Friday, November 9, 2007

Help from Local Engineers and Draftman

In our quest to figure out what occurred geologically at Port Tobacco, Bolton & Associates (an engineering company) and Sean Phelan, draftman, have come to the rescue. They placed our digital drawing on a larger topographic map of the area. The drawings are compatible and both are in three dimensions. Hopefully we will have some 3D images to share before too long. For now, here is a view of Port Tobacco and part of the valley from overhead.

(Click image for larger view.)

The dark red lines to the right of the drawing illustrate the steep slopes east of town and the green arrows show the ravines through which water and sediment flowed into town. Fan deposits of alluvium appear at the bottoms of the ravines, and it takes little to imagine how those sediments buried earlier deposits.

The symbols on the map (e.g., WoA) represent soil types, the descriptions of which appear on the right side of the drawing. The Soil Conservation Service described the Woodstown soils as "somewhat gravelly" in 1974; but they were describing a general class of soil that appears all over Charles County, and the specifics of their description are for an area near Newtown, some miles to the east on Budd's Creek. The Woodstown sandy loams at Port Tobacco are very gravelly, probably because they derive from the eroded, gravelly soils of the adjoining uplands.


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