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Earth and Environmental Sciences Home > Rappahannock Watershed Information > Research Projects > Ancient Fluvial Deposition throughout Fredericksburg, Virginia

The Extent of Ancient Fluvial Deposition Throughout Fredericksburg, Virginia

Dana Hamel , Brad White, Jenny Burger, David Webster

Introduction

Results

Interpretation and Discussion

Bibliography

 

Introduction (Back to Top)

The town of Fredericksburg has coexisted with the Rappahannock River for hundreds of years. Despite the centuries of human contact with the Rappahannock, little effort has been made to uncover the nature of the River before settlement. The geographic extent of the river prior to settlement is virtually undocumented. By studying the terrace morphology throughout Fredericksburg, and the nature of the terrace sediment, we have determined the locations of the major historic floodplains, as well as their relative ages.

The study area is located in Fredericksburg Virginia, approximately 45 miles south west of Washington, DC. Fredericksburg is located amidst the rolling hills of the Atlantic coastal plain, and is predominantly underlain by sand and gravel. The soils in the study area are extremely fertile, and typically have thick A horizons ranging from 6 to 12 inches. The climate of the area is temperate, with a mean annual precipitation of 41 inches. The average mean temperature is 56 F. The western side of Fredericksburg is built on the remains of an ancient river terrace, at an elevation of approximately 150 feet. From this terrace, the town gradually slopes eastward toward the modern Rappahannock River channel, covering two smaller river terraces located in the center of the town. In the unsettled areas of the town, the land is covered in secondary growth deciduous forest, including maples, oaks, American holly, and dogwoods. A thick network of undergrowth is typically associated with these secondary forests.

Results (Back to Top)

The contours in the USGS topographic quadrangle of Fredericksburg, VA. suggest that the Rappahannock River has not always flowed within its current channel. After consulting the map, we located several strategic sites for sampling. Upon examination of these samples, we theorize the river has flowed in areas other than its present course.

Site 1 is located on an ancient river terrace at an approximate elevation of 140 feet, one and a quarter miles from the current river channel. All samples in this area exhibit the same characteristic gravel size, and soil type. Evidence of fluvial deposition occurs approximately 30 feet below the soil surface, including an assortment of rounded quartzite gravel, and rust colored sandy clay. This is characteristic of the entire terrace, and runs in a NE to SW direction, flanking the entire length of the city on the western side.

Site 2 is located at an elevation of 60 feet. This terrace runs in a NE to SW direction, parallel to the 140-foot terrace above. Site 2 is ¾ mile west of the current river channel. It is important to note that site 2 has a particularly thick soil horizon. This is due to the erosion and deposition of the hill slope material from the adjacent terrace. Below the thick soil horizon lies a 4-½ foot layer of reddish clay. This clay has virtually no sand content, and is extremely cohesive. Below this layer of clay lies a gray and very gritty layer of sandy clay, possessing large quartzite sand grains.

Site 3 is located at an elevation between 40 and 50 feet. As with the other sites, it lies in a terrace running the same NE / SE direction. Site 3 is divided into subpart A and subpart B due to the nature of the samples taken in this area. The sample in subpart A was drilled to a depth of 7 feet, and yielded a uniform layer of red clay. Nothing else was found in the sample. The sample in subpart B was taken 30 feet to the west of site A, and yielded sandy clay and rounded quartzite pebbles at a depth of approximately 4 feet.

Site 4 was taken on the remains of an older eroded river terrace 180 feet east of site 3A. The terrace occurs at an elevation of 60 feet, approximately the same elevation of site 2. Samples here yielded sandy clay and gravel at a depth of 3 ½ feet.

Site 5 occurs at an elevation between 40 and 50 feet, and is approximately 1/10 mile west of the current river channel. A thick layer of rounded quartzite gravel was encountered 4 feet below a top layer of red clay.

Interpretation and Discussion (Back to Top)

The presence of sand and rounded gravel in the majority of the samples shows that the Rappahannock River has flowed as far as 1 ¼ miles from its present course. Furthermore, these samples show us that the Rappahanock has played an important role in the creation of the area morphology. The uniform slope of Fredericksburg is a result of river incision during the late Pleiocene, (Markewich, 1987) and would be nonexistent were it not for the presence of the Rappahannock River.

According to the article by Markewich, et.al 1987, the land in the Fredericksburg area has been in a process of constant uplift. As the land has been uplifted, the Rappahannock has incised into the bedrock, and cut its way down to its current location. In terms of relative terrace age, this theory dates the site 1 terrace as the oldest terrace in the town of Fredericksburg. The soil in site 1 is very developed, and is so mature that much of the minerals in the profile have been leached out.

The Markowich et. al article also states that the redness in soil is a function of increasing age. This statement does not hold true for the profile in the oldest terrace. The difference in color in this profile is due to the amount of chemical weathering it has encountered. This terrace has actually lost color as a result of mineral leaching from the profile. Aside from this departure in color at the site 1 terrace, the remaining terraces in Fredericksburg conform to this trend.

Over the millennia, the Rappahannock eroded its way to the elevation of sites 2 and 4, and deposited sand and gravel that is now about four feet below the current soil surface. The clay in this terrace is redder than the more mature 140-foot terrace. The minerals in the clay of sites 2 and 4 have not been as drastically leached from the profile, hence the redder hue.

The difference between the samples in site 3 can be attributed to the location of the ancient river channel (Figure 1). Sample 3A yields uniform clay down to 7 feet, suggesting the

Fluvial Terraces of the Rappahannock River

Figure 1: Rappahannock Fluvial Terraces

presence of an ancient river channel. The contour lines on the map in the area of this site indicate that the river has flowed along the bases of the terraces containing sites 2 and 4, where site 3 is located. Once the ancient river channel was abandoned, it slowly filled in with mud and silt, leaving the layer of clay existing there today.

In order to support this hypothesis, we took samples about ten meters from the proposed river channel, and found sandy clay and gravel about four feet down. The contents of this sample are the results of the deposition of gravel and sediment over the banks of the old channel in times of flooding.

Site 5 is the closest to the modern day channel, and contained the largest layer of rounded gravel. The gravel was overlain by a four-foot thick layer of silty clay. There was virtually no transition from the clay layer to the gravel layer. The presence of so much gravel this close to the surface indicates that this has been the most recent area occupied by the ancient Rappahannock.

We have used several related studies of soils and terraces for guidance. The Markewich study examines the soils of the Rappahannock terraces and the coastal plains of Virginia and Maryland in general. This study explained the nature of the soils in the area, and what they represent. Other studies have been conducted on terraces in various parts of the United States, such as the Deweyville terraces around the Gulf Coast (Blum, 1990; Blum, et al, 1995). These studies tended to go into more detail than was necessary for our purposes and sought to interpret climate and sea level change from terrace data, but provided valuable information concerning the nature of terrace formation.

Bibliography (Back to Top)

Blum, Michael D. 1990. Climactic and Eustatic Controls on Gulf Coastal Plain Fluvial Sedimentation: An Example From the Late Quaternary of the Colorado River, Texas.

GCSSEPM Foundation 11 th Annual Research Conference.

Blum, Michael D., et al. 1995. Deweyville Terraces and Deposits of the Texas Gulf Coastal Plain. Gulf Coast Association of Geological Societies Transactions. v. 45.

Feldman, Howard R., Wagner, John R. 1991. Pleistocene Braided Stream Deposits From Northwestern South Carolina. Southeastern Geology. v. 31, No. 4, p. 223-234.

Luft, Stanly J. 1986. The South Fork of the Licking River—Eastern Kentucky’s Major Late Tertiary River? Southeastern Geology. v. 26, No. 4, p.

Markewich, M. J., et al. 1987. Age Relations Between Soils and Geology in the Coastal Plain of Maryland and Virginia. U.S. Geological Survey Bulletin: Pedologic Studies in the Eastern United States, Relations to Geology.

Rust, B. R. 1972. Pebble Orientation in Fluvial Sediments. Journal of Sedimentary Petrology. v. 42, No. 2.

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