Geomotphology, Vegetation, and Patowmack Canal Constmction Problems: Great FaDs Park, Potomac River, Virginia Potomac River Gorge, Virginia July t 3, t 989 Field Trip Guidebook T236 Leader: Nancy M. Milton Associate Leader: Robert S. Sigafoos American Geophysical Union, Washington, D.C. Published 1989 by American Geophysical Union 2000 Florida Ave., N.W., Washington, D.C. 20009 ISBN: 0-87590-582-X Printed in the United States of America COVER Photograph of the Great Falls of the Potomac River from the Visitors Center, Great Falls Park, Virginia. Leader: Nancy M. Milton U.S. Geological Survey 927 National Center Reston, VA 22092 Associate Leader: Robert S. Sigafoos U.S. Geological Survey 927 National Center Reston, VA 22092 IGC FIELD TRIP T236: GEOMORPHOLOGY, VEGETATION, AND PATOWMACK CANAL CONSTRUCTION PROBLEMS: GREAT FALLS PARK, POTOMAC RIVER, VIRGINIA N. M. Milton AND R. S. Sigafoos U.S. Geological Survey The spectacular landscape of the of the uplands, relict terrace deposits, Great Falls of the Potomac River, including rounded boulders and gravel, approximately 25 kilometers northwest of are preserved. Washington, D.C., is the result of Vegetation reflects the differences prolonged geological and biological in the physical environment. Some tree processes. The slow downcutting of the and shrub species are widespread, but resistant bedrock by the river, many grow only on selected types of controlled in part by faults and terrain. Sycamore, silver maple, box jointing, the weathering of the rock, elder, and green ash grow only on active and the continuous growth of the forest flood plains. Chestnut oak, post oak, have interacted to produce the present and red oak are found where bedrock is landscape. Superimposed on these long, exposed on terraces. Pin oak, willow continuous processes has been the oak, swamp white oak, and red maple are effects of 200 years of human activity. present in swamps. Forests on rolling Great Falls lies near the eastern uplands near Great Falls have been edge of the Piedmont physiographic repeatedly cut for firewood and lumber, province, which adjoins the Atlantic some of the land has been cuIt ivated, Coastal Plain province to the east and much of the soil has been severely (Figure 1). The boundary between these eroded. These upland sites support a two provinces, commonly called "the Fall large number of tree species, but yellow Line," extends approximately from New poplar, beech, white oak, and black oak York City southwestward through are most common. Part of the bedrock Philadelphia, Baltimore, Washington, terrace close to the old Patowmack Canal Richmond, and beyond. Each of these supports many introduced species of cities was founded at the upstream end trees, vines, shrubs, and ornamental of the navigable waters of a major flowers. river, where industries prospered by The Patowmack Canal was conceived as applying the hydrologic power of the a method of transporting people and falls. materials around Great Falls, to provide The rocks underlying the Piedmont the uninterrupted river travel from the province at Great Falls are highly Chesapeake Bay to the upper reaches of metamorphosed shale, muddy sandstone, the Potomac River which would enable and volcanic rocks forming schists, development of the west. The project metagraywacke, and amphibolites of required the confederated states of Cambrian age (Drake, 1987). Granite, Maryland and Virginia to agree to free lamprophyre, and quartz ve ins were trade on the Potomac River. Use of the intruded during the Acadian Orogeny. canal would require water from the Slow uplift of the Piedmont and the river, which belonged to Maryland, to Appalachian Mountains to the west flow into Virginia. Negotiations beginning in the Miocene Epoch resulted fostered by the Patowmack Canal Company in downcutting of rivers and deposition added an incentive for the adoption of of sediments on the Coastal Plain to the the United States Constitution and the east. As sea level lowered during the establishment of 13 United States in Pleistocene, the Potomac River eroded place of 13 independent states (Garrett, deeper into underlying bedrock. The 1987) . configuration of the channels at Great George Washington, general of the Falls is the result of local differences army and first president of the United in bedrock, the locations of joints and States, had earlier surveyed the Potomac a major fault, and periodic river River basin and promoted a river trade flooding. The effects of these local route west into the Appalachian differences can be seen in active flood Mountains. He advocated the use of plains, bedrock terraces now rarely boats and a system of canals and lift flooded, and rolling uplands. On some locks to circumvent waterfalls. The T236: /" MARYLAND / / '" " / / "" / / t / ~ /// WASIIINGTON,D. C. @ / / 4-- / To Culpeper Basin VIRGINIA and / Dulles Airport / ~ N / W 0'\ .. K Street / F\) / 'd Convention / Center Theodore Roosevelt Island PIEDMONT ~/COASTAL PLAIN LOCATION DIAGRAM -------------- -~.~VA\ MARYLAND f"" 7\ " v'"-- O I 2 3 4 5 Miles I I ! ! VIRGINIA 1"/ \?~, ~I I! ,,I I 0 I I ~ 3 4 5 Kilometers I",, ! IIIII FIGURE 1 Map of the field trip area in northern Virginia with locations of stops 1-3. Patowmack Canal at Great Falls was the of the Piedmont province. most ambi tious phase of his canal From the Convention Center, travel construction project. The canal was west on K Street, follow signs to the plagued with severe construction Whitehurst Freeway, and cross the problems from the start in 1185 until Potomac River on the Key Bridge. completion in 1802. The problems were, Immediately turn right (west) onto in part, the result of insufficient site George Washington Parkway and proceed study prior to construction. The 4.5 kilometers (2.1 miles) to the second proposed canal route was selected by overlook. surveyers trapped within a cabin during a snow storm that completely obliterated Stop 1: Overlook on George Washington the landscape. The route selected was Parkway the shortest in distance, but because of the geologic setting was difficult to The horizon directly across the river develop. Had the swamp to the south and is the Piedmont province. Directly below west of Glade Hill (Figure 2) been is the modern flood plain, which surveyed, the canal could possibly have consists of a thin layer of fine-grained been bu i I t in much less than 11 years alluvium deposited on resistant and at a fraction of the cost. bedrock. The fine alluvium is present Operations of the canal ceased in 1828 because minor, frequent floods, which when transportation was assumed by the normally would wash the alluvium continuous Chesapeake and Ohio Canal downstream, have been diverted upstream located across the river. of the bridge to the left by a diversion Between 1936 and 1985 five major channel of the metropolitan water supply floods covered all or part of the reservior. The diversion channel was Patowmack Canal. If the river had cut into the bedrock under the flooded during 1185-1828 as frequently, Chesapeake and Ohio Canal on the or to the levels, that it has in the Maryland side of the river. The flood last 50 years, construction might have plain downstream of the diversion been terminated before completion of the channel floods an average of only once canal in 1802, or canal operations might in 2 years, and water velocity is have ended before 1828. diminished by the high tides of the Chesapeake estuary. Upstream from the Topographic Maps: 1:24,000 scale (11 bridge, canal locks were built in the minute quadrangles): Washington West, late 18th century as part of the D.C., Falls Church, Va., Vienna, Va., Patowmack Canal system, another segment Rockville, Md., Seneca, Md., Va. of which will be seen at Great Falls. 1: 100,000 scale: Washington West D. C.- The route to stop 2 follows the older Md.-Va., and Baltimore, Md. rock-cut terraces bordering the river channel and drops occasionally into En Route From the Convention Center to narrow tributary valleys. Continue west Stop 1. on the Parkway 6.1 kilometers (4.0 miles) to the entrance to Turkey Run The most impressive geologic features Park. Proceed to parking area C. of Washington, D.C., result from widely differing bedrock. The eastern two- Stop 2: Turkey Run Recreation Area thirds of the area is within the Coastal Plain province, where poorly The rolling hills in the picnic area consolidated, nearly flat-lying gravel, are typical of the Piedmont. These sand, silt, and clay stratigraphically knolls and locally gentle slopes are overlap an east-sloping basement of interspersed with narrow, steeply metamorphic and igneous rocks. The sloping valleys. A short distance to western third of Washington is on the the north, the land drops steeply, more Piedmont Plateau province, where the than 60 meters to the river. The resistant metamorphic and igneous rocks underlying bedrock consists of Cambrian are exposed. metamorphic rocks, metadiorite, gabbro, The route from the Convention Center and amphibolite with numerous quartz crosses Pleistocene gravel, sand, and veins (Drake, 1981). silt deposits of the Coastal Plain The knolls and steeply sloping province before ascending onto the valleys precluded heavy agricultural Piedmont province. The route crosses land use in the past. The absence of the river upstream of Roosevelt Island, gullies, which are common in even­ a Holocene terrace at the eastern edge sloping cultivated areas underlain by T236: 3 MARYLAND t-3 f\.) W 0'\ .:= • Bedrock Terrace VIRGINIA ~~:~:~:~:):~t Up Iand o 0.5 I Mile I , ! I I a 0.5 I Kilometer I I ! , I FIGURE 2 Detail of a portion of Great Falls Park showing field trip stops 3-9. thick saprolite, indicates that mills, built approximately 150 years cultivation of crops probably never ago, took advantage of the steep occurred here.