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Earth and Environmental Sciences Home > Rappahannock Watershed Information > Research Projects > The Effects of Structural Deformation on the Fall Line of the Rappahannock River near Fredericksburg

The Dramatic Effects of Lithology, Faulting, and Joints on the Fall Zone of the Rappahannock River, Fredericksburg, Virginia

Amy Friant - Senior Geology and Environmental Science Major

Adam Schultz - Junior Geology Major

ABSTRACT (Back to Top)

The Rappahannock River in the Fredericksburg area leaves an area of crystalline piedmont rocks and enters a region of coastal plain sedimentary rocks. This transition zone is the Rappahannock River’s fall line/zone. In this fall zone, the river crosses a series of rapids and drops 60 feet in elevation. Strong field evidence suggests that this dramatic change in river geography is not solely the result of a change in lithology, but a result of local uplift. This uplift can be attributed to high angle reverse faults in the vicinity of the fall zone.

INTRODUCTION (Back to Top)

The fall line/zone is the area that separates the Piedmont Plateau and the Atlantic Coastal Plain (Encarta, 1997). To the west of the fall zone is the Piedmont Plateau, which is a region of gently rolling upland. This area is part of the Appalachian Highland Province, and has average elevations ranging from 300 to 1800 feet (1997). East of the fall zone is the Atlantic Coastal Plain. This is an area of flat low-lying topography near the ocean, with a maximum elevation of 300 feet (1997). The coastal plain is an extension of the continental shelf, and has subsided and emerged several times since the Mesozoic era (1997).

The fall zone is the area of transition between these two areas. In this zone, streams and rivers flow from areas of bedrock to the softer rocks and soils of the coastal plain (1997). This zone has a steeper gradient than the two adjacent areas, and as a flowing body of water moves from the rolling hills of the piedmont to the low-lying coastal plain, it passes through an area of rapids. These rapids and the steeper gradient can be attributed to a change in lithology, from more resistant granites and high-grade metamorphic rocks to easily erodeable sands, clays, and shales (1997).

The fall zone of the Rappahannock River at Fredericksburg, Virginia illustrates these features. Up river of the fall zone are Paleozoic age granites while below the fall zone are Cretaceous age sedimentary rocks. The region of the fall zone has high-grade metamorphic rocks as the transition between the other lithologies. This area also shows a sharp change in gradient. The fall zone can also be strongly influenced by the presence of structural features. The steep gradient can be attributed, not only to lithologic changes, but also to the presence of fault and joints in this region. The Fredericksburg area is greatly influenced by the Stafford Fault System. Figure 1 from Mixon and Newell (1978) shows this fault system. The two faults closest to Fredericksburg are the Hazel Run Fault and the Fall Hill Fault. Both of these faults are high angle thrust faults where older Paleozoic granites and gniesses are thrust over younger, Cretaceous age sedimentary coastal plane formations (Mixon and Newell, 1978). As these two faults run in close proximity to the fall zone, they serve as the focus of our study.

METHODS (Back to Top)

This study focuses on the fault and joint systems, as well as the lithology in the Fredericksburg area, and their affect on the fall zone. In order to study these system, four sites were chosen along the Rappahannock River and two sites along Hazel Run. The site locations will be discussed starting with the up river initiation of the fall zone, and continuing down river to its termination near the US Route 1 bridge. They can be seen on the two topographic map sections (1994 - Map 1 and Map 2 ) of the Rappahannock River in the Fredericksburg vicinity.

The first site along the Rappahannock River is located along a drainage creek that runs from Motts Run Reservoir to the Rappahannock River. Exposures studied here are located approximately 100 feet south of VA Route 618 along the east side of the drainage creek. Site two is located near VA Route 639. The site lies near the intersection of two drainage channels, approximately 500 feet north of VA Route 639 from its intersection with an industrial park driveway. The third site is located on a small island on the Rappahannock River and the continued exposure on land. This site is about 600 feet downstream of Lauke’s Island off of Caroline Street. Site four is Falmouth Park, located on the north bank of the Rappahannock River, below the US Route 1 bridge. This site can be accessed from River Road in Stafford County.

The first site along Hazel Run is located about 400 feet west of US Highway Alternate 1 near the southbound crossing of Hazel Run. The second site on Hazel Run is located in Alum Spring Park. This site is an extensive exposure along the south bank of Hazel Run within the park.

Measurements of fault and joint set orientations were taken at each site. This evidence was then used to find local stress orientations and possible controls on the features of the fall zone. Rock samples were also taken to compare the lithologies present at the different stages of the fall zone.

DISCUSSION (Back to Top)

Evidence collected from the study sites was consistent with initial ideas and definitions concerning the formation of a fall zone. Site one along the Rappahannock River showed signs of a northeast trending fault ( Pict. 1 & 2 ). This fault is visible in a rock face that runs along side the drainage creek in which crystalline rock is thrust at a high angle over younger coastal plain rock. Fault gouge and cataclasite is present along this fault contact. Joints in this site trend dominantly in a northwest direction and are largely responsible for exposing this rock face ( Pict. 3 ). The joint and fault orientations at this site are indicative of a compression zone in which the s 1 direction was northwest. The river in this area is entering its first set of falls down stream, and no falls are visible in field or map view upstream from this site ( Pict. 4 ). This indicates that this point is the beginning of the fall zone.

Site 2 along the Rappahannock River shows evidence of another fault also trending in the northeast direction. This fault has been identified in earlier studies of the area, (Mixon and Newell, 1978) as the Fall Hill Fault. A cross section view of this fault can be seen in figure 4 (1978). This fault is no longer visible in the site studied by Mixon and Newell (1978) due to newer construction in the area. However, the site two outcrop can be traced in the field directly to the area in which they had identified the fault ( Pict. 5 ). At this site, the fault has displaced older piedmont crystalline rock more than 100 feet over younger coastal plain formations ( Pict. 6 & 7 ) This displacement can be seen where the river drops 20 feet upon its intersection with the fault. This intersection is located just upstream from Lauke’s Island. Jointing along this fault trends in a dominant northwest direction and once again all fault and joint evidence indicates a compression zone with a s 1 direction trending northwest. The large drop in river elevation upon crossing this fault indicates that in this area, faulting is the primary cause for this local fall zone feature.

Site 3 along the Rappahannock River contains no direct fault exposures but does exhibit a large number of joint sets trending dominantly in the northwest direction ( Pict. 8 & 9 ). Rocks in this area exhibit more compression evidence than in the previously mentioned sites. The rock outcrops in this area have foliations trending in a northeast direction ( Pict. 10 ). The joints and foliations at this site serve to support evidence collected at the previous two sites of a compression zone in which the s 1 direction was oriented northwest. This site is bounded on one side by the river and falls are visible upstream and downstream from this area indicating that the site is well within the boundaries of the fall zone ( Pict. 11 ).

Site 4 along the Rappahannock River shows no signs of jointing or faulting, and no falls are present at this location ( Pict. 12 ). Falls are visible, however, about 500 feet upstream from this site ( Pict. 13 ). There are no rock outcrops in this area, and the soils are made up of sand bars directly along the river ( Pict. 14 & 15 ) and muddy and clayey deposits below sand bars and farther away from the river ( Pict. 16 ). This would indicate that this site is just beyond the fall zone boundary. Evidence collected at these sites shows the beginning and end of the fall zone and illustrates the relationship between geological structures and the behavior of the river within this area of study. The sites along Hazel Run serve to support evidence, gathered along the Rappahannock River, of a compression zone and its influence on the fall zone.

Site one along Hazel Run shows evidence of a fault. Evidence of this fault includes fault gouge and cataclasite ( Pict. 17 & 18 ). This northeast trending high angle reverse fault has been identified in earlier studies of the Hazel Run Fault (1978). A cross section of this fault can be seen in figure 9 (1978). At this site piedmont crystalline rocks are thrust 120 feet over coastal plane deposits (1978). The direction this fault trends is indicative of a compression zone in which the s 1 direction was trending northwest, consistent with evidence in the Rappahannock sites. Site two along Hazel Run showed no faulting, but displayed joint sets dominantly trending northwest. Younger rock in this area, sediments with petrified wood layered throughout, are overlain by older crystaline rock. This reverse layering indicates that the area was affected by a reverse fault. The joints indicate a compression zone in which the s 1 direction was northwest. The evidence collected from Hazel Run is consistent with and supports all evidence collected at the Rappahannock Sites.

CONCLUSIONS

The consistent northeast trend of the faults and the dominant northwest trending joint sets indicate that within the fall zone the river crosses a historical compression zone. Other studies have dated and identified this compression as being caused by the Allegheny Orogeny (1978). The uplift caused by this compressional event ranges in places from 60 to 120 feet along the thrust faults. When examining the fall zone, this aspect of the region can not be ignored. Some down cutting along the Rappahannock River has occurred due to a change in lithology, but the change in geographic elevation due to uplift is more likely to be the cause of the dramatic falls and riverside rock formations present along the Rappahannock River fall zone.

Figure 1: Stratigraphic Cross Section of Stafford and Brandywine Fault Systems

Figure 4: Coastal Plain Cross Section