The incision history of the Great Falls of the Potomac River—The Kirk Bryan field trip
Published:January 01, 2015
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Paul Bierman, 2015. "The incision history of the Great Falls of the Potomac River—The Kirk Bryan field trip", Tripping from the Fall Line: Field Excursions for the GSA Annual Meeting, Baltimore, 2015, David K. Brezinski, Jeffrey P. Halka, Richard A. Ortt, Jr.
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Measuring the rate at which rivers cut into rock and determining the timing of incision are prerequisite to understanding their response to changes in climate and base level. Field mapping and measurement of cosmogenic 10Be in 106 rock samples collected from the Great Falls area of the Potomac River show that the river has cyclically incised into rock and that the position of the knickzone, now at Great Falls, has shifted upstream over the later Pleistocene. Exposure ages increase downstream and with distance above the modern channel. The latest incision began after 37 ka, abandoning and exposing a strath terrace (the old river channel) hundreds of meters wide beginning at Great Falls and ending at Black Pond, 3 km downstream. This incision was coincident with expansion of the Laurentide ice sheet. Exposure ages of samples collected down the walls of Mather Gorge downstream of Great Falls indicate incision, at rates between 0.4 and 0.75 m/k.y., continued into the Holocene. The 10Be data are more consistent with continued channel lowering through this 3 km reach than the steady retreat of a single knickpoint. Prior to 37 ka, the primary falls of the Potomac River were likely at Black Pond. Ongoing incision siphoned water away from these paleofalls, leaving them high and dry by 11 ka. Downstream of Black Pond, the strath terrace surface is covered with fine-grained sediment, and the few exposed bedrock outcrops are weathered and frost-shattered from periglacial processes active during the Last Glacial Maximum.
This, the 2015 Kirk Bryan field trip, is designed to stimulate discussion about why, when, and in what way large rivers and their tributaries on passive margins incise into rock by focusing on the Potomac River and the geomorphology of the Great Falls area. More than 100 10Be exposure ages provide chronologic control and fodder for our discussion, which will explore changing boundary conditions over the Pleistocene and Holocene including variations in climate, changes in sea level, and glacial isostatic land level adjustment.
Kirk Bryan was one of the most influential geomorphologists in the first half of the twentieth century. He was a field geomorphologist and for that reason, it seemed logical that the annual Quaternary Geology and Geomorphology field trip dedicated to inspiring discussion and inquiry centered on an outstanding field site be named after him. We have been doing the Kirk Bryan trip since 2006 (Philadelphia meeting). 2015 is the 10th anniversary!
Over the course of the day, we will make four stops (Fig. 1) in close proximity to see the river upstream of the falls (stop 1), Great Falls (stops 1 and 3), Mather Gorge and the strath terrace downstream of the falls (stops 1 and 2), and Black Pond, the site of a paleofalls abandoned during the last glaciation (stop 4). At stop 4, we will also consider the geomorphic effect of Potomac River incision on the morphology of tributary streams. During the day, we will visit several outcrops sampled for 10Be exposure dating that are representative of those sampled elsewhere along the Potomac River.
Geography and Background
The Potomac River drops more than 15 m over Great Falls just 20 km upriver of Washington, D.C. (Fig. 2). Above the falls, the river’s bedrock channel is shallow and > 500 m wide (Fig. 3). Below the falls, in Mather Gorge, the river is constrained to a deep, narrow channel < 50 m wide and flanked by a wide, bare-rock strath terrace that is hundreds of meters wide and covered by fluvially rounded outcrops of at most lightly weathered rock (Fig. 4). This terrace is best expressed on Bear Island just downstream of the falls (Fig. 1). At Black Pond, 3 km downstream of Great Falls, there are plunge pools and chutes left high and dry above the river by channel incision (Fig. 5). The strath terrace continues > 10 km downstream of Black Pond, mostly on the Maryland bank of the river; however, the terrace downstream of Black Pond is largely sediment-mantled and the outcrops, where observed, are frost-shattered and generally more heavily weathered than those upstream (Fig. 6).
The surrounding area is underlain by metamorphosed and deformed Proterozoic and Cambrian sedimentary rocks of the Piedmont physiographic province intruded by igneous rocks of different ages and compositions. The geology of the Great Falls area has been mapped by several workers and was compiled and presented in a publically accessible format by Southworth et al. (2000) and Southworth et al. (2008).
The Potomac River basin was not glaciated during the Quaternary; however, the basin climate was affected by glacial-interglacial climate fluctuations and by permafrost during glacial maxima (Braun, 1989). The closest long-duration (> 100 k.y.) paleoclimate record is from Hybla Valley, ~20 km south of Washington, D.C., adjacent to the Potomac River estuary (Litwin et al., 2013). The pollen record suggests cold, boreal forests were common from ca. 16 ka to 31 ka; before that time, climate alternated between warmer and cooler periods.
On both banks of the Potomac River, human impacts figure prominently in the geomorphology. On the Virginia bank, the Patowmack Canal was built in the late 1700s at the instigation of George Washington; its construction involved the earliest extensive use of black powder blasting in the Americas. The canal bypassed Great Falls using a series of locks allowing trade between the eastern colonies and the western frontier, but only when water levels were optimal. Parts of the canal remain visible today. On the Maryland bank, the C&O Canal was built starting in 1828. The 184-mile-long canal was not completed until 1850, and from then until it closed in 1924, it was beset by issues related to repeated flood damage. At its peak, in 1871, 500 boats carried nearly a million tons of freight through the canal, much of which was coal (www.nps.gov/nr/twhp/wwwlps/ lessons/10cando/10facts2.htm, accessed 2 June 2015).
The area examined by this trip is mostly within park boundaries and is heavily visited by the public. In 2014, over five million people visited the C&O Canal National Park unit on the Maryland bank of the Potomac River (https://irma.nps.gov/Stats/, accessed 3 June 2015). This ranks the C&O Canal National Park ninth in the nation in terms of visitation. In 2014, ~250,000 people visited the smaller Great Falls National Park unit on the Virginia bank of the river.
The Potomac River at Great Falls has been studied previously and the geomorphology described in a series of papers. Tourmey (1980, 1988) summarizes early research, provides textural data on terrace soils, describes channel morphology, and suggests that knickpoints are more prevalent on tributaries downstream of Great Falls than above the falls. Reed et al. (1980) and Reed (1981) describe Great Falls, the terrace sequence below the falls, and other, older Potomac River gravels in detail, including the relative weathering of both the terrace cover sediments and underlying rock in an attempt to understand better the chronology of river incision. The first numerical geochronology was provided by Reed (1981), who cites two 14C ages on organic material in sediment deposited on the Bear Island terrace surface. These ages, 9.5 and 16.0 14C k.y. B.P., are minimum limits for the exposure age of the surface. Bierman et al. (2004) and Reusser et al. (2004) both provide a small set of cosmogenic 10Be age estimates for rock surfaces and use these ages to calculate incision rates (those data are included in the data set underlying this field guide). Zen (1997) provides detailed observations of the terraces and their geomorphology below Great Falls as well as consideration of tributary stream response to Potomac River incision. Zen and Prestegaard (1994) describe potholes that are common along the rock terraces of the Potomac River downstream of Great Falls.
Between 1999 and 2003, we collected 108 samples of rock along and above the Potomac River on both the Maryland and Virginia banks using a hammer and chisel. Sample sites stretch along ~10 km of river length. The sample sites are at varied elevations (0-36 m) above the present-day, low-flow water level. We designed our sampling plan to retrieve samples from a range of elevations at different distances downstream (Fig. 7A). We sampled outcrops of quartz-bearing schist that preserved fluvially rounded forms with the exception of a single large quartzite boulder collected from the summit of Glade Hill (EZ1). At several locations along the river (Great Falls, Cow Hoof Rock, Black Pond; see Fig. 1), we collected samples along vertical transects in order to calculate incision rates.
Samples were ground and the quartz purified at the University of Vermont. In situ produced 10Be was extracted and 106 isotopic ratios were measured at Lawrence Livermore National Laboratory. Some of the data were published in Bierman et al. (2004) and Reusser et al. (2004). For this field guide, we have recalculated all of the ages using the CRONUS online calculator (Balco et al., 2008) considering the standards used for isotopic analysis and using the currently accepted global average production rate.
Sample locations were surveyed using a variety of GPS systems including Garmin handheld, beacon-corrected Trimble ProXR, Trimble RTK4400, and Trimble ProXH with extended occupancy times and HStar post processing. We use the ProXH and 4400 data for elevation control. Most replicate measurements, made years apart, agree to decimeters in the x and y coordinates and within a meter for elevation. A few samples, those under deep forest cover, have less accurate elevation control.
Google Earth Professional was used to plot the samples geographically. The distance downstream was measured projecting the sample site perpendicular to the Potomac River thalweg. The height of the sample above the channel elevation at low flow was determined using the Google Earth DEM.
Quartz extracted from samples collected along the Potomac River contains different concentrations of in situ 10Be (0.18 to 11.81 × 105 atoms/g); thus, the outcrops and boulder we sampled have widely varying exposure ages (3.8-278 ka). The distribution of exposure ages is multimodal (Fig. 8). A prominent mode between 22 and 37 ka reflects the abandonment of the broad strath terrace surface on Bear Island and adjacent island summits. The modes at 53 ka and 74-83 ka include high points scattered along the length of the sampled area, whereas the oldest mode (170-200 ka) includes only samples collected from relatively high points far downstream. The modes at 37 ka, 53 ka and 74-83 ka correspond to sea-level highstands and relative warm periods as indicated by ice core and deep sea-oxygen isotope records (Siddall et al., 2008).
There is spatial patterning to the distribution of exposure ages. Overall, outcrop elevation (m a.s.l. [above sea level]) is unrelated to exposure age (Fig. 7B). However, elevation above low water is positively correlated to exposure age (Fig. 7C), with the oldest samples being located farthest above today’s channel margin. The maximum measured exposure age increases linearly downstream (Fig. 7D). Outcrops near the waterline at low flow have the youngest exposure ages (3.8-6.6 ka, GF10-13 and GF71-72). The oldest outcrop exposure ages are found far downstream (138-198 ka, GF87-89 and GF60-61) that we will not visit on this trip. The boulder from Glade Hill has the oldest exposure age (EZ1, 278 ka) although there is no way to know how much inherited 10Be the boulder carried at the time it was deposited.
Samples collected along elevation transects show progressive incision of the Potomac River over time. Just below Great Falls, a vertical transect from south of the viewing platform on the Maryland bank (Fig. 9) indicates that incision there proceeded at 0.64 m/k.y. between 26 ka and 4 ka (Fig. 9A). At Cow Hoof Rock, 1.5 km downstream, incision between 38 ka and 8 ka occurred at a rate of 0.75 m/ k.y. (Fig. 9B). At Black Pond, 3 km downstream of Great Falls, incision was slower, at 0.41 m/ k.y., between 34 ka and 11 ka (Fig. 9C). Continued incision over tens of thousands of years at each site seems inconsistent with the migration of a single knickpoint upstream.
Logistics and Hazards of the Field Trip
To see the geomorphology of the Great Falls area, one must walk. The trip outlined here involves ~10 km of walking on a mixture of trail types ranging from well-compacted, flat gravel paths to scree slopes with no formal path. Some of the trip involves scrambling over steep, rocky outcrops that are slippery when wet. Supportive, closed-toed shoes are a necessity. Faunal hazards include ubiquitous deer ticks that carry Lyme disease as well as rarer poisonous snakes (copperheads) and spiders (black widows). Poison ivy is everywhere; it seems to thrive in the disturbed floodplain environment with old-growth vines covering trees and exceeding several inches in diameter. Water is available at both Great Falls stops and at Maderia but likely not at Carde-rock. Bathrooms are available at all stops.
The first three stops are on public lands controlled by the National Park Service, the C&O Canal National Park on the Maryland bank and the Great Falls Park on the Virginia bank. Both charge admission fees. The last stop is on private land owned by the Maderia School. This site is only accessible with permission obtained in advance from the school.
Stop 1. C&O Canal National Park, Great Falls (-77.247034°, 39.001587°, WGS84) Duration: 2.5 hours
We begin the trip on the Maryland bank of the Potomac River at Great Falls. The purpose of this stop is to view the falls, contrast the geomorphology of the river and its banks above and below Great Falls, and introduce the approach we took to dating river incision.
Begin at the traffic circle just beyond the park entrance, cross the C&O Canal at the first lock, and walk north on the towpath, heading upstream. In ~700 m, turn left (west) onto an unmarked but well-established trail through the woods and walk < 100 m to the river bank where there is a view of the wide, shallow channel of the Potomac River upstream of the falls. Look for small outcrops of rock that extend above the water surface at low flow. Similar outcrops, sampled on the Virginia bank, gave 10Be ages of 30 and 32 ka (GF53, GF54) suggesting that they were exposed the same time that Mather Gorge and Black Pond began to incise (see below).
Walk downstream along the river’s edge until you come to the viewing platform (adjacent to the towpath) from which you can look across the Potomac at the low head dam that diverts water from the river for use by the capital area. Built by the Army Corps of Engineers starting in 1853 and repeatedly modernized, the water system includes 12 miles of conduit, several reservoirs, and modern filtration systems.
Continue 500 m down the towpath until the sign for Olm-stead Island trail. Turn right (west) here and cross a series of bridges and fenced boardwalk paths across floodplain forest and numerous, fluvially molded and abraded rock outcrops. You are headed to the observation platform for a view of Great Falls. On the way, you will cross two channels that carry some of the river flow unless the water levels are exceptionally low. Look closely at these channels and adjacent high points because on the last stop of the day, at Black Point, we will observe analogs, now left high and dry above the Potomac River by incision. Once at the observation platform, you will see schist exposed and an excellent example of a pothole. The creation and merging of potholes has been proposed (Zen and Prestegaard, 1994) as a means by which channel erosion and knickzone retreat can occur.
After a discussion at the overlook, walk back toward the tow path but turn right into the woods just after the last bridge and before the towpath. Walk ~50 m south and stop at sample site GF113 (Fig. 10). The 34 ka exposure age of this outcrop is similar to three other samples nearby at similar elevations (GF112, 38 ka; GF46, 29 ka; GF65, 26 ka). These ages are similar to instream outcrops above the falls (30 and 32 ka, GF53 and GF54, respectively) indicating that incision exposed these sample sites to cosmic radiation between 38 and 26 ka, coincident with the expansion and maximal extent of the Laurentide Ice Sheet (Balco et al., 2002; Balco and Schaefer, 2006) and the lowest, maximum glacial sea level (Lambeck et al., 2014).
From here, continue down the towpath ~500 m to the Billy Goat trail which is well signed on the right and traverses Bear Island, an extensive strath terrace. About halfway to the trail head, a view opens from the towpath west across to the Virginia bank of the river, which clearly shows the accordant height of the strath terraces on both sides of the river, as well as the island tops in the river. These surfaces represent the channel bottom of the Potomac before it incised ca. 37 ka.
Walk past sample sites GF114 and GF115 with exposure ages of 35 and 43 ka, and continue downstream to an overlook (-77.24718705°, 38.99070248°, WGS84) that provides a clear view of Mather Gorge below as well as the paired strath terrace surface on the Virginia bank (Fig. 4). Other nearby samples on Bear Island have exposure ages of 39 ka and 37 ka (GF116 and GF117). After this stop, return to the visitor center, backtracking along the Billy Goat trail and the towpath.
Stop 2. C&O Canal National Park, Carderock (-77.205490°, 38.976345°, WGS84) Duration: 1.5 hours
We continue the trip at Carderock examining the most prominent terrace 7 km downstream. The purpose of this stop is to see an older landscape. Here, the terrace morphology is far more subdued. Outcrops, where present, are weathered and frost-shattered, a qualitative indication of their age. Much of the terrace surface is covered with fine-grained sediment to a depth of several meters. Where exposed in stream cuts, this sediment has been reddened by pedogenic processes, another indication of relative age.
Exit to Carderock from the Clara Barton Parkway. Park at the far upstream parking lot and follow the signs to the Billy Goat trail. After < 100 m, turn right on the trail and then left toward the cliff top and sample site GF83 (Fig. 6). Despite significant surface weathering, this sample has an exposure age of 121 ka.
Either walk the Billy Goat trail downstream to the picnic area at Carderock (most downstream parking lot) or take the bus to the picnic shelter (depending on time, weather, and fatigue level). If walking, notice the smoothness of the trail and the paucity of outcrops downstream. Those outcrops that are visible are heavily weathered. If riding the bus, look out the window and notice how few outcrops you can see.
After lunch, walk to the downstream end of the large field passing sample site GF86 (70 ka) and take a set of stairs down ~100 m to a bridge crossing a deeply incised creek. Rock crops out at the bottom of the creek below meters of fine-grain covered sediment that is reddened. A sample from the rock (GF108) gives an age of 11 ka, indicating the rock surface was rapidly buried after initial exposure.
Stop 3. Great Falls National Park, Virginia Duration: 1 hour
The afternoon begins in Great Falls National Park on the Virginia bank, where we will stop in order to observe Great Falls, the predominant Bear Island terrace surface, and the vertical sample transect extending from the terrace to the base of Great Falls. We will also consider inundation by contemporary floods.
Cross the Potomac River on the I-495 bridge and exit going west on Georgetown Pike. Go west ~5 km and enter the Great Falls National Park. Park and walk under the visitor center (which has an interesting display on the history of the area if there is time, as well as restrooms). Walk downriver 150 m to observation platform 3. From here, look across Great Falls to the Maryland bank where the vertical transect (Fig. 9A) was collected. You can see accordant surfaces on the islands that match the in-channel outcrop elevations upstream and the Bear Island strath terrace surface downstream. On the way to the observation platform, you will notice a pole erected by the National Park Service that shows the heights of various historic floods. In the last century, six floods have overtopped the strath terrace: 1936, 1937, 1942, 1972, 1985, and 1996. Of these, the 1936 flood was the largest (484,000 cfs). The paucity of terrace-inundating floods clearly demonstrates that the strath and its outcrops are hydraulically disconnected from the modern river system.
Walk upstream to observation platform 1. Here you have a view upstream at the falls and can compare the view with a photograph taken at the maximum flow (167,000 cfs) resulting from Hurricane Isabel in September 2003 (Fig. 11).
Stop 4. Maderia School and Black Pond, Virginia Duration: 2.5 hours
The trip concludes at the Maderia School providing access to Black Pond, which likely marks the location of a significant falls before incision of Mather Gorge. The purpose of this stop is to view the impact of Potomac River incision on the geomorphology of tributary streams and to see the abandoned falls at Black Pond.
Leave the Great Falls Park and return to Georgetown Pike. Go east 3 km and turn left (north) into the school. This is private property and can only be entered with advance permission from the school. We will park in the first lot beyond the entry gate (on the right) and walk several kilometers downhill on gravel roads to Black Pond, which we interpret as a trough eroded by river flow (a deep) during the time prior to the incision of the Bear Island strath terrace and the formation of the contemporary Great Falls. On the way down to the pond, notice the deep incision of the small tributary streams as they try and adjust to falling base level (the incising Potomac River).
Once at the pond, look across at the high points that would have been emergent when the Potomac flowed through the low area now occupied by Black Pond. You can traverse a steep scree slope that defines the southern margin of the pond. Walk west over rough terrain for ~200 m. Note the frost-shattered nature of the rock, which likely results from exposure to permafrost during the Last Glacial Maximum. At the end of the slope, turn right onto the bare rock. The high point here is sample site GF96 with an exposure age of 56 ka. Below the high point on a transect leading to the low point (paleochannel bottom) are rock outcrops with exposure ages regularly decreasing to 18 ka (Figs. 12 and 9C). Arrows on Figure 12 show paleochannels through which water flowed before the Potomac River incised. Sample GF104 has an exposure age of 11 ka, dating the last substantial flow of water through the Black Pond area. Just downstream of the transect and at higher elevation are a number of surfaces with older ages (60-98 ka), highpoints around which the Potomac once flowed.
QUESTIONS FOR DISCUSSION
What is the spatial pattern of incision? Did discrete knick-zones retreat upstream or was incision accomplished by channel-bed lowering along kilometers? How does the river upstream of Great Falls “know” of incision downstream (similar outcrop exposure ages).
What is the meaning of 10Be exposure ages? How do we know if they are accurate or if they over-or underestimate the age of strath surfaces? What role might sediment cover and exposure deep under the surface to low rates of muon-induced 10Be production play in observed ages?
What are the drivers of incision? Is incision driven directly by changing climate or indirectly by climate-mediated sea-level change, glacial isostatic adjustment, or changes in runoff and sediment load?
What was the incision history of the Potomac River prior to the Last Glacial Maximum, and how might we discern that history?
What role, if any, does channel sediment cover have in the creation and exposure of strath terraces and control of outcrop exposure ages?
I thank the National Park Service and the Maderia School for access and the U.S. National Science Foundation for support under NSF-0003447. C. Massey, M. Pavich, L. Reusser, J. Reuter, J. Larsen, and M. Jungers assisted with fieldwork and guided my thinking about the field area as did M.G. Wolman. I dedicate this field guide to E-an Zen who spent hours with me in the field and on email discussing the terraces of the Potomac River and the formation of Great Falls (Fig. 13).
Figures & Tables
Tripping from the Fall Line: Field Excursions for the GSA Annual Meeting, Baltimore, 2015
Prepared in conjunction with the 2015 GSA Annual Meeting in Baltimore, Maryland, this volume contains guides to field trips in this historic region. Emanating from the Fall Line city of Baltimore, these trips reflect the diversity of geological features in the mid-Atlantic region including the Piedmont, Appalachian Mountains, and Coastal Plain, and the importance of geology on the development and construction of the Baltimore-Washington, D.C., metropolitan area. Trips to the core of the Appalachian orogen concern themselves with the tectonic and metamorphic history, early Paleozoic carbonate platform development, Devonian paleoclimate, and coal-mine fire hazards. Excursions to the Coastal Plain examine various aspects of Cenozoic stratigraphy, structure, barrier island formation, and wetland and ecosystem development. A variety of trips also explore urban geology, including building and monument stones of Baltimore and Washington, D.C., urban hydrogeology, and Civil War battlefield geology.
- alkaline earth metals
- exposure age
- Fairfax County Virginia
- field trips
- incised valleys
- landform evolution
- last glacial maximum
- Montgomery County Maryland
- Potomac River
- radioactive isotopes
- road log
- United States
- upper Pleistocene
- Black Pond
- Chesapeake & Ohio Canal National Historical Park
- Great Falls National Park