Wooster Geologist in northern Virginia

Alexandria, Virginia —Last August I retired from The College of Wooster after 43 years of service. It was difficult to detach from the wonderful Earth Sciences department after planting such deep roots in this extraordinary community of teacher/scholars. Then two weeks ago, my wife Gloria and I moved from Wooster to northern Virginia to be close to our daughter, son-in-law and 13-month-old grandson. It has been quite a transition, leaving so many friends to go to a new home on the East Coast — to go from the rural Midwest to the urban Mid-Atlantic. As with all such changes, though, there are opportunities to explore unfamiliar communities, ecosystems, and, of course, geology. I plan to continue contributing to this blog from my new perch just ten miles south of the White House on the west shore of the storied Potomac River (pictured above at Belle Haven Park).

I began my new chapter with questions about the geological context of where we now live. We’re now part of a great hive of people in a condominium complex in Alexandria south of its Old Town. This is our building. Very different from our house on Forest Drive in Wooster! The complex (which has three other residential units like this one) is on a flat-topped hill called Mount Eagle.

The blue dot on this Google aerial image shows our condo in one of the four main buildings. The complex is surrounded by woods, which make a nice buffer from the busy roads just outside. Despite 2200 people living here, the first impression of the grounds is deep greenery.

The red arrow on this USGS topographical map points to Mount Eagle, which rises about 50 meters from its base to a plateau on the top.

The veneer of woods around us on Mount Eagle is so thick it is hard to see just how deep and steep the ravines are on the north and south sides of the hill.

So what is the underlying geology of Mount Eagle? There are no rocky outcrops. Where the vegetation is thin we see pebbles, cobbles and small boulders like these.

Rarely this gravel is cemented into thin layers of coarse sandy conglomerate, like this.

The clasts have their own geological histories. For example, this is a cross-bedded quartzose sandstone boulder. The cross-beds were not formed here but are instead derived from some distant source rock for the gravel.

Occasionally there are even trace fossils evident in these boulders. These burrows were formed in a fine sandstone that was eroded to produce this boulder deposited later on what is now Mount Eagle.

At this point a geologist now turns to geologic maps for information about a spot of local geology. Virginia has a wondrously diverse and complicated geology. The geology of Ohio is much simpler! Virginia is deeply tectonized in the west (as seen in the Blue Ridge Mountains) and dominated by extensive sedimentary basins in the coastal plain of the east. I have a lot to learn here. The star on the map shows the location of Alexandria. We need a more detailed map to sort out the geology of Mount Eagle.

Now with this map of the surficial geology of Alexandria and its region we can see the detail we need to make sense of our local cobbles and pebbles. The red square contains Mount Eagle. Along the south side of the square is a triangular patch of orange. On the key of the map that color represents the “Beverley Hills terrace (Pliocene?)”, which is mostly “coarse cobble gravel …” (This map and the next are modified from maps on an excellent geology site by the city of Alexandria.)
This is a map showing the terraces of the Alexandria area. The Beverley Hills Terrace is the easternmost, and Mount Eagle is conveniently under the “Tt5” label. These terraces are fluvial terraces carved and deposited by the ancient Potomac River.

This Wikipedia image of fluvial terrace deposits shows how they form along the banks of a down-cutting river.”

So there we go. This coarse gravel was deposited roughly 2-3 million years ago by the ancient Potomac River. The quartz-rich clasts are classic survivors of a long erosion, weathering and transportation process from their original deposition locations in what are now the Blue Ridge Mountains far to the west. I’ve learned something about the geology of Mount Eagle and the geomorphic history of this region.

Next: How did this geological context affect the human history of Mount Eagle?