Gravel sheets in the suburbs of Washington, DC

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.

Fig. 1. The first site, in a far from pristine
location in the town of Suitland, resisted
many of our attempts to dig below 13cm due
to the presence of numerous cobbles in the
soil column. (Photo: Deborah Painter.)

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.

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