Research Projects
Spring Sapping at Caledon National Park
Post-Settlement Deposition along Rappahannock River near Falmouth
Coastal Erosion at the Rappahannock River Mouth
Using Paleo-Indian Sites to Study Rappahannock River Course
Riparian Forest Buffers (RFBs)
Geologic Influences on Horsepen Run (Upper Rappahannock Basin)
Ancient Fluvial Deposition throughout Fredericksburg, Virginia
First Order Stream Hydrology
Lower Terraces along the Rappahannock River below Fredericksburg, Virginia and the Effects on Terrac
The Effects of Structural Deformation on the Fall Line of the Rappahannock River near Fredericksburg
Wetlands: Characteristics, Rappahannock Basin Case Study, Outlook, and Legislation
Gen'l Environmental & Geomorphic Effects of Dams, & the Significance of Embrey Dam on Rappahannock
Soil and Drainage Survey of the Hazel Run Drainage Basin
Water Quality in Massaponax Creek Drainage Basin
Lower Terraces along the Rappahannock River below Fredericksburg, Virginia and Effects on Terrace Deposition
Anne Witt, Junior Geology Major
Sand and gravel pits along the Rappahannock River , topographic maps, soil maps, provide evidence of at least two or more lower fluvial terraces located in Spotsylvania , Stafford , and King George Counties . These terraces are located between 40 and 70 feet in elevation and represent a time when the Rappahannock was a meandering gravel and sand river. The Rappahannock terraces formed no later than the Quaternary, and their deposition has been estimated during the Pleistocene between 500,000 to 100,000 years.
These terraces are predominately composed of sands and gravels. Changes in bar and trough morphology, as detected in the Goodloe Pit, indicate a lateral accretion of the Rappahannock to it's original position. These terraces were formed in a depositional environment between a glacial retreat and an interglacial period. Changing weather patterns during this time caused periods of high channel flow and increased deposition of sediments originating from the Appalachian Mountains . These sediments are markedly different from modern deposits along the Rappahannock , which are mainly composed of fine sand, silt, and clay. Only during periods of flooding are larger sands, gravels, and cobbles deposited.
Terraces are landforms caused by a series of aggregation and degradation of a river. When a river deposits sediments as part of a terrace, low areas fill in with sediment and become unusually flat and broad. These deposits later form the remnants of an abandoned floodplain. Following a period of sediment deposition, erosion may occur due to tectonics or changes in climate. These factors may cause a change in base level that may reduce the sediment load and allow the river to incise into the recently deposited sediments. After several periods of dissection and deposition, a step-like appearance of terraces forms (Miall, 1996). This same process of terrace formation has been documented along the Rappahannock River .
The motivation for this study came from a few pertinent questions about the local sequences of fluvial terraces along the Rappahannock River . First, there are several sand and gravel pits in the area which follow the banks of the Rappahannock below Fredericksburg . Why are these gravel pits at these strategic positions and are not located at points further from the river? From an initial inspection of local topographic maps, these terraces seem to be located on a 40 to 70 foot terrace. Second, these deposits of sand and gravel are very different from modern river deposits. What kind of environment and river type was necessary to move these sediments downstream? Do these sand and gravel deposits represents a change in paleoclimate and if so, how long ago were these sediments deposited?
The Rappahannock River is located in eastern Virginia , south of the Potomac River and north of the York River , in a humid-temperate climate zone (Dunford-Jackson, 1975). The Rappahannock originates in the Blue Ridge Mountains and flows southeast, through the Piedmont region of Virginia , and onto the Coastal Plain where it empties into the Chesapeake Bay . River terraces are found upstream in the Piedmont and downstream throughout the Coastal Plain. Upstream terraces are not as well defined and are smaller than the Coastal Plain terraces which have been well preserved (Colman, 1983). This study focuses on lower fluvial terraces found below Fredericksburg , Virginia , specifically in Spotsylvania , Stafford , and King George Counties (Figure 1). Also, this project focuses on the terrace sediments found in the two Goodloe Sand and Gravel pits and their correlation to other low terrace sediments downstream.
Figure 1: Fluvial Terraces below Fredericksburg, Virginia (Colman, 1983).
Sand and gravel pits are located along the Rappahannock River throughout the Coastal Plain. These sediments are mainly used to make concrete and asphalt because they are more angular than marine sediments and are not coated by salt (Calver, 1973). The Goodloe Sand and Gravel pits are located off of Route 3 cutting into a terrace located at a contour interval of 50 to 60 feet. The "Old Pit" is a quarter-mile from the Route 3 bypass and is about 40 acres wide and between 15 to 20 feet deep. Sediments are no longer being removed from the pit and it is being backfilled and reclaimed for environmental reasons. The "New Pit" is located four miles down Route 3 from the Old Pit. Goodloe has only started mining these sand and gravel deposits recently. Incidentally, the New Pit is much smaller than the Old Pit, only about five acres. Eventually, it will be excavated down to the same depth as the Old Pit.
Pavich (and others, 1989) and Colman (1983) both identified six terraces along the Rappahannock , below Fredericksburg and the Fall Line. The sediments for these deposits originated from igneous intrusive rocks from the Blue Ridge and the Piedmont region as well as from sedimentary and mafic intrusive rocks in Mesozoic basins (Markewich and others, 1987). The lowest two terraces are located at 40 feet and 70 feet. The 40 foot terrace (represented by the "E" in Figure 1) preserves the depositional environment of the paleoriver channel in the form of ridge-and-swale topography (Colman, 1983). Above Fredericksburg , Dunford-Jackson (1975) also identified six terraces but these probably do not correlate exactly with the terraces farther down river.
Field Study
Terrace sediments in the Goodloe pit fluctuate from brown, fine-grained sands to coarse, reddish-brown sands with inclusions of rounded gravel. These gravels range from cobbles to large, boulder sized blocks averaging about a foot to three feet in width. The representative rock types are slate, quartzite, chert, and other miscellaneous rocks.
River deposits are best preserved in the New Pit. Several examples of bars, troughs, and chute channels could be seen on the walls of the pit. Bars are deposits that are caused by the deposition of sediment along the banks, or in the centers, of streams. These accumulations of sediments form cross-beds and troughs as part of fluvial "dune" deposition (Miall 1996). The beds dip close to 40 degrees in a due east direction (Figure 2). Chute channels are represented by concave lenses of gravel formed when a channel is filled in by the gravely bed load of a river. These river deposits were probably formed as a result of river migration during lateral accretion.
Figure 2 : An Example of the Cross-Bedded Bar Deposits in the New Goodloe Pit.
The deposits found at Goodloe are very different than those found in modern Rappahannock River deposits. Along Old Mill Park , which is located adjacent to the Rappahannock in downtown Fredericksburg , fluvial deposits are a very fine-grained, light brown sand. These deposits indicate that the modern river is not moving at a velocity great enough to move coarse-grained sediments except in times of high flow such as during floods. Across from Old Mill Park in Falmouth Park , there are some local patches of gravels and coarser sands that were deposited during flooding. Modern flow deposits, though, are very fined grained sediments.
Mapping
Before these deposits are mined at Goodloe, the A and B horizon of the soil is removed. These are the remains of weathered sands and gravels. The deposits are mainly clayey to sandy reddish loams with inclusions of deteriorating rounded gravels. These are the deposits that are likely to be labeled on county soil maps since they are the uppermost soil layer. By correlating these uppermost layers with similar soils found on county soil maps, a correlation can be made between the terrace deposits at Goodloe and those elsewhere.
From the Spotsylvania (Elder, 1985), Stafford , and King George (Isgrig and Srobel, 1974) soil surveys, these soils can be identified as Wickam loams. The Wickam soils were then mapped using the soil surveys of the three counties and appropriate topographic maps. Only terrace deposits between 40 and 70 feet were mapped. Figure 3 shows that lower terraces can be traced along the modern river and a pattern of ridge-and-swale topography can be seen. This mapping correlates with the mapping of terrace and river morphology completed by Colman (1983).
Figure 3: Lower Terraces Occurring in Spotsylvania, Stafford, and King George County.
Interpretation and Discussion (Back to Top)
To interpret the data collected, several fundamental facts should be known about the terrace deposits. One of these is the relative ages of sediments and when they were deposited. With precise dates, terrace morphology and the depositional environment can be determined with better accuracy. Unfortunately, much of the literature gives conflicting dates, which can range from 180,000 years to over one million years, for the lower terrace deposits. What is known, is that these sediments were deposited in the Pleistocene during a period of interglacial high sea stands.
Dating
Several ranges of ages have been proposed for the lower terraces along the Rappahannock River . Mixon (1982) attempted to date Pleistocene terrace deposits near the mouth of the Rappahannock based on the uranium-series dating of fossils found in these deposits. He found that these fossils were around 184,000 + 20,000 years old and correlated these Pleistocene deposits with those further upriver. This approach may be faulty, though, as no fossils from the immediate area below Fredericksburg have been dated specifically.
Pavich (and others, 1984) used the radioactive isotope beryllium (Be), to attempt to date terrace deposits at the 23 meter contour (75.5 feet) and the 12 meter contour (39.4 feet). By measuring the accumulation of Be over time, it is theorized that relative dating of soils could be accomplished. While Pavich and his team did find problems in their study with using Be to date soils, they dated the 12 meter terrace at 180,000 years and the 23 meter terrace at 1.5 million years old. There seems to be a significant discrepancy between these two dates. Based on it's location in proximity to the lower terrace, the 23 meter terrace is probably not this old.
In Markewich's (and others, 1987) study, the soils of the 23 meter terrace are assigned a different age of 500,000 years. Both Markewich and Colman (1983) decided to group the lower 12 meter terrace with the 184,000-year-old sediments along the mouth of the Rappahannock based on Mixon's study.
Based on the estimations of these four articles, one can assume that no one really knows exactly what the ages are of the lower Rappahannock terrace deposits and further study is necessary to determine their exact age. A likely estimate for the lower terraces is between 100,000 and 500,000 years old.
Effects on Terrace Deposition
During the Pleistocene, the formation of the Rappahannock terraces could occur in three ways: a change in climate, a change in base level, or due to continuing tectonic uplift and faulting of the Appalachians (Dunford-Jackson, 1975). Climate change occurred due to several glacial and interglacial periods that disrupted climate patterns in North America . In the glacial periods, water was stored in the ice of the glaciers and caused a drop in sea level and a change in base level. A change in base level often causes rivers to incise into their terraces. During interglacial episodes, sea level rose, as the glaciers melted and receded, and base level dropped causing rivers to deposit their sediment loads without significant erosion (Colman and Mixon, 1988). According to Blum (and others, 1995), studies on oxygen isotopes in deep-sea cores have indicated that there have been at least seven major fluctuations in climate during the past 700,000 years (Figure 4). Indeed, there have been several chances for terrace formation to occur.
Figure 4 : Changes in Sea Level over the Past 120,000 years. (Blum and others, 1995)
Tectonic uplift of the area also could have changed the base level of the Rappahannock forcing the river to incise. Tectonics were probably only a minor influence on terrace formation compared to climate (Dunford-Jackson, 1975).
Blum (and others, 1995) present on of the most likely explanations for terrace formation during the Pleistocene in their study of Pleistocene Deweyville Terraces in Texas . Blum suggests that accompanying the recession of the glaciers were changes in vegetation, precipitation intensity, and temperature. This would have allowed mature sediments from the uplands, previously undisturbed during glacial episodes to erode away as warm, moist air shifted northward and brought an increasing amount of rainfall. Thus, the erosion of this sediment occurred during a transitional phase between the glacial maximum and the interglacial. The erosion would have left the uplands relatively bare, consisting of bare bedrock.
What occurred in Texas seems a very likely scenario for Virginia . The Piedmont area has a great deal of exposed bedrock and would have also experienced an influx of warm, moist air from the gulf as weather patterns shifted. Combined with increased precipitation, tectonic uplift, and a change in base level, coarse-grained sediment could have migrated down river to deposit as terraces.
Based on the mapping of lower terrace soils and the sands and gravel found in the Goodloe sand and gravel pits, the Rappahannock River has changed over the past 500,000 years. From the appearance of ridges-and swales and sand and gravel bars, it can be interpreted that the Rappahannock would have been a sand and gravel meandering river with much wider, sinuous meanders and a large bed load (Miall 1996). It also would have had a greater flow velocity to move these larger sediments downstream due to an increase in precipitation and changes in base level.
Today, the Rappahannock is still a meandering river, but it is not as actively meandering and seems "caught" in it's current position. A possible reason for this is that sea-level rise during the Holocene forced seawater into the lower Rappahannock causing it to be tidally influenced. This influx of ocean water slows down the flow of water moving downriver, acting like a dam and preventing further wild meandering and deposition of larger sediments.
References Cited (Back to Top)
Blum, Michael, Robert Morton, and James Durbin. 1995. "'Deweyville' Terraces and Deposits of the Texas Gulf Coastal Plain." Gulf Coast Association of Geological Societies Transactions . v. 55, p 53-60.
Calvert, James. 1973. Virginia Division of Mineral Resources. Geologic Studies, Coastal Plain of Virginia . Bulletin 83, Part 3. Richmond : Department of Purchases and Supply. 153 p
Colman, Steven. 1983. "Progressive Changes in the Morphology of Fluvial Terraces and scarps along the Rappahannock River, Virginia." Earth Surface Processes and Landforms .. v.8, p. 201-212.
Colman, Steven, and Robert Mixon. 1988. "The Record of Major Quaternary Sea-Level Changes in a Large Coastal Plain Estuary, Chesapeake Bay , Eastern United States ." Paleogeography, Paleoclimatology, Paleoecology . v. 68, p. 99-116.
Dunford-Jackson, Carey Stanly. 1975. The Geomorphic Evolution of the Rappahannock River Basin . Published M.S., University of Virginia : Charlottesville . 92 p.
Elder, John. 1985. Soil Survey of Spotsylvania County , Virginia . Soil Conservation Service. 171p.
Isgrig, Dan and Adolph Strobel. 1974. Soil Survey of Stafford and King George Counties , Virginia . Soil Conservation Service. 124 p.
Miall, A.D. 1996. The Geology of Fluvial Depositis . Springer-Verlag: New York . 582 p.
Markewich, H.W. et al. 1987. Age Relations Between Soils and Geology in the Coastal Plain of Maryland and Virginia . Washington : GPO. p.34.
Mixon, R.B. et. al. 1982. Uranium-Series Dating of Mollusks and Corals, and Age of Pleistocene Deposits, Chesapeake Bay Area, Virginia and Maryland . Washington : GPO. 18 p.
Pavich, Milan , et. al. 1984. "Be accumulation in a soil chronosequence." Earth and Planetary Science Letters . v. 68, p 198-204.
Pavich, Milan , Robb Jacobson, and Wayne Newell. 1989 "Geomorphology, Neotectonics and Process Studies in the
Rappahannock River Basin , Virginia ." Washington : American Geophysical Union : p. 22.