channel. Remnants of the former floodplains were left as gravel terraces. The Wisconsinian ice age came to a close. During the Holocene epoch, eustatic sea level changes (uplift and subsidence) occurred. The Potomac and its tributaries responded by further shifting of the channel and further gravel terrace deposition within the floodplain.
The Potomac now flows from northwest to southeast, with the Virginian cities of Arlington and Alexandria along its southern shoreline. The river flows south of the capital of the United States. In the Miocene and Pliocene, it had an even more southerly direction of flow. During the Miocene and Pliocene epochs, the main channel flowed through what we now call northern Fairfax County in Virginia, down to what we now call southern Maryland several more kilometres to the southeast, and thence to the Chesapeake Bay. Massive amounts of coarse grained materials were deposited during the entire Quaternary Period and especially during the Miocene and Pliocene.
A series of migrating stream channels, including but not limited to the Potomac, deposited the coarse-grained materials that cut downward as they migrated, carrying sediments largely from eroded and weathered crystalline igneous and metamorphic rock of the Appalachian Mountains to the west. These migrating channels accounted for loose boulders of sandstone, chert and quartzite, forming gravel sheets in the beds (Fig. 11). The beds would one day be the foundation for forests, small office buildings, homes, places of worship and petrol stations during the Holocene.
Interesting changes had already made their presence known here since the days when the Potomac flowed under where our feet now stood. No one who wanders through the wooded areas of Oxon Hill without an understanding of the geological history of this area would expect that, ten million years ago, the forest was the bottom of a wide and deep river. Ten million years from now, the changes will be greater still. Will the Potomac shift its main stem once again? If it does not, will the cobbles and boulders present in Prince George’s County become totally worn away from erosion and weathering? The rocks will tell the story, and that is the most interesting (and yes, exciting) thing about geology… the stories the rocks tell. If I may be forgiven for saying so, it makes a geology enthusiast exclaim, “Cool!”
About the author
Deborah Painter is an ecologist and general environmental scientist specialising in transportation and industrial development planning to minimise deleterious environmental impacts. She lives in the United States.
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