The River and 

the Rocks

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The Geologic Story 

of Great Falls and the 

Potomac River Gorge

Depository

^il LIBRARIES

GEOLOGICAL SURVEY BULLETIN 1471
OHIO STATE

UNIVERSITY

LIBRARIES
The Geologic Story 

of Great Falls and the 

Potomac River Gorge
The River and th

^ BP^-" ^ ihn ulrtMP' 
tocks

The Geologic Story 

of Great Falls and the 

Potomac River Gorge

By JOHN C. REED, JR., 

ROBERT S. SIGAFOOS, 

AND GEORGE W. FISHER

U.S. Geological Survey Bulletin 1471

UNITED STATES

GOVERNMENT PRINTING OFFICE,

WASHINGTON: 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress catalog card No. 80-600023

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402

Stock Number 024-001-03280-5
Foreword to the Second Edition

The Great Falls of the Potomac River has figured prominently in the purposes of 

men since prehistoric time. Long before John Smith reached the falls in 1608, 

groups of Indians from East and West met at this great river barrier to trade and 

perform ceremonies in honor of the spirit of the "Roaring Waters." As early as 

1754, George Washington visualized the Potomac River as an important avenue 

of trade and communication with the interior.

Records show that, with the exception of Mount Vernon, Great Falls was 

perhaps as intimately associated with George Washington's everyday life as any 

other place in the country. As first President of the "Patowmac Canal 

Company," Washington frequently visited the working parties as they constructed the canal and lock system which skirted the treacherous falls on the 

Virginia side. Matildaville, a town of about 16 hectares named after the wife of 

"Light Horse" Harry Lee and consisting of various dwellings, grist mill, market 

house, forge, sawmill, and tavern, sprang up along the banks of the canal.

In 1802, the Patowmack Company canals were essentially completed, and hundreds of boats plied the river, bringing corn and wheat, coal and limestone, flaxseed, and furs downstream from the mountainous region around Cumberland. 

Many of the boats were sold for lumber in Georgetown, thus sparing the 

boatmen an arduous upstream journey.

After the establishment of the Nation's Capital, Great Falls became a popular 

scenic attraction for residents and visitors alike. But Great Falls was not always 

as easily accessible as it is today. In 1845, a newspaper columnist, after praising the beauty and historic interest of the region, added, "... the access to this 

interesting spot is, on both sides of the river, by the most infamous roads 

and the accomodations for visitors anything but what they ought to be!"

Visitors to Great Falls now number more than one-half million annually 

and, because of this continuing and mounting interest, the U.S. 

Geological Survey has joined with the National Park Service in preparing this booklet for better understanding and enjoyment of the Great 

Falls of the Potomac River. The original edition was published in 1970. 

The current edition has been revised and expanded to incorporate new 

information, an enlarged trail log, and changes in trail locations. 

Because the policy of the Geological Survey is to foster use of the International System of Units, all measurements in this edition are in the 

metric system.

Contents

v Foreword

1 The falls today

3 The river and the land

8 The origin of the Potomac River valley and

	the carving of Great Falls 

13 The origin of the rocks 

20 A few words about the forests 

24 Trail log Great Falls, Virginia 

49 Trail log Great Falls, Maryland

74 Suggested reading

75 Acknowledgments

75 Metric-English conversion

VII
 *.;*-

_jf f

'5:

Average flow stage in summer

,,_£

w Flow in flood stage
The Falls Today

The Great Falls of the Potomac River is the most spectacular landscape 

feature of the Washington metropolitan area. It has influenced the lives 

and fortunes of man in the Potomac Valley for centuries. Countless 

generations of Indians used it as a place to gather, to trade, and to fish. 

For the early settlers it was a barrier to river navigation, an obstacle that 

canal builders struggled to overcome. The river above the falls has been 

the principal source of wafer for the city of Washington for more than 

100 years.

In its seaward course, the Potomac River crosses many small rapids and 

cascades, but these are insignificant in comparison with the foaming 

fury of Great Falls, where the river drops 12 meters in about 180 meters 

and is channeled into a narrow rock-walled gorge less than 25 meters 

wide in places. In the summer the flow may be less than 38,000 liters a 

second, but during floods the flow commonly reaches 40 million liters a 

second. The average flow pouring over the falls is 349,000 liters of water 

every second and in a year, more than 9.5 trillion liters enough water to 

flood the entire District of Columbia to a depth of 55 meters, converting 

the Washington Monument (169 meters high) into a tall lighthouse.

During three periods of recent flooding in 1936, again in 1942, and still 

again in 1972 water covered the towpath of the Chesapeake and Ohio 

(C and O) Canal on the Maryland side of the river and spread across the 

area of the Visitor Center and parking lots on the Virginia side. Floods of 

this size are unusual; but about once every 2 years, floodwaters rise to 

the brink of the gorge at the falls, reaching to within 4 to 6 meters of the 

trails and overlooks along the cliffs. There have been 20 such floods 

since 1930.

Today the Great Falls of the Potomac, the Mather Gorge, and the surrounding scenic area administered by the National Park Service provide 

splendid opportunities for outdoor recreation. But perhaps most important of all, they serve as a unique outdoor laboratory for nature studies 

and a place to reflect on the impact of man on his natural environment.

Such studies require an understanding of the landscape the geologic 

story concealed behind the scenery. This booklet presents a brief account of the geology of Great Falls, summarizing what is known of the 

events that formed the rocks and shaped the land.

1

The River and the Land

All landscapes are the result of a continuing struggle between the deepseated earth forces that raise the land above sea level and the forces of 

erosion that gradually wear down the land surface. Erosion begins with 

the weathering of rocks the chemical decay and solution of mineral 

grains and the mechanical disintegration of rocks by frost. The resulting 

debris is carried into streams and rivers by running water, again aided 

by frost action and the slow creep of soil down the slopes. Once in the 

streams the debris is transported seaward, partly in solution, partly as 

mud and silt in suspension, and partly as sand, gravel, and boulders 

rolled or bounced along the bottom. The Potomac River is estimated to 

carry more than 1.45 million metric tons of sediment and about 1 

million metric tons of dissolved material over the falls each year. This 

amounts to 80 metric tons of material removed from each square 

kilometer of the river's drainage basin above the falls.

The Potomac, like other Atlantic seaboard rivers, rises in the Appalachian Mountains and flows eastward to the Atlantic Ocean. The 

river cuts through the Blue Ridge at a narrow gap below Harpers Ferry 

and across Catoctin Mountain, the easternmost ridge of the Appalachians, through a similar gap at Point of Rocks. From there to its 

mouth at Point Lookout on Chesapeake Bay, the river flows across three 

major landscape provinces: a broad lowland between Point of Rocks 

and Seneca that is called the Frederick Valley in Maryland and the 

Leesburg Basin in Virginia; the Piedmont Plateau, between Seneca and 

Washington; and the Atlantic Coastal Plain, between Washington and 

Point Lookout.

The nature of the landscape and river valley in each of these provinces 

is determined largely by the nature of the underlying rocks. The 

Frederick Valley and Leesburg Basin are underlain principally by red 

sandstone and shale that were deposited during the Triassic and 

Jurassic Periods, about 200 million years ago. Because these rocks are 

rather easily eroded, the land's surface is nearly flat; slopes are gentle 

and smooth. The river is wide, sluggish, and shallow, and is flanked by 

broad flats frequently covered by floodwaters.

The Piedmont Plateau is a rolling, hilly upland underlain by hard rocks. 

Although these rocks are very resistant to erosion, they are subject to 

chemical decay, which in places extends to depths of 30 meters or more
Pal.

nd sh

Metamorphosed sedimentary 

and volcanic rocks, chiefly 

schist and metagraywacke

Metamorphic and 

igneous rocks

Sand, gravel, and 

clay of Tertiary anc 

Cretaceous age

and produces saprolite (rotten rock) and deep, red clayey soils. The 

smaller streams on the uplands have not cut through the blanket of soil 

and soft decayed rock and thus flow in broad valleys and wide flood 

plains, but the larger streams have cut through to hard rocks and flow 

within narrow steep-sided valleys having constricted flood plains.

The nature of the valley of the Potomac across the Piedmont Plateau is 

quite variable. Above Great Falls the valley is steep sided but wide, and 

in most places the river is broad, shallow, and placid. In a few places, 

however, riffles and rapids break the quiet water as it passes across 

resistant ledges of rock. The numerous islands are composed of sand 

and gravel laid down by the river, and none of them rises much above 

the level of the flood that occurs about once every 2 years. At Great 

Falls the character of the river changes abruptly. From there to 

Theodore Roosevelt Island, it flows within a series of narrow rockgirded channels twisting between cliffs and flat-topped bedrock islands

4
wned ice-age 

nnel now filled 

h silt and clay

BLOCK DIAGRAM OF THE 

WASHINGTON-BALTIMORE REGION 

SHOWING PHYSIOGRAPHIC PROVINCES 

AND GEOGRAPHIC 

AND GEOLOGIC FEATURES

that rise well above the level of the highest known floods. Passing over 

a series of rapids and low falls, including Yellow Falls, Stubblefield 

Falls, and Little Falls, the river reaches the level of the sea at Chain 

Bridge.

As the river flows beneath the Arlington Memorial Bridge, it leaves the 

Piedmont Plateau and enters the Atlantic Coastal Plain. The Coastal 

Plain is underlain by layered deposits of sand, clay, gravel, and shells 

laid down in and along the edge of the sea when it encroached onto the 

eastern edge of the continent at various times during the past 100 

million years. In cross section, these deposits resemble a gigantic wedge 

ranging from thicknesses of a few meters near the inland edge to nearly 

3,000 meters along the Maryland coast. Chesapeake Bay and its system 

of estuaries are ancient valleys cut in the soft sediments of the Coastal 

Plain when sea level stood much lower. When the sea rose to its present 

level, the valleys were flooded. The Potomac River occupies one of
. A..S. S 1C S A' N p_S T .6

'.: ;y''-.' AND SHALE : /: '. WLEESBURG BASIN 15 KILOMETERS ABOVE GREAT FALLS NEAR SENECA 

sw NE

PIEDMONT PLATEAU 8 KILOMETERS ABOVE GREAT FALLS NEAR VIOLETS LOCK

SW N t

PIEDMONT PLATEAU 6 KILOMETERS BELOW GREAT FALLS NEAR CARDEROCK

Sand and gravel deposited by river 

while cutting its valley NE

v'xv. vs. , i t,

FALL LINE, ARLINGTON MEMORIAL BRIDGE

Sea- .- - -±

-----------------------~-~-~-"-~-~-"-"-~-'- SOFT COASTAL-PLAIN ROCKS - -~-----~-'-~-------------------------~-----

ATLANTIC COASTAL PLAIN, ALEXANDRIA, VIRGINIA

CROSS SECTIONS OF POTOMAC RIVER VALLEY

SHOWING TYPICAL SHAPE IN EACH LANDSCAPE PROVINCE

Each cross section is approximately 6 kilometers long

6
these "drowned" valleys between Washington and its mouth at Point 

Lookout. The broad river, here affected by tides and navigable by 

oceangoing ships, is flanked by wide low terraces and, in a few places, 

by wave-carved bluffs in the soft Coastal Plain rocks.

The boundary between the hard rocks of the Piedmont Plateau and the 

soft, easily eroded rocks of the Coastal Plain is called the Fall Line, 

because it is the line along which falls and rapids are first encountered 

in ascending the major rivers. It marks the head of navigation for 

oceangoing ships and the farthest point downstream that the streams 

can be crossed by fords or small bridges. For this reason, many of the 

cities of the eastern seaboard from Trenton, N.J., to Macon, Ca., were 

established along the Fall Line.

Although the nature of the river valley is largely controlled by the properties of the underlying rocks, these alone are not sufficient to explain 

the origin of the falls and the Potomac Gorge. If they were, one would 

expect Great Falls to lie at the Fall Line, rather than 25 kilometers 

upstream in the middle of the Piedmont Plateau. The reason for the present location of Great .Falls must be sought through studies of the 

geologic history of the Potomac River valley.
The Origin of the Potomac

River Valley and 

The Carving of Great Falls

As the sea finally withdrew eastward from the Washington area.between 6 and 7 million years ago, streams draining from the Appalachian 

Highlands spread a blanket of silt, sand, and gravel over the Coastal 

Plain deposits and nearby parts of the Piedmont Plateau. This blanket 

was not laid down by a single major river, but by numerous streams that 

constantly shifted their courses back and forth to form a complex series 

of coalescing fan-shaped deposits. Remnants of this blanket are still 

preserved capping some of the highest hills in the Piedmont near Tysons 

Corner, Va., 8 kilometers south of Great Falls.

Continued slow uplift of the Piedmont Plateau and the Appalachian 

Highlands to the west increased the slope of the land surface, causing 

the streams to deepen their valleys and eventually to combine into a 

river which was to become the Potomac. As this river deepened its 

valley, scattered remnants of its former flood plains were left at various 

levels as gravel-covered terraces. About 2 million years ago the river 

had carved a broad, open valley in approximately its present position.

With the beginning of continental glaciation in the Pleistocene 

Epoch about 2 million years ago sea level was lowered, and the 

Potomac River began deepening this early valley. As water was 

withdrawn from the oceans to form the great ice sheets on the land, sea 

level around the world fell by as much as 150 meters. Most of the continental shelf off the Eastern United States was exposed, and the 

shoreline lay as much as 120 kilometers east of its present position. Actually, continental glaciation occurred not just once, but at least four 

times in the last 2 million years. The last glacier advanced into central 

New Jersey as recently as 18,000 years ago, and glaciers disappeared 

from much of Canada only 12,000 years ago.

8
I

As sea level fell, the river cut correspondingly deeper into the floor of 

its former valley. The valley was rapidly deepened in the soft, easily 

eroded materials in the Coastal Plain, but in the hard rocks of the Piedmont Plateau the downcutting was much slower. It was this downcutting into the hard bedrock floor of the older wider valley that produced 

the spectacular rocky gorge of the Potomac between Little Falls and 

Great Falls. At Great Falls the river encounters a series of thick rock 

layers that are particularly resistant to erosion, and these hard ledges 

have slowed the progress of valley cutting. The river valley above Great 

Falls thus remains essentially the unmodified, original pre-Pleistocene 

valley, but below the falls the river flows in a gorge excavated within 

the last 2 million years. Along the gorge the original valley floor can be 

recognized as a flat gravel-covered bench 15 to 18 meters above the 

present river level. MacArthur Boulevard follows this bench from Cabin 

John to Anglers Inn. Some of the details of the cutting of the gorge and 

the sculpturing of Great Falls are illustrated in the block diagrams.
A A. The Great Falls area as it probably appeared before the Ice Age. The river occupies a 

broad valley, and the future site of the falls is marked by rapids and ledges of resistant 

rock. Gravel-covered benches (T) are remnants of an older, higher valley floor. Difficult 

Run (D-D') flows across a broad, flat flood plain to empty into the Potomac River.
STAGES IN THE CARVING OF GREAT FALLS. The area of the block diagram covers about 

the same area as that shown on the map. Refer to the map for scale and names of 

geographic features.

A B. The sa.-tie landscape late in the Ice Age. The lowered sea level has caused the river to cut 

deeply into the floor of its former valley. Floods at this time may have been even higher 

and more frequent than they are today because of greatly increased precipitation and 

more frequent ice jams. The downcutting has been relatively rapid in the slightly softer 

rocks below Great Falls and especially rapid where the rocks are broken along a fault 

(F-F'J. At F the river encounters a series of closely spaced fractures or joints. This zone of 

weakness has caused the river to cut laterally (J-)'). Diversion of water into the deeply cut 

channel along the fault and fracture zone has caused the river to abandon several channels on the Maryland side, including the one now occupied by Widewater (VV-W). The 

channel around Glade Hill (G-G') still carries water, but it is rapidly being cut off. Continuous flow of water through this channel ceased about 9,500 years ago. Deepening of the 

river valley has caused Difficult Run to correspondingly deepen the lower part of its valley 

(D-D'), destroying its old flood plain and building a new lower one.

C. Great Falls today. Continued erosion along the fracture zone (]-)') has diverted all the 

water from the channel around Glade Hill (G-G'). Channelways are now being cut upstream 

from the fracture zone, leaving Olmsted (O) and Falls (F1) Islands above river level. Construction of the dam (X-X") to divert water into the Washington aqueduct has further 

modified the details of the landscape above the falls. Part of Conn Island (C), for example, 

has been built since the construction of the dam. The abandoned channel north of Bear 

Island (B) has been flooded by the building of dams at W and W so that it could be used as 

part of the C and O Canal. Difficult Run (D-D') continues to deepen its valley so that its old 

flood plain survives only above the first rapids (D).

11

The Origin of the Rocks

As the Potomac River strips away the soil and cuts into the underlying 

bedrock, it lays bare a fascinating record of events stretching back 550 

million years or more. In order to reconstruct this story, we must look 

beyond the obvious cracks and fractures that control the form of the 

river gorge and study the more subtle features of the rocks. A brief 

search around the Great Falls area reveals exposures of five distinct 

kinds of bedrock: mica schist, metagraywacke, amphibolite, granite, 

and lamprophyre. The mica schist, metagraywacke, and amphibolite 

are rocks that have been deeply buried within the Earth's crust and 

transformed (metamorphosed) by heat and pressure to their present 

state. Prior to metamorphism the schists were mudstones or shales, the 

metagraywackes were beds of muddy sandstone, and the amphibolites 

were sheets of once-molten rock similar to basalt lava.

The shales and muddy sandstones were deposited on the bottom of an 

ancient ocean, probably in a deep trough near the coast of a rugged 

landmass. Farther east, near Chain Bridge, the metamorphosed shale 

and sandstone grade into metamorphic gneiss formed from thick 

deposits of muddy sandstone containing jumbled pebbles and boulders 

of quartz, amphibolite, granite, and other rocks. These materials were 

deposited by huge submarine landslides that cascaded down the steep 

eastern side of the trough. The muddy sandstones at Great Falls were 

probably formed from thick slurries of mud and sand that moved 

downslope from the main slides and came to rest in the bottom of the 

trough. The shales at Great Falls were originally muddy sediments that 

were carried seaward by ocean currents and settled to the bottom during the quiet intervals between the slides. Soon after deposition of these 

sediments, molten lava was injected in thick sheets parallel to the 

layers of sandstone and shale and cooled to form the layers of basalt 

that later became amphibolite. The high landmass from which these 

sediments were eroded has long been obliterated, but it probably lay 

only a few kilometers to the east. It was made up largely of granitelike 

rocks similar to those exposed today near Baltimore, which are about 

1,100 million years old.

The exact time when the sediments were deposited is uncertain because 

no fossils have been found in them, but dating of a rare radioactive 

mineral (zircon) indicated that the basalt layers were intruded about 

550 million years ago, and the sediments are probably about the same 

age.

13
Sediments are deposited on the sea floor 

and eventually become layers of mudstone 

and muddy sandstone. Sheets of basalt (not 

shown in diagram) are injected parallel to 

the sediment layers.

The layers of rock are folded and the 

mudstones recrystallize to form mica schists. 

The flakes of mica grow in parallel planes to 

form a new structure schistosity which 

cuts across the original layering. Basalt 

sheets are folded and metamorphosed to 

form amphibolite.

The growth of large crystals of kyanite and 

andalusite push the planes of schistosity 

aside.

The layers are folded again, and new planes 

of schistosity are formed at angles to the 

first.

Rocks are fractured and molten material 

wells up along the cracks to form granite 

intrusions. Kyanite and andalusite are altered to sillimanite. Still later lamprophyre 

dikes (not shown) are emplaced.

STRUCTURAL AND METAMORPHIC HISTORY 

OF THE ROCKS NEAR GREAT FALLS
Since they were first formed, these rocks have undergone a full and 

varied history. As younger sediments accumulated above them, they 

became accordingly more deeply buried. They were later compressed, 

folded, metamorphosed, and finally intruded by granite. The different 

stages in this complex sequence of events are outlined in the accompanying figure. The total thickness of sediment that accumulated can be 

inferred from laboratory experiments which show that some of the 

minerals that grew in the schists when the granites were intruded require temperatures of more than 650° C and pressures corresponding to 

depths of 20 to 30 kilometers. The exact age of this deformation and 

recrystallization is unknown, but dates on mica from granite near Great 

Falls indicate that the granite was emplaced about 470 million years 

ago, and most of the deformation must have been accomplished by that 

time.

Thus far in the history of these rocks, events were controlled by forces 

deep within the earth which led to the downbuckling of the sea floor, 

accumulation of sediments, and later compression and folding. After 

the emplacement of the granite, however, a fundamental change took 

place; the Earth's crust stopped moving down and slowly began to rise 

again, carrying the Great Falls rocks back toward the Earth's surface, 

where we find them today. As the crust rose, overlying rocks were eroded and the resulting debris was transported westward, where it accumulated as thick sedimentary deposits which now make up the main 

part of the Appalachian Mountains. Erosion was very rapid during the 

initial stages of this uplift, suggesting that the Piedmont area at that 

time was very mountainous, perhaps like the Rocky Mountains today.

As erosion followed uplift, temperatures in the rocks at Great Falls 

gradually declined, and the rocks became fractured as they cooled. 

Along some of the fractures, molten rock material of a different type 

rose and solidified to form the sheets or dikes of lamprophyre, another 

dark lavalike igneous rock. Dating of the minerals in the lamprophyre 

dikes shows that they were intruded about 360 million years ago. Still 

later, movement occurred along other fractures, shattering and 

crushing the rocks along them and displacing the rocks on either side. 

Zones along which such movement has taken place are called faults. 

The straight narrow section of the gorge below Rocky Islands is cut 

along one such fault. The river follows the fault zone because the 

broken rock along it is much more easily eroded than the unbroken 

rocks on either side.

Along some of the faults, hot solutions have risen and deposited thick 

veins of white quartz. Some of these veins carry small amounts of gold,

15
TIME IN

GEOLOGIC AGE MILLIONS OF PRINCIPAL EVENTS

YEARS

: QUATERNARY

£ -...^.TERTIARY ,,,

0

\Carving of the Potomac I 

Valley.

Deposition of sediments on 

f Atlantic Coastal Plain.

Deposition of sedimentary 

rocks in Leesburg Basin and 

Frederick Valley.

I Final uplift of Appalachian 

J Mountains.

PRECAMBRIAN

-11OC

^ Intrusion of lamprophyre 

dikes (365 million years)

Intrusion of granite 

470 million years)

Metamorphism and folding 

.'. of older sedimentary rocks 

and basalt.

Intrusion of basalt j|

Deposition of sedimentary 

rocks, chiefly shale and 

sandstone.

Formation of Baltimore 

Gneiss.

GEOLOGIC TIME CHART
The rocks of Great Falls

Name Description Color

Mica schist Metamorphic rock containing Silvery gray; 

abundant coarse flakes of mica weathers to 

oriented along parallel planes. reddish brown. 

Thin veinlets of quartz cutting 

the schist stand out as a delicate 

network of ribs on weathered 

outcrops.

Original rock

Shale or 

mudstone.

Metagraywacke Metamorphic rock composed of 

fine-grained quartz, feldspar, 

and some mica. Has a sugary 

texture on weathered outcrops; occurs interstratified 

with mica shist in layers a few 

centimeters to a meter in 

thickness.

Light bluish 

gray to buff.

Muddy 

sandstone.

Amphibolite Metamorphic rock composed of 

dark green to black crystals of 

hornblende in white to pink 

feldspar; occurs in layers half 

a meter to 35 meters in thickness.

Dark green 

to black.

Basalt.

Granite Intrusive igneous rock composed 

of quartz and feldspar with 

mica; occurs as dikes and irregular masses.

White to pink. Original rock 

essentially 

unchanged.

Lamprophyre I ntrusive igneous rock composed 

of chiefly mica and feldspar; 

occurs as crosscutting dikes.

Dark green. Original rock 

essentially 

unchanged.

17
River-scoured outcrop west of Black Pond showing folded beds of metagraywacke cut by 

dikes of light-colored granite(g). The conspicuous broad fold belongs to the youngest fold 

generation; earlier tight folds in the beds are labeled F1. Schistosity formed during the 

younger folding is labeled S; schistosity formed during the earlier folding is nearly parallel to 

the beds.

18
which was mined sporadically in the area from 1867 until as recently as 

1941. Many of these faults and quartz veins have the same orientation 

as faults of Triassic age (about 200 million years old) a few kilometers to 

the west, and the faults at Great Falls may be of the same age.

The Triassic faults appear to have developed in response to a final stage 

of uplift, a regional arching and stretching of the Piedmont rocks, which 

produced a series of fault-bounded basins by subsidence of keystoneshaped blocks. These basins were then filled with the red shales and 

sandstones which crop out a few kilometers west of Great Falls in the 

Leesburg Basin and Frederick Valley.

At this stage in the geologic story, we have come full circle; the record 

of the rocks merges with the origin of the river and the development of 

its valley. But the chronicle of the river and the rocks is told best not by 

words and sketches on a printed page, but^by the face of the land and 

by the rocks themselves. To read the story for yourself, you are invited 

to follow one of the trails described in the trail logs.

19
A Few Words About the Forests

The original forests of Great Falls Park have been almost completely 

cut over since 1790; only a few trees on the hill north of Difficult Run remain from presettlement times. After repeated cutting and clearing, 

fires, and high water, most of the forest lands now support only secondor third-growth trees.

The various types of plants growing in the area reflect differences in 

landscape features and the availability of water. Forty-three species of 

trees have been recognized; their distribution and the associated groupings of shrubs and wild flowers for the five major subdivisions of the 

forest landscape are summarized in the table.

The distributions of several tree and shrub species are puzzling. For example, red oak is common on the bedrock terrace but is not found on 

the islands and flood plains above the falls, whereas its closest relative, 

the Shumard oak, is found there. Post oak is common on the bedrock 

terrace and is rare on the upland. Black oak is common on the upland 

but absent over much of the bedrock terrace. Why do these (and others) 

grow only in certain places?

Even more puzzling is the distribution of the woody, yellow-flowered 

St. John's Wort, Hypericum densiflorum. It is common in open areas on 

the bedrock terrace and on rocky islands downstream to Chain Bridge. 

It does not grow elsewhere in the local area but is common in the Appalachian Mountains south to North Carolina. The moss, Crimmia 

laevigata (Stop 4, Virginia trail log; and Stop 2, Maryland trail log), grows 

only on the cliffs below the bedrock terrace here and in similar places 

downstream. It, too, is common in the mountains. Other such strange 

distributions could be mentioned. What factors control the distribution 

of these species?

The vegetation of the Great Falls area was different in the past. A partial history of the changes in the forest during the last 16,000 years has 

been pieced together by microscopic study of pollen grains preserved 

in local swamp deposits. The relative number of pollen grains of various 

kinds in the sample tell us much about the forest that produced the 

pollen. Core samples were taken from two places in the Great Falls 

area. One core, 1.2 meters long, was taken from the swamp west of 

Glade Hill; and the other, 3.7 meters long, was taken from a bog south 

of Anglers Inn. The organic material at the bottom of the Glade Hill

20
Landscape features and forest subdivisions

Landscape 

features

Flood 

plains

Bedrock 

terrace

Piedmont 

uplands

Swamps

Areas of 

past use

Prominent species

Sycamore 

Green ash

Post oak 

Red oak

White oak 

Black oak

Red maple 

Pin oak 

Willow oak

Boxelder 

Elm 

Black locust

Forest subdivisions

Other species

Elm 

Boxelder 

Silver maple 

River birch 

Cottonwood

Virginia pine 

Pignut hickory 

White oak 

Chestnut oak 

Redcedar

Red oak, Scarlet 

oak/Chestnut 

oak, Southern 

Red oak, 

Yellow poplar, 

Virginia pine, 

Shortleaf pine

Swamp white 

oak, Sourgum 

Yellow poplar

Yellow poplar 

Honey locust 

Black walnut

Small trees and shrubs

American hornbeam 

Spicebush 

Pawpaw 

Silky cornel

Blueberries, Dogwood 

American holly 

Fringetree, Blackhaw 

St. Johns's wart 

Pawpaw

Mountain-laurel 

Dogwood, Shadbush 

Mapleleaf viburnum

Spicebush 

Southern arrowroot 

Poison-sumac 

Alder

Spicebush 

Japanese honeysuckle 

"Escaped" cultivated 

shrubs

A

W

S

V

D

B
swamp is about 9,500 years old; that in the bog near Anglers Inn is about 

16,000 years old. Ages are determined by measuring the amount of 

radioactive carbon (C-14) that the organic matter contains.

The fossil pollen has been studied only for the samples from the deeper 

bog. It indicates that about 16,000 years ago, the hills surrounding Great 

Falls were covered with a spruce-jack pine woodland growing in a 

subarctic climate. Jack pine now grows only as far south as the northern 

tier of States. Spruce grows not only in the northern tier of States but 

also down the backbone of the Appalachian Mountains to Tennessee.

Sometime between 12,000 and 13,000 years ago a few scattered oaks, 

hemlocks, and alders began to grow. By 8,000 or 9,000 years ago the 

forests began to resemble those we see today. Spruce and jack pine 

were still present, but chestnut, hickory, elm, maple, and walnut were 

present along with increasing numbers of oak and Virginia pine. The 

oak-pine chestnut forests, as seen by the first settlers to this area, have 

been here only 5,000 years or less.

What caused these major changes? At least four times between 2 

million and 12,000 years ago, much of the Northern Hemisphere was 

covered with vast ice sheets. The last glacial advance reached its 

southern limit, from Long Island across Pennsylvania and central Ohio, 

about 18,000 years ago. Although the glaciers never reached as far 

south as the Potomac River, the climate was much colder than it is now; 

in fact, this vast amount of ice had a marked cooling influence on the 

climate that affected the vegetation at least as far south as Florida and 

Louisiana.

The catastrophic changes in the forests wrought by the early settlers are 

recorded by the pollen. As the region was settled, the area of forest land 

and the numbers of trees and shrubs declined drastically, while 

ragweed and grasses markedly increased. The settlers cut down and 

burned trees and plowed the land, thus encouraging the growth of 

grasses and weeds. Henry Lee, who was granted land now called Sully 

near Dulles Airport, wrote in 1732 that he was happy to see brush fires 

built by freeholders to remove the unwanted trees and shrubs. The 

Great Falls area was farmed extensively for about 120 years, but 

gradually, as cultivated fields were abandoned, forest plants again 

started to grow. At first pines were dominant but hardwoods also were 

present. Pines grow more rapidly and reach maturity sooner than hardwoods and produce more pollen. Thus, the fossil record shows a 

predominance of pine. However, the forests bordering the Potomac near 

Great Falls have tended to return gradually to their presettlement com22
position. Grasses and weeds, which grow in pastures, parkland, and 

backyards and along highway rights-of-way, are still more abundant 

than before settlement. The relative proportions of tree species in the 

present forests approach what they were in the presettlement forests. 

The exception, of course, is the American chestnut, which disappeared 

during the blight of the 1920's and 1930's and was replaced by oaks and 

hickories. The sketches of leaves shown in the trail logs have been added to assist in identifying the numerous species of trees growing in the 

park.

mockernut hickory Virginia pine

poison-ivy pitch pine

23
Trail log-Great Falls, 

Virginia

This walking tour is designed to acquaint you with the important 

geological, botanical, and ecological features that can be seen from the 

park trails. The entire tour is about 7 kilometers long and takes about 6 

hours. It includes two steep climbs. The tour can be shortened to 3.7 

kilometers, however, by a shortcut that avoids the two steepest climbs. 

Good hiking boots and rough clothing are recommended.

Kilometers

0.00

0.12

FLOOD-PLAIN 

TREES

box elder

From south entrance of Visitor Center, walk about 40 

meters south to first footbridge across old Patowmack 

Canal. Cross bridge and follow trail toward river to 

overlook at brink of Great Falls.

Stop 1. A CLOSEUP OF GREAT FALLS. The total drop 

of the river from the uppermost rapids to the channel 

below this overlook is about 12 meters; the cascade in 

front of you is 5 meters high. The narrowest part of the 

channel below the falls here is 23 meters wide at normal stages of the river. The foam commonly seen 

below the falls is detergent suds evidence of water 

pollution from towns and cities upstream.

The rock here is metagraywacke, a type of muddy 

sandstone that has been altered (metamorphosed) by 

heat and pressure into hard metamorphic rock. The 

conspicuous layers in this outcrop are the original 

layers in which the mud and sand were deposited on 

the sea bottom but which were turned on edge as the 

rocks were folded. The sand grains in some of the 

layers are as large as BB's, but most of the sand is 

much finer.

The same layers are plainly visible in the rocky islands 

in the middle of the falls. Note how the river channels 

follow the rock layers.

Notice that in many of the metagraywacke layers the 

largest sand grains are concentrated at one side of the 

layer and the grains gradually become finer toward

24
the other side of the layer. This is called graded bedding. Its origin and how it allows us to distinguish tops 

and bottoms of metagraywacke beds are explained in 

the discussion of Stop 1, in the trail log on the 

Maryland side of the river. Notice that most layers 

here have tops to the west, but there are a few with 

tops to the east. This suggests that the layers have 

locally been folded completely back on themselves. 

Can you find any other evidence of tight folds in the 

metagraywacke layers in this vicinity?

The straight fractures that cut across the rock layers 

are joints. They formed as the rocks were uplifted and 

cooled. The most conspicuous joints are nearly vertical and perpendicular to the folded metagraywacke 

layers. These joints break the rock into blocks, making 

it easier for the river to quarry away the resistant 

layers.

silver maple

Return to path along old Patowmack Canal, turn left 

and walk south (downstream).

0.35 Marker here shows relative levels of 1936,1942,1972, 

and 1937 floods. At normal stages the water in the 

channel below the falls is about 10 meters deep. In 

1936 the water in the middle of the river was more 

than 30 meters deep. Turn left here and walk about 50 

meters to overlook.

0.40 Stop 2. VISTA OF THE FALLS AND VIEW OF THE OLD 

RIVER BED. The level surface on which you stand was 

part of the bed of the river before the gorge was cut 

and the falls were formed. From here you can trace 

the surface upstream and see that it becomes the present riverbed a few hundred meters above the falls.

green ash

Notice the nearly right-angle turn in the river at this 

point. Above this point the river is following the 

metagraywacke layers; here it turns to follow a zone 

of weakness caused by closely spaced joints like 

those visible at Stop 1. This zone is J-J' in the block 

diagram.

25
Return to path along Patowmack Canal and continue 

south (downstream).

persimmon

0.62 Pass into woods.

0-70 Large outcrop of rock in trail. This rock is mica schist 

containing lenses and pods of white quartz. Notice 

how the layers are crumpled and folded. This occurred 

during metamorphism that converted the rock from 

shale to schist.

0.77 Trail fork. Follow left fork. Trail follows top of an old 

stone wall. This wall held a large pond that served as a 

collecting basin for water needed to operate the 

Patowmack Canal. The pond also provided room for 

mooring barges waiting to be locked through. This 

area held water until 1830, less than 150 years ago. 

The basin is now filled with mature trees, indicating 

that when man's activities cease, the forests return. As 

part of the restoration of the Canal, these trees will be 

removed and the basin refilled with water.

American hornbeam

0.91 Trail junction. Continue about 10 meters straight 

ahead to break in wall below which is a small stream 

channel. This was the waste weir that drained excess 

water from the collecting basin. A saw mill driven by 

an undershot water wheel once straddled the weir. 

Retrace your steps about 10 meters and turn right on 

trail.

0.97 Trail fork. Follow blue-blazed trail to right and down a 

series of steps into an 8-meter-deep stream valley.

1-00 Cross creek. The valley on your right as you face 

upstream has been cut by the stream along a series of 

thin lamprophyre dikes, several of which are exposed 

in the small waterfall about 15m upstream from the 

trail crossing. This stream drains the waste weir you 

just saw; the location of the stream was controlled by 

the presence of the dikes and the waste weir was 

placed to take advantage of the stream.

26
Lamprophyre dikes (indicated by white dotted lines) on the Maryland side of the river as 

seen from Stop 3 on the Virginia side. Location of Stop 4 on Maryland side is shown.

Continue on blue-blazed trail up steep slope on south 

side of valley. About 15 meters after trail emerges 

from valley, turn right and follow blue-blazed trail 

south (downstream).

In 30 meters, trail reaches top of rock rib. After crossing rib, walk 3-4 meters downstream to a faint trail, 

turn left and scramble out to overlook above river.

1.06 Stop 3. THE MATHER GORGE. The cliff below is 

about 20 meters high. The river here is about 65 

meters wide and is 8 meters deep at normal stages.

river birch

27
sycamore

1.27

Almost directly across the river are three lamprophyre 

dikes, 20 to 40 centimeters thick, filling some of the 

conspicuous joints that dip steeply upstream. These 

are the same dikes as those exposed in the small 

stream valley you just crossed. The dikes look as 

though they should project across the river to about 

this point, but the extension of them on this side of the 

river is about 25 meters upstream. This offset is caused 

by a fault that lies beneath the river. The straight 

steep-sided gorge here has been cut by the river in the 

crushed and broken rock along the fault, which is 

more easily eroded than the solid rocks on either side.

The rock here is mica schist. About 8 meters to the 

north (upstream) is a vein of quartz more than 30 centimeters thick. Gold was mined from similar veins at 

several nearby places on the Maryland side of the 

river.

The trees here are eastern red cedar, Virginia pine, red 

oak, pignut hickory, fringe tree, ash, and blackhaw.

Return to blue-blazed trail and turn left (downstream).

Follow blue-blazed trail downstream along brink of 

gorge past river-smoothed outcrops of mica schist.

Trail junction. Blue-blazed trail turns right. Continue 

ahead 40 meters gently downhill to twin-trunked ash 

tree on left side of trail. This tree is living testimony of 

the flood of 1889 (the year of the first Johnstown 

flood). The original trunk grew vertically until it was 

tipped over by floodwaters. The two vertical trunks 

started to grow at that time. Their age, determined by 

counting rings, tells us the year the tree was tipped. 

Records show that the 1889 flood was as large as that 

of 1936 and easily would have reached this level. 

Other trees in this vicinity are ash, chestnut, red oaks, 

and bitternut hickory (the large tree with light-gray 

ridged bark).

Flood felling of trees continues. In the small depression that the trail follows for the next 50 meters are 

many inclined trees with vertical sprouts. These were

28
felled by the flood following hurricane Agnes in June 

1972, when water was at least 3 meters deep in the little valley. Most of these trees are eastern hophornbeam.

Retrace steps to trail junction; turn left on blueblazed trail. For about 40 meters the trail crosses the 

lower collecting basin for the old Patowmack Canal, 

once held by still-apparent retaining walls.

Ash tree showing damage from 1889 flood. Man is drilling to take core used 

to determine year in which the vertical sprout began to grow.

III

BEDROCK-TERRACE 

TREES

red oak

AM

«^v- '.**&** 
1.34 Trail junction. Follow blue-blazed trail to left.

post oak

pignut hickory

1.38 Ruins of lift lock on Patowmack Canal. The lock is 

built chiefly of reddish-brown sandstone that was 

quarried near Seneca, 13 kilometers upstream, and 

carried down river by boat. The same "Seneca Stone" 

was used in many of the older buildings in downtown 

Washington, including the Smithsonian Castle on the 

Mall.

At trail fork at lower end of lock, follow right (upper) 

branch of trail. Blue-blazed trail goes left.

1.48 Junction with blue-blazed trail. To your left about 10 

meters down the blue-blazed trail is a twin-trunked 

white oak tree. The trunks, each almost 70 centimeters in diameter, sprouted from a stump. They 

began to grow before 1818. The original main trunk 

may have beeqxrut for timber for the lock gates on the 

Patowmack Canal shortly before that time.

Continue straight ahead.

Between this trail junction and the next stop, you pass 

through a red oak and post oak forest, typical of the 

bedrock terrace. Also growing here are Virginia pine, 

cedar, pignut hickory, white oak, and chestnut oak. 

This type of forest is also present on the flat surface 

above the cliffs across the river.

About 140 meters from the junction is a shallow pond 

west of the trail. The pond occupies a depression that 

was once a side channel of the river.

In early spring a metallic, discordant chorus of wood 

frogs can be heard coming from the pond. The fern 

that blankets some of the nearby rocks, especially on 

the far side of the pond, is the common polypody. It 

grows only on the rocks along the river from Turkey 

Run to Blockhouse Point.

The next stop is along the trail opposite the south 

(downstream) end of the pond.

30
'"3"H.

Pothole in schist at Stop 4. This hole, about 30 centimeters in diameter and 65 centimeters 

deep, was ground by pebbles and cobbles churned by swirling currents when the river 

flowed at this level.

1.64 Stop 4. POTHOLES. Here are several unusually good 

examples of potholes circular holes ground in the 

rock by pebbles and boulders churned by the river 

when it was at this level.

All of the rocks in this area were smoothed and polished by the river at that time. Although an occasional 

extreme flood still reaches this level, the potholes and 

waterworn rock surfaces date from a much earlier 

time, prior to the cutting of the gorge and formation

31
SWAMP TREES

sourgum

swamp white oak

of the falls. Notice that, although the general shapes 

of the rock outcrops are rounded, all of the rock surfaces have been roughened by weathering since they 

were polished by the river. Some of the pebbles left by 

the river can still be seen in the potholes and wedged 

in crannies in the rock outcrops.

Leave trail and walk carefully over to edge of gorge. 

Large potholes can be seen at the present river level 

and many smaller ones at the top of the cliffs on the 

Maryland side.

Looking upstream you can see evidence of the effect 

of flooding on plant life. Near the tops of the cliffs the 

rocks are rough and sharp and are covered by several 

species of gray lichens. Some are leaflike; others are 

tight crusts cemented to the rock. The large ones that 

look like warty, gray potato chips are Umbilicaria. The 

light-gray, spreading ones are Parmelia conspersa. The 

gray crust that requires close examination to see is 

Lecanora cinera. The lichens are alive. If watered they 

turn soft and green in 5 to 10 minutes. Floods reach 

these levels only rarely, perhaps once every 10 to 20 

years.

About 6 to 9 meters below the cliff edge is an irregular 

band marked by dark-olive-green patches of moss 

(Grimmia laevigata). This species is common on many 

kinds of rocks in the Great Smoky Mountains and 

southern Blue Ridge; here it is growing at its northernmost limit. This zone is flooded about once every 2 

years. Below the moss and extending down to 3 to 6 

meters above normal water level, the rocks have a 

yellow cast which is due to lichens (Candelariella 

vitellina). This zone is flooded several time each year.

Near the base of the cliffs, just above normal water 

level, the rocks are covered with a metallic purplishblack coating that consists of iron and manganese oxides. This stain is the result of acid mine drainage from 

coal mines in the upper Potomac River basin.

Return to trail and continue south (downstream). Trail 

follows narrow trough between rock rib at edge of

32
river gorge and steep hillside on right. About 20 

; meters before the trail reaches the south end of the 

' rock rib is a large rounded boulder on the left. This is

Stop 5.

1-74 Stop 5. A GEOLOGICAL PUZZLE. Along the trail are

: many rounded pebbles, cobbles, and boulders left

behind by the river when it flowed at this level. This

boulder, 1.5 meters in diameter and estimated to

weigh over 4 metric tons, is the grandaddy of them all! willow oak

Diabase boulder at Stop 5.
ROCKVILIE 11.4 KM.\. POTOMAC 3.1 KM. CAPITAL BEITWAY INTERCHANGE 15 3.9 KM../

............................................mmmm......

%PROf\\.t OF THE POTOMAC RIVER SHOWING RELATION OF OLD FLOOD PLAIN ABOVE 

£ GREAT FALLS TO BEDROCK TERRACE AND YOUNG FLOOD PLAIN BELOW GREAT FALLS 

Profjl^ofyiver bottom largely diagrammatic, but based in part on soundings and obsery; vations at extreme low water. Vertical scale greatly exaggerated SEA 

LEVEL

MAP EXPLANATION

1 KILOMETERS ABOVE 

ANGLERS INN

Old flood plains

Flood plains of fhe river and ifs 

tributaries above Great Falls 

and of fhe Difficult Run above 

ifs falls. Th.jse flood plains 

occupy valley boftoms fhaf 

were not deepened during fhe 

cuffing of fhe gorge. They 

ore underlain by silt, sand, 

and gravel deposited by floods 

fhaf sfifl cover these areas 

every few years'

Young flood plains

Flood plains of the river below 

Great Falls. These flood 

plains have formed since fhe 

cutting of fhe gorge and the 

development of Greaf Falls. 

They are underlain by materials 

similar to those of the old 

flood plains and are also 

covered by floods every few 

years

Bedrock terrace

Areas of bare rock and rock 

thinly veneered with sand and' 

gravel. These areas are remlf 

nanfs of the old valley floor 

scoured bore during cutting 

of fhe gorge. The upper 

surface corresponds in altitude 

and age to the valley floor 

occupied by the old flood 

plain above Greaf Falls

Piedmont uplands 

Well-drained uplands underlain 

by thick clayey soils and deeply 

decayed crystalline rocks similar to the fresh rocks exposed 

along the river and on the 

bedrock terrace. Exposures 

of fresh rock are very rare, 

except on steep hillsides at the 

edge of the river valley

Swamps

In fhe uplands swamps generally 

occur at the edge of flats 

bordering small 'streams; on 

the bedrock terrace they occupy 

depressions between bedrock 

 ribs and knobs. The large 

swamp west of Glade Hill 

occupies an old channel of the 

river that was abandoned as 

the gorge was cut

Self-guiding trail showing 

numbered stops

Other trails

TRAIL MAP OF THE GREAT FALLS AREA SHOWING LANDSCAPE SUBDIVISIONS
red maple

It is composed of a dark, heavy igneous rock called 

diabase. The nearest outcrop of such rock is near 

Blockhouse Point, more than 11 kilometers upstream. 

This boulder is far too large to have been moved by 

even the largest modern flood. Could it have floated 

in an ice jam or in a tangle of logs? Or does it indicate 

even larger and more violent floods, perhaps during 

the Ice Ages when the cutting of the gorge took place?

Continue downstream. After about 20 meters the trail 

skirts a deep cleft down which you can see directly to 

the river, 15 meters below. The trail continues along a 

narrow bench that is a continuation of the bedrock 

terrace. The bench here is slightly below the terrace 

on the opposite side of the river and probably marks 

the bottom of a channel cut in the terrace.

1.79 Most of the rocks are mica schist, but here is a dike of 

pegmatite, a coarse-grained rock with the same 

mineral composition as granite. It is composed of 

glassy gray quartz, pink to white feldspar, and conspicuous plates of silvery mica (muscovite). The black 

prismatic mineral is tourmaline. A sample of mica 

from this dike has been dated as 470 million years old 

by the potassium-argon method of radiometric dating.

1.90 Trail passes behind (west of) rock rib, no doubt once a 

narrow rocky island. The sand in the bottom of the 

depression between the rock rib and the hillside to the 

west was deposited by modern floods. It is littered 

with small shells identical to those of snails that live 

on the aquatic plants in the sluggish parts of the river 

above Great Falls. These shells will probably not survive more than a few decades of weathering. A few 

 meters to the north, however, you passed large 

boulders that were probably carried down when the 

main river flowed at this level. Among them is a 80- by 

40-centimeter boulder of Catoctin Greenstone, an ancient volcanic rock from the core of the Blue Ridge.

1.93 Trail skirts left of small pond. Note quartz veins and 

intricately folded layers in schist on south side of outcrop on left. These folds were formed during the main 

period of folding of the schist and metagraywacke.

36
2.00 Trail emerges onto an improved gravel road used by 

rescue vehicles. The beach 50 meters to the left at 

river's edge is known as Sandy Landing. It was once 

the site of a ferry to the low area across the river. During the early operation of the Patowmack Canal, when 

all improvements had been made except for the completion of the locks at Great Falls, goods from upriver 

were portaged around the falls to this beach and loaded into barges to continue the trip downriver.

Turn right, away from river, on gravel road for about 

10 meters.

2.01 Junction. Blue-blazed trail turns left. If you wish to 

return to Visitor's Center by a shorter route, continue 

straight ahead on road for about 260 meters, passing 

through an old stone quarry, now largely overgrown. 

Just beyond point where road emerges from quarry 

and 40 meters before it joins main graded road, turn 

right on well-beaten horse trail. This junction is at 5.56 

kilometers of the trail log. Follow remainder of log 

from that point to Visitors Center.

If you wish to continue around complete loop, follow 

blue-blazed trail to left (down river).

2.06 Cross small stream. Follow trail along sandy bench 

with scattered outcrops of mica schist.

Note the abundant flood debris piled on the

downstream sides of trees and other obstructions and

that many small trees are leaning in an upstream

direction. Trees were felled and the debris was

deposited by the flood following hurricane Agnes in

June 1972. Water at the crest of the flood was at least pin oak

15 meters above normal in the channel. Huge volumes

of water roared through the gorge and struck the

hillside ahead of you, where part of it was diverted to

the right and swirled back upstream creating a huge

eddy.

2.24 Pitch pine 45 centimeters in diameter and 20 meters 

tall. The tree started to grow before 1817 and has survived the floods of 1889, 1936, 1937, 1942, and 1972. 

Note the nearby trees leaning upstream.

37
PIEDMONT UPLAND 

TREES

yellow poplar

2.27 To the left of the main trail are the exotic Forsythia 

shrubs, day lilies, and Wisteria vines. These nearly 

hide the remains of an old chimney. This probably 

marks the site of someone's vacation fish camp. It was 

destroyed either by the 1936 flood or possibly by an 

earlier one. Whoever built it thought that the large 

trees belied the hazard from floods. Who would have 

expected the normally placid river to reach such 

heights?

2.28 Trail junction. Well-beaten but unmarked trail leads 

straight ahead up hill. Follow blue-blazed trail to left. 

Five meters beyond the junction, the trail crosses a 

small stream issuing from an old cement spring box, 

which, no doubt, was the water supply for the cabin 

marked by the ruined chimney.

2.35 Trail passes behind (west of) prominent rock knob that 

affords good view up Mather Gorge. Rock is mica 

schist with several dikes of granite. Faint foot trail 

leads right (uphill). Follow blue-blazed trail straight 

ahead.

2.41 Trail begins steep climb away from river. At this level 

(9 meters above the river) the rock outcrops still retain 

their rounded shapes, although their surfaces are 

somewhat roughened by weathering since the river 

polished them. As you climb, notice that the rocks 

become rougher and more angular. Notice also that 

the soil along the trail is no longer river-deposited 

sand, but is reddish-brown clay produced by deep 

chemical weathering of the schist bedrock. This is the 

typical soil of the Piedmont upland.

2.44 Stop 6. COWHOOf ROCK. This bold outcrop 34 

meters above the river affords an unexcelled view of 

the Mather Gorge. The straight section above the 

right-angle bend on your left is cut along the fault 

zone, which must project into the ridge behind you. 

Below the bend, the river follows prominent joints 

that cut nearly at right angles across the rock layers. 

Notice the potholes and river sculpture on the rocks 

across the river. At normal stages the river here is 

about 18 meters deep, whereas, elsewhere in the

38
,. ,-.,.;,

The Mather Gorge from Cowhoof Rock: A (A) At normal water level.

f (B) During flood of June 1972.
chestnut oak

gorge, it is 7 to 8 meters deep. This is one of the 

deepest points in the river above tidewater and may 

mark the position of temporary falls or rapids during 

cutting of the gorge.

The June 1972 flood completely covered the cliff opposite you and extended into the trees on the top of 

Bear Island. At the crest, the water was 30 meters 

deep.

The evergreen shrubs are mountain laurel, which 

display beautiful pinkish-white flowers in May. The 

evergreen tree is eastern hemlock, one of the few 

specimens in the Park. The gnarled, twisted tree to 

your left is a chestnut oak. Continue up hill on narrow 

trail.

2.46 Thicket of small trees. A summer cabin was located 

here until the early 1950's. The small trees have grown 

since the cabin was removed. The peculiar soil with 

peanut-sized lumps is derived from weathering of amphibolite. A small block of unweathered amphibolite 

lies in the middle of the trail.

2.50 Top of hill. The ground is littered with angular pieces 

of white quartz derived from a large quartz vein, probably related to the gold-bearing veins to the north on 

the Maryland side of the river. Cavities in the quartz 

are due to weathering of pyrite and other sulfide 

minerals.

2.58 Trail junction. Turn left and follow graveled road. The 

forest that you are walking through is typical of the 

Piedmont upland. It has been cut over, but a few trees 

are old. The predominant trees are white, black, 

southern red, chestnut, and scarlet oaks. Also present 

are tuliptree, mockernut hickory, and Virgina and 

pitch pines. Dogwood and American holly are the 

small trees. The evergreen shrub is mountain laurel, 

and the small one with maplelike leaves is mapleleaved viburnum.

American chestnut was a common tree here and elsewhere in the Piedmont uplands before it was killed 

by blight in the 1920's and early 1930's. Chestnut

40
did not grow in swamps, on the bedrock terrace, on 

the flood plain, or on the rocks of the Leesburg Basin. 

Many of the stumps along the'trail in this area are 

chestnut.

Chemical decay of rocks underlying this ridge and 

others similar to it has produced a material called 

saprolite to depths of 30 meters or more. Saprolite is a 

relatively soft soil-like material that can be dug with a 

pick and shovel but still retains the structure and texture of the parent bedrock. Rocks this far above the 

river have been weathering for 1.5 to 2 million years. 

Saprolite is not present on the bedrock terrace 

because it was scoured away when the river flowed at 

that level, and new saprolite has not had time to form 

since the river has cut below the level of the terrace.

2.75 Trail junction at saddle in ridge. Well-beaten but unmarked trail to right. Follow road straight ahead up

2.88 Top of hill. Gravel was hauled in to surface the road 

and confound geologists it is not a natural deposit.

3.26 Outcrops of granite along road.

3.37 Trail junction 15 meters before end of road. Follow 

blue-blazed trail right (south). Trail descends steeply 

for about 20 meters.

3.42 Trail junction. Turn right on well-beaten trail.

3.53 Trail junction. Well-beaten and deeply gullied side 

trail leads left down to Difficult Run. Gulley at trail 

junction exposes saprolite formed by deep weathering 

of schist. Follow trail straight ahead.

3.58 Trail junction. Follow narrow but distinct trail 30 

meters left (downhill) to brink of cliff, crossing beaten 

trail near top of cliff.

southern red oak

41
3.61 Stop 7. DIFFICULT RUN GORGE. This picturesque 

gorge with its cascades and falls was cut by Difficult 

Run in its efforts to keep pace with the Potomac as 

the river cut its gorge.

The rocks here are chiefly mica schist with a few beds 

of metagraywacke and numerous small dikes of 

granite. Notice the potholes in the bottom of the 

gorge some are as large as those along the river.

Difficult Run Gorge from Stop 7.
Looking downstream you can see uprooted trees and 

deeply eroded bank caused by the 1972 flood.

Return to main trail and turn left (upstream).

Trail follows bed of old wagon road along valley side. 

This is part of Towlston Road, built by Bryan Fairfax in 

the late 18th century, leading from a water-driven saw 

mill on Difficult Run to the river. The mill and the 

road were abandoned by 1815.

3.75 Trail emerges on flat rock about 3 meters above 

stream level. Most of the flat rock face is granite, but 

several angular pieces of schist that spalled off the 

walls and were incorporated in the still-liquid granite 

are visible.

Continue upstream on rough rocky trail, no longer on 

old road bed. The rock exposed along the trail is 

predominantly coarse-grained amphibolite, composed 

of dark-green hornblende and cream-colored feldspar. 

The hornblende crystals are typically 0.5 to 1 centimeters in diameter and give the rock a characteristic 

mottled appearance.

3.86 Trail comes to bank of Difficult Run where prominent 

rib of rock extends almost across stream. At low water 

you can walk directly along the edge of the stream to 

the rock, but at higher water you must scramble over 

one of the steep narrow trails leading across or over 

the steep bank that projects into the stream.

3.87 Stop 8. A VARIETY OF ROCKS. This outcrop, scoured 

clean by the Agnes flood of 1972, contains all the major rock types of the Park except metagraywacke.

The coarsely mottled, light-gray rock is mica schist. 

Some of the fresh flat faces display conspicuous darkgreen spots 1 to 2 centimeters in diameter that may be 

altered relicts of metamorphic minerals, possibly 

garnet or staurolite.

The darker greenish-gray, fine-grained rock that forms 

a 2-meter-thick band in the center of the outcrop is

white oak

43
..-ft.

S&SaJ

S*2

Altered relicts of garnet or staurolite in schist at Stop 8. Red oak leaf about 15 centimeters 

long gives scale.

amphibolite. It is not nearly as coarse grained as that 

exposed along the trail below, either because the 

original basalt from which it was derived crystallized 

too fast for large crystals to form, or because the 

crystals were crushed and broken during metamorphism.

Both the amphibolite and the schist are cut by irregular, rudely tabular bodies of streaky light-gray 

granite. The streaks, which are due to concentrations 

of black mica, probably are the result of differential 

flow of the pasty, crystal-rich granite liquid as it forced 

its way into joints and fractures in the enclosing 

rocks. The fact that both the contacts of the granite 

dikes and the flow structure within them cut across 

the foliation in both the schist and amphibolite shows

44
that the granite was emplaced after the enclosing 

rocks were folded and metamorphosed.

Near the top of the outcrop, the largest granite dike is 

cut by an irregular body of pegmatite 5 to 15 centimeters thick. The pegmatite is composed of bluishgray glassy quartz, salmon-pink feldspar, and silverywhite mica in crystals several centimeters in diameter. 

The pegmatite probably crystallized from water- and 

vapor-rich fluids that migrated into a crack or irregular opening in the rocks when the granite had 

almost solidified. Radiometric dating of mica from 

this pegmatite indicates that it is about 470 million 

years old. This is probably the age of emplacement of 

the granite.

Granite dike at Stop 8. Note streaks in granite parallel to the contact with the enclosing 

schist and how the contact cuts across foliation in the schist. Post oak leaf about 12 centimeters long gives scale.

scarlet oak
4.04

Continue upstream along trail following road. Notice 

thick saprolite exposed in face of old quarry on right.

Trail junction 10 meters beyond where small stream 

crosses road. Turn right on well-beaten but unmarked 

trail that leads west up a tributary valley.

4.27 Road junction. Turn left on gravel road.

4.44 Trail junction. Blue-blazed trail turns right to 

Cowhoof Rock. Continue straight ahead on road.

4.47 Trail descends gently into saddle in ridge. Close examination of the woods in this area reveals small bits 

of charcoal, charred tree bases, and logs. Many of the 

small trees and shrubs appear dead. The fire, however, 

killed only the tops and regrowth appeared as sprouts. 

This fire occurred during the summer in the 

mid-1960's. Farther along on the slope on the left is a 

common, evergreen ground cover, tree clubmoss 

(Lycopodium obscurum).

black oak

4,65 Many fragments of white quartz derived from another 

vein like the one above Cowhoof Rock.

4.97 Junction with old carriage road. This road was a main 

access to Great Falls around the turn of the century. It 

was abandoned in 1935 when the electric trolley line 

was removed and Old Dominion Drive was opened to 

auto traffic. The road is cut 3 to 4 meters deep into 

schist saprolite. Turn right on road. On the slopes 

beside the road are light-gray, smooth-barked 

American beech and abundant mountain laurel.

5.46 Road crosses small stream that drains a swamp. This 

swamp, once a channel of the river, contains peat 

deposits at least 9,500 years old. The peat rests on 

rounded pebbles and boulders, deposited by the river, 

and the peat must have accumulated after the river 

abandoned this channel. The surface shortly before 

9,500 years ago probably looked much like the rocky 

surface along the river at Chain Bridge, which is swept 

by floodwaters several times a year.

46
Vegetation in the swamp is markedly different from 

any other vegetation in the Park. Trees are red maple, 

black tupelo, and pin and willow oaks. Shrubs are 

sweet pepper-bush; swamp azalea with its white, 

sticky flowers; poison sumac; common elder; alder; 

and the ubiquitous greenbrier. Skunk cabbage is common along with lizard's tail. Skunk cabbage is the first 

plant to bloom in the spring. Flowers appear as early 

as January or February. The large broad green leaves 

persist into early summer. When broken, they yield a 

strong skunk odor.

5.53 Main road bends left; side road branches right. Follow 

side road 30 meters.

5.56 Cross gravel road leading up out of old quarry and 

take well-beaten horse trail straight ahead. Ten meters 

up horse trail is juction with unmarked but wellbeaten foot trail. Continue straight ahead (up hill) on 

horse trail. You are climbing Glade Hill, which was an 

island when the river flowed through the 9,500-yearold channel now occupied by the swamp.

The forest on Glade Hill is typical of the uplands. 

White, black, scarlet, and southern red oaks grow with 

mockernut hickory, tuliptree, and Virginia pine. Scattered shortleaf pines also grow in the upland woods.

Notice the deep gullies that run parallel to the trail. 

The trail here follows part of the old road to 

Matildaville. Like many heavily traveled dirt roads in 

the Piedmont, this one was subject to severe erosion. 

Water running down ruts left by wagon wheels eroded 

gullies in the deep clay soil. When the ruts became 

too deep, it was easier to move the road to one side 

than to repair the gullies. In some places this happened 

several times, leaving a series of parallel gullies.

5.69 Stop 9. AN ANCIENT GRAVEL DEPOSIT. The ground 

here is littered with gravel and large rounded 

boulders, part of a river deposit even more ancient 

than those along the trail on the bedrock terrace 

below. This deposit lies almost 40 meters above the 

present level of the river and must have been

SHRUBS AND SMALL 

TREES

pawpaw

47
spice bush

deposited during a very early stage of the cutting of 

the valley, perhaps 2 to 3 million years ago.

Follow trail north (upstream).

5.75 Trail junction. Well-beaten side trail to right leads 

steeply downhill about 100 meters to join blue-blazed 

trail. Continue straight ahead.

Deep gullies crossing trail were probably caused by 

drainage from old field, now completely overgrown, 

which once occupied top of Glade Hill to left. This is 

well shown on the terrain model on display at the 

Visitor's Center Museum.

5.85 The area of this field is marked by a wire fence 

embedded in tree trunks and by the presence of 

Virginia pine. The pines are dying and falling, thus permitting the hardwood trees to grow more rapidly. In 

and along the gullies, yellow poplar, a valuable 

timber tree, is predominant. This species is common in 

places where soil was severely eroded in the past.

6.21 Trail junction. Turn right about 20 meters. Chimney 

and ruins of buildings at Matildaville. The town, 

perhaps one of the first subdivisions in the area, was 

established in 1790 and leased to "Light Horse" Harry 

Lee in 1793. It included a sawmill, forge, gristmill, 

store, and storage buildings and provided lodging and 

services for the canal traffic. The forest from near the 

base of the hill to the Patowmack Canal is typical of 

areas that had heavy land use. Black locust with short 

stout spines, elm, and boxelder are common. The tree 

bristling with many-branched thorns is honeylocust, 

found only here. Many cultivated plants such as daffodils, day lilies, privet, and others indicate human activity. At Matildaville, ruins follow right fork of trail 

about 15 meters.

6.27 Junction with trail along old Patowmack Canal. Turn 

left.

6.36 Trail junction. Turn right across footbridge, then left.

6.49 Trail junction. Continue straight ahead.

6.96 Visitors Center.

48
Trail log-Great Falls, 

Maryland

This walking tour is designed to acquaint you with the important 

geological, botanical, and ecological features that can be seen from the 

Park trails. Part of the route is over a rough trail and requires some 

scrambling over rock ledges, so wear suitable clothes and sturdy hiking 

shoes. Time'required for the full trip is 3 to 4 hours, total distance 5.6 

kilometers, but the trip can be cut short at several points. The first three 

stops are near the C and O Canal towpath and can be visited in about an 

hour with an easy round-trip walk of about 1.8 kilometers.

Kilometers

0.00 Leave Great Falls Tavern Visitor Center; cross C and O 

Canal. The low dam across the river diverts about 

9,500 liters of water per second into an underground 

aqueduct which empties into Dalecarlia Reservoir 14 

kilometers away for treatment and distribution to the 

District of Columbia and nearby suburbs. Turn left 

(downstream) on towpath.

The towpath here is about 4 meters higher than the 

water level above the dam. The elevation of the 

towpath is about 52 meters above sea level. On your 

right is a narrow channel of the Potomac River that 

separates the Maryland shore from Olmsted Island. 

The main part of Great Falls is out of view about 0.7 

kilometers to the west.

To the right (west) of the towpath and down to water's 

edge are many small trees that have been overturned 

by floods. Notice how the vertical stems have 

sprouted from the prostrate trunks. Sprouting occurred 

during the first growing season after the flood. To 

determine the year of the floods, we can core a sprout 

at its base and count the rings. Most of these trees 

were damaged during the 1936 flood; the smaller 

ones, in 1972. These sprouts look like branches but are 

different. Branches originate from buds on year-old 

twigs, so a branch is always one year younger than the 

larger branch from which it grows. Sprouts grow from 

dormant buds under the bark of trunks, so the sprout 

can be many years younger than the trunk.

SHRUBS AND 

SMALL TREES

maple-leaf 

viburnum

49
southern arrowroot

Four hundred meters from the Tavern, turn right on 

short trail leading a few meters west towards remains 

of old trail bridge.

0.40 Stop 1. METAGRAYWACKE. In this area are good outcrops of metagraywacke, the resistant rock that has 

impeded the downcutting of the river and caused 

Great Falls. The material that formed this rock was 

originally a mixture of mud and sand deposited on the 

sea floor. It has subsequently been altered (metamorphosed) by heat and pressure to produce the tough 

metamorphic rock you see here. The conspicuous 

layers 10 centimeters to 1 meter thick were originally 

nearly horizontal beds but are now tilted to nearly 

vertical and trend approximately north-south. In 

several places especially on the south (downstream) 

side of the outcrop, beds in the metagraywacke 

display conspicuous folds. On the north (upstream) 

side of the large outcrop that is circled by the trail, 

these layers are especially well displayed. In some 

places, original sand grains 2 to 4 millimeters in 

diameter are visible in the metagraywacke. Notice 

that in each layer the grains become progressively 

finer to the east. There is then a sharp discontinuity 

and another zone of coarse sand grains at the base of 

the next layer.

The gradation from coarse to fine sand in each individual layer is called graded bedding. It is 

characteristic of sedimentary deposits laid down by 

underwater "avalanches" of sand and mud. As the 

avalanche comes to rest, the largest sand grains stop 

first and the finer sand, silt, and mud gradually settle 

out of the water in order of decreasing size. If the 

layers are steeply tilted by later deformation, as in 

this outcrop, the graded bedding makes it possible to 

determine which are the tops and which are the bottoms of the beds. In this outcrop, the tops of the beds 

are toward the east, and the older beds are, therefore, 

on the west. The many knots and thin veins of white 

quartz represent material that was dissolved by hot 

solutions moving through the rock during metamorphism and deposited in open cracks or local areas of 

low pressure. Notice how the quartz stands out from

50
Metagraywacke outcrop at Stop 1 showing graded beds. Tops of the becl^ are to the east 

(left).

51
0.70

American holly 

(evergreen)

the surface of the outcrop, in some places°by as much 

as a centimeter. The surface of this outcrop was once 

smoothed and polished by the river, but it has been 

roughened and pitted by weathering. The quartz, being more resistant to chemical attack than the enclosing rock, has been etched into relief. This suggests 

that the outcrop is no longer polished by the river except during the highest floods.

Most of the trees here are typical of the bedrock terrace surface farther downriver on Bear Island. They 

are red, chestnut, and post oaks and pignut hickory. 

Fringe tree, a small tree that bears conspicuous sprays 

of pale yellow-green flowers in the spring, is common.

Behind you, toward the Tavern, is a channel containing much debris deposited by f loodwaters. Trees there 

are American elm and a few slippery elms, which can 

be easily distinguished from each other only by their 

distinct seed. Others are sycamore with its white bark, 

ash, and black locust. These, as well as a few other 

tree species, are common on flood plains and are rare 

on uplands.

Return to the towpath, turn right, and continue 

downstream.

Stop 2. ROCKY ISLANDS CHANNEL. The towpath 

passes high above an intermittent river channel encircling Rocky Islands. Nearly dry in the low-water 

stages of summer and fall, this channel is often filled 

to a depth of 10 to 12 meters by winter and spring 

floods. The towpath here is at an elevation of about 

41 meters; you have descended about 10 meters since 

leaving the Tavern. Notice that the river, which was 

about 4 meters below the towpath at the Tavern, is 

now nearly 17 meters below the towpath. This gives 

you some idea of the drop at Great Falls. The upper 

surface of the rocky outcrops visible here is at almost 

exactly the same level as the flood plain of the river 

above Great Falls.

This surface marks the river bed prior to the cutting of 

the gorge and the establishment of Great Falls in its

52
View of Rocky Islands Channel from Stop 2.

present position. The steep rock faces that slope 50 to 

70 degrees to your right (north) are joints in the 

bedrock caused by relief of pressure as the rocks were 

uplifted and exposed during erosion. A zone of closely spaced joints provided a zone of weakness along 

which the river worked to cut the channel that extends westward from this point. This zone is indicated 

by J-J' on the block diagrams. The steeply sloping rock 

faces nearly at right angles to these joints formed by 

splitting of the rock parallel to metagraywacke beds.

Notice the small shrubs growing in rocks at the bottom of the channel. These are sycamore, river birch,

53
chinquapin

and ash that normally grow to be large trees, but here 

they are kept small by the frequent battering of flood 

torrents.

The evergreen trees on the islands are Virginia pine 

and eastern red cedar, typical of the bedrock surface. 

Also present are red, post, and chestnut oaks, and 

pignut hickory. Many of the trees on the island were 

battered by the flood following Hurricane Agnes in 

1972. Water covered the towpath to a depth of 1.5 

meters where you are standing.

The moss halfway down the cliffs (Crimmia laevigata) 

is green when wet and black when dry. Here it grows 

only on the cliffs at the level that floods reach about 

every 2 years. In the Washington area it is found only 

on rocky cliffs between Great Falls and Chain Bridge, 

but it is widespread in the Appalachian Mountains. 

Continue south (downstream) along towpath.

0.84 Trail junction about 20 meters before towpath 

reaches stoplock (where wooden steps lead up embankment to bridge across canal); follow Billy Goat 

Trail (marked by blue paint blazes) to right 

(southwest). Along the trail and to your right are many 

bent trees that were felled by the 1936 flood, which 

was 3 to 4 meters deep here.

The prominent levee to the left of the trail was designed to divert floodwaters from the canal bed and 

return them to the main channel of the river.

0.91 Stop 3. A FLOOD FELLED TREE. About 65 meters from 

the towpath is a nearly horizontal tree that was felled 

by a flood similar to that which felled the trees at Stop 

1. The age of the nearly vertical sprout at the end of 

the inclined trunk tells us that the flood occurred in 

1936. The maximum known flood on the Potomac 

River was on March 19th of that year.

The rounded outcrop about 4 meters east of the trail 

is thinly layered metagraywacke with many small 

upright folds. Veinlets of white quartz as much as half 

a centimeter thick follow the beds around the fold

54
noses. Some of the quartz veinlets have been etched 

into relief of as much as 5 millimeters since this outcrop was smoothed by the river.

Beyond the flood-felled tree is the ubiquitous and 

troublesome Japanese honeysuckle vine that has 

nearly smothered the wildflowers and shrubs. 

Honeysuckle was introduced into the United States 

from eastern Asia to help control soil erosion. The prominent shrub here is spice bush. It is covered with 

small yellow flowers in early spring and red berries in 

the fall. A crushed leaf produces a spicy aroma.

This forest is typical of the bedrock terrace. Prominent trees are post oak, with its almost Maltese crossshaped leaves; red, white, and chestnut oaks; and 

pignut hickory; along with Virginia pine and eastern 

red cedar.

Lamprophyre dikes (indicated by white dots) on the Virginia side of the river as seen from 

Stop 4 on the Maryland side. Outcrop of dike in foreground is indicated by arrow Location 

of Stop 3 on the Virginia side is also shown
dogwood

Along the trail to the next stop and beyond, you will 

constantly encounter these trees. Interestingly 

enough, two common upland oaks, black and 

southern red, are extremely rare on Bear Island. These 

species are fairly common, however, on the higher 

islands down river from Anglers Inn. Small trees that 

never get as large as the oaks and pines are fringe tree, 

with its sprays of white, showy flowers in the spring; 

and blackhaw, with saucer-shaped clusters of white 

flowers in late spring and blue-black berries in the fall. 

Possumhaw, a deciduous holly, grows in open rocky 

areas on Bear Island but is found nowhere else in the 

Washington area. It grows from here to Florida and in 

the midwest.

Continue southwest along the Billy Goat Trail past 

outcrops of metagraywacke similar to those at Stops 1 

and 3. Several other flood-felled trees similar to that 

at Stop 3 are obvious.

1.06 Trail junction. Rough unmarked trail leads right (west) 

down into lower end of same river channel viewed 

from Stop 2. The Billy Goat Trail continues left 

(south). A scramble down the side trail to the bottom 

of the channel (dry at low water) gives you an opportunity to examine the material in the channel bed. 

Note that most of the rocks here are large pieces of 

metagraywacke and schist identical to the rocks that 

form the walls of the channel. These large pieces, 

some as large as several meters in diameter, are 

somewhat smoothed by water. However, their angular 

shapes and sharp edges indicate that they have not 

been transported very far by the river. Only the 

smaller cobbles (less than 30 centimeters) are conspicuously rounded by transport in the river, and only 

the smaller pebbles and cobbles are composed of 

rocks that can be identified as having come from any 

great distance upstream. Evidently even the floodwaters that at times fill this channel to depths of as 

much as 12 meters do not carry the larger boulders 

far. Scramble back up to blue-blazed Billy Goat Trail 

and turn right. Trail passes across bare outcrops of 

metagraywacke and mica schist, and in about 100 m 

descends into a shallow wooded valley about 3

56
meters deep. Walk west in valley about 20 meters to 

small rock outcrop overlooking river.

1.16 Stop 4. LAMPROPHYRE DIKES. Across the river 

diagonally upstream, notice a series of nearly vertical 

clefts in the steep rock wall of the gorge. These clefts 

mark the outcrop of two 20- to 40-centimeter-thick 

dikes of lamprophyre, a dark lavalike rock that was injected as a liquid in sheets following the steeply inclined joints in the metagraywacke and schist. The narrow valley here is eroded along the same set of dikes 

on this side of the river. Just to the right (upstream) of 

the outcrop of metagraywacke in the center of the 

valley overlooking the river, the dike rock is exposed. 

The natural surface is weathered and thickly covered 

with green lichen, but a few freshly broken pieces can 

be seen. The rock is dark green and filled with glittering flecks of black mica (biotite). Another dike is exposed about a meter farther upstream. Close examination of the upstream dike shows that the biotite 

crystals are much larger in the center of the dike than 

they are near its walls. This is because the molten rock 

material cooled faster close to the cool rock walls 

than it did toward the center, thus preventing the 

growth of larger biotite crystals. The straight walls of 

the dikes and the chilled margins show that they must 

have been injected long after the metagraywacke and 

schist had been folded and metamorphosed. Since the 

dikes have been dated at about 360 million years, we 

know that the folding and metamorphism of the 

enclosing rocks occurred before that time.

Notice that the dikes on the other side of the river are 

not directly on the projection of the narrow valley, 

but are about 25 meters upstream from where you 

would expect to find them. The rocks on the two sides 

of the river have slid past each other along a fault 

which lies beneath the river at this point. The straight 

steep-sided gorge is cut in the crushed and broken 

rocks along the fault, because they are more easily 

eroded than the solid rocks on either side. Since the 

fault offsets the dikes, it must have moved since 360 

million years ago.

fringetree

57
The common trees here are pignut hickory with stout 

twigs and compound leaves and red, post, and 

chestnut oaks. Shrubs are southern arrow-wood, 

maple-leaved viburnum, and low blueberry.

Return to Billy Goat Trail, turn right, and continue 

downstream.

Between Stops 4 and 5 the trail is nearly level and 

passes in and out of forest typical of the bedrock terrace. In about 400 meters the trail emerges from the 

woods and weaves over bare rock ribs and knobs to 

the crest of a rocky ridge near the edge of the gorge.

1.68 Stop 5. POTHOLES. The rock here is mica schist containing many thin streaks and small veins of quartz, 

most of them parallel to crumpled discontinuous 

layers that probably represent the remains of thin 

beds in the original muddy sediment that was 

metamorphosed to form the schist. 

Potholes in schist at Stop 5.
Large cobbles are trapped in a 

depression in the streambed where 

they can be rolled by the current 

but seldom lifted out.

The depression is enlarged and 

deepened by the grinding action of 

the rolling cobbles. As the cobbles 

are reduced in size, they are removed and replaced by new ones.

f

Stages in the cutting of a pothole.

When the depression has been 

deepened sufficiently, small rocks 

are also trapped and the grinding 

continues.

This area is part of the bedrock terrace that represents 

the river bed before the gorge was cut. The same terrace level is marked by the prominent rock bench on 

the other side of the river. The conspicuous large circular depressions in the rock are potholes that were 

cut by the grinding action of pebbles and cobbles carried by swirling currents when the river flowed at this 

level. Trapped in these potholes are large rounded 

boulders of rocks derived from many kilometers 

upstream. One pothole contains a boulder of rustybrown diabase that must have been carried from at 

least 11 kilometers upstream near Blockhouse Point 

and also boulders of hard white and brown quartzite 

that came at least from Point of Rocks (55 kilometers) 

and possibly from Harpers Ferry (70 kilometers) 

upstream. Other potholes here contain cobbles of 

Catoctin Greenstone, a green metamorphosed 

volcanic rock that crops out nowhere closer than 

Point of Rocks. These far-traveled boulders are much 

larger than any of those in the dry channel near Stop 3.

mountain-laurel 

(evergreen)

59
Does this indicate that the river was much swifter and 

powerful when its bed was at this level?

blackjack oak

Notice that the rock surfaces of the schist, while still 

smoothed and rounded in general outline, are in detail 

much roughened by weathering. This roughening must 

have taken place after the gorge was cut and the river 

no longer reached this level except during the very 

highest floods. Rock surfaces around potholes at the 

present river level are perfectly smooth and polished 

by the constant wear of sand and silt carried by the 

river.

The light-gray lichens plastered to the rocks are 

Parmelia conspersa, which are common on all exposed rocks at this height above the river. Lichens 

generally will not grow in cities because they accumulate sulfur from the atmosphere, which eventually kills them.

This stop also provides an excellent view of a 

remarkably straight and steep-sided gorge cut in the 

crushed rocks along the fault. On the walls of the 

gorge, horizontal color bands produced by different 

species of lichens and by chemical staining reflect different levels of flooding. These are described at Stop 

4 of the Virginia trail log.

The trail continues across bare rock ledges for about 

120 meters, then descends to the left into the woods. 

After about another 30 meters it is possible to scramble down to a sandy beach at the river's edge and, at 

low water, to walk out onto a small rocky peninsula 

projecting into the river. On this peninsula, verticalstanding beds of metagraywacke, with gradations 

from coarse sand grains at their bases (on the west) to 

fine sand at their tops (to the east) are beautifully 

displayed. Beds range in thickness from 5 to 20 centimeters and are separated by beds of mica schist of 

about the same thickness. The metagraywacke and 

schist are cut by one irregular dike of light-colored 

granite 20 to 30 centimeters thick. In places the 

granite is composed of very large crystals of feldspar 

and mica, irregular grains of quartz, and is referred

60
2.30

to as pegmatite. Return to Billy Coat Trail and continue right downstream.

Trail crosses a small stream in a valley eroded along a 

fault zone. The water in the stream and rocks in the 

streambed are heavily stained with iron, probably 

from pyrite (iron sulfide) deposited by solution percolating along the fault zone. The pyrite has been 

dissolved by the creek as it cuts its way down into the 

crushed rocks, and the iron reprecipitated as iron oxide.

This mineralized fault zone is one of a series of such 

zones which extends northward from here for the next 

3 to 4 kilometers. It was along these zones that the 

gold deposits were mined in about half a dozen small 

gold mines near Great Falls. The fault beneath the 

river in the gorge is probably one of the same set of 

faults.

The white-barked sycamores in this valley are growing 

with ash, elm, and boxelder on fine soil deposited on 

the flood plain of this small stream by high water. A 

little-used trail turns left along the east side of the 

brook and leads 0.3 kilometers to the C and O Canal 

towpath.

Continue across valley and follow trail up cleft in rock 

slabs.

For about 350 meters the trail winds through the 

woods among outcrops of metagraywacke; it then 

turns to the right and descends toward the river. Stop 

6 is near the point where the trail emerges from the 

woods and descends through a small sandy brushfilled valley to bare rock outcrop near the river.

Stop 6. ECHO CLIFFS. The high bluffs across the river 

are Echo Cliffs. Locally they are as much as 45 meters 

high.

The central part of the cliffs just downstream from 

this stop is composed chiefly of metagraywacke 

similar to that exposed on the small peninsula on this

red cedar

61
r»:

* if

Pothole

N<*

*S- \

Echo Cliffs. Near the upstream end of the cliffs, the river bottom is only 7 meters above sea 

level. At normal river stages the water surface is at an elevation of about 22 meters, but 

during extreme floods it may rise as much as 16 meters higher. Elsewhere in the gorge the 

bottom is generally about 15 meters above sea level. Rocks exposed in the cliff are indicated as follows: a, amphibolite; m, metagraywacke; s, schist; g, granite; p, pegmatite; q, 

quartz vein. Also shown are fault zones, fz. In lower right is photograph of a pothole 

exposed in cross section where a large rock block has fallen away near the lower (east) end 

of Echo Cliffs.

62
Cowhoof 

' Rock

£V
black walnut

side of the river just in front of you. The dark green 

rock in the cliffs farther downstream is amphibolite, 

an extension of the amphibolite body on which you 

are standing. The rock at both the upstream and 

downstream ends of the cliffs is mica schist containing very abundant thin veinlets of quartz and feldspar 

fo/med during metamorphism. All of the rocks in the 

cliff are cut by dikes and irregular bodies of granite, 

similar to the granite that forms the prominent lightcolored knob behind you. The amphibolite in the cliff 

is probably all part of a single 20- to 30-meter-thick 

layer that was injected as a sheet of molten material 

parallel to the beds of mudstone and sandstone now 

represented by schist and metagraywacke. Dating of 

zircon, a rare mineral with a trace of radioactivity, indicates that the molten rock material cooled and 

crystallized about 550 million years ago. When the 

sedimentary materials were metamorphosed to form 

schist and metagraywacke, the basaltlike rock was 

metamorphosed to form amphibolite. The am-^ 

phibolite layer has been folded along with the enclosing rocks, and so the layer that appears in the 

upstream part of the cliff is folded down beneath the 

metagraywacke in the central part and reappears near 

the downstream end of the cliff. The dikes of granite 

and pegmatite injected the amphibolite and the 

enclosing rocks about 470 million years ago after this 

folding was largely completed.

Two fault zones, one containing a 1-meter-thick 

quartz vein, are exposed near the downstream end of 

the cliff. The vein probably connects with the large 

body of quartz near the river's edge a few meters 

south of this stop. Notice the enormous potholes near 

the downstream end of the cliffs.

The outcrops within a few meters of this point afford 

an opportunity to examine the amphibolite and 

metagraywacke in some detail. The amphibolite 

characteristically displays dark-green knots 0.5 to 1 

centimeter in diameter. These are clusters of hornblende crystals that have formed during metamorphism, replacing crystals of pyroxene (an iron64
magnesium-aluminum-silicate) in the original igneous 

rock. The light-colored mineral between them is 

feldspar. Notice that the hornblende knots are largest 

near the center of the large amphibolite body and 

become finer at both sides. This suggests that the 

original igneous rock chilled rapidly against the 

enclosing sediments and crystallized so fast that large 

pyroxene crystals had no time to form.

Just upstream from the amphibolite, numerous beds 

of metagraywacke 2 to 20 centimeters thick and a 

1.5-meter-thick sheet of amphibolite are interlayered 

with mica schist. Graded bedding in the 

metagraywacke indicates that the tops of beds are to 

the east. These rocks display many of the details of 

the folding and metamorphism outlined earlier. Most 

rocks display a prominent alinement of platy minerals 

(schistosity) generally parallel to the layering; a few 

contain folds which apparently formed at the same 

time as the schistosity, because the schistosity in their 

noses cuts through the layering. Hornblende clusters 

in the amphibolite are crushed and strung out parallel

Early folds in metagraywacke near Stop 6. Schistosity cuts across bedding in the noses of 

folds, but everywhere else the two are parallel.
Intricately folded veinlet of quartz cutting across layers in mica schist near Stop 6.

66
:o the schistosity, showing that the amphibolite must 

have been intruded and metamorphosed before these 

early folds formed. The large fold exposed in the cliff 

across the river formed later, because the schistosity 

is bent around it.

The rocks are cut by many thin veinlets of quartz, 

most of which are intricately folded and must have 

formed prior to the early folds and the schistosity. But 

the small veins and irregular pods of light-colored, 

coarse-grained granite cut across the schistosity, 

showing that the granite must have been intruded 

after the folding. Several of the mica schist layers contain elongated (5 centimeters) crystals of light-colored 

kyanite; they too must have formed after the early 

folding, because they clearly pushed the schistosity 

aside as they grew.

Altered crystals of kyanite in mica schist near Stop 6. The crystals are as much as 5 centimeters long.

elm
honey locust

Continue southeast along trail across bare outcrops of 

thinly layered metagraywacke and schist showing occasional folds and many irregular splotches and 

streaks due to growth of metamorphic minerals. Small 

dikes of granite and pegmatite are abundant in this 

area. In about 80 meters the trail descends from a 

rock rib and crosses a sandy beach. It then climbs 

across another series of rock slabs and skirts to the 

left around a high rock knob. Stop 7 is in a sandy 

hollow just east (downstream) of this knob.

2.50 Stop 7. LATE FOLDS. The flat rock slab near the north 

side of the sandy hollow displays a good example of 

one of the late folds, a miniature version of the large 

fold exposed in Echo Cliffs.

The early schistosity, parallel to the metagraywacke 

layers, is bent around the fold; and a second, weaker 

schistosity has formed at an angle to the first.

From here the trail descends into a small valley which 

follows the sinuous form of a folded amphibolite 

sheet, more easily eroded than the schists and 

metagraywackes on either side. This fold is another 

larger scale example of the late folds, like the one at 

Echo Cliffs.

One of the early folds may be seen about 60 meters 

farther along the trail where it passes near the edge of 

a pond. In the rock face about 10 meters to the left of 

the trail is a metagraywacke bed that is folded back 

completely on itself. The doubled-back bed has then 

been refolded as part of the later folding displayed at 

Stop 7. Across the pond from the trail, sycamore, boxelder, ash, and silver maple trees are growing on fine 

soil deposited by the river during high water. The trail 

continues around the north side of the pond, then 

climbs a rocky slope with metagraywacke outcrops, 

and passes through a jumble of boulders and outcrops 

of amphibolite.

2.74 Trail crosses small stream. An unmarked but wellbeaten side trail leads left (north) 290 meters to the 

C and O Canal towpath 2.4 kilometers southeast of

68
Late fold in interlayered mica schist and metagraywacke at Stop 7. An old schistosity lies 

parallel to the bedding; the new schistosity, formed during development of this fold, cuts 

across the bedding in the hinge of the fold. Details are partly obscured by lichen growth 

(light gray in photograph).

69
Great Falls Tavern and 0.85 kilometers west of the 

parking lot at Anglers Inn. The blue-blazed Billy Goat 

Trail continues right (south) across a low rocky ridge 

(chiefly schist and metagraywacke, some granite and 

pegmatite).

2.82 Trail descends steep rock outcrop into small open 

valley with scattered outcrops of amphibolite (probably part of separate layer from that exposed in Echo 

Cliffs and around the pond near Stop 7).

The trail crosses another rib of schist and 

metagraywacke, then crosses more large blocks of 

amphibolite, and emerges on an open flat covered 

with fine soil. To the left is a large outcrop of mica 

schist.

black locust

Block diagram showing folded amphibolite near Stop 7.

MICA SCHIST AND 

METAGRAYWACKE

\

EARLY FOU

70
 v

2.92 stop 8. A CONTRAST IN ENVIRONMENTS. Scramble 

over blocks in the bottom of a small draw to the top 

of the outcrop. Here is another rare plant prickly 

pear cactus.

In this region it grows only on Bear Island, along the 

coast, and in certain areas in the Shenandoah Valley. 

The presence of cactus here seems to be in contradiction with the nearby bogs where red maple and black 

tupelo thrive. Black tupelo is a tree with smooth gray 

bark and nearly horizontal branches. Red maple also 

has light-gray bark, but the twigs, buds, and leaves 

grow opposite each other. These two are the most 

common trees in swamps on Bear Island.

The contrast is due to the virtual absence of soil on 

much of the irregular surface of the bedrock terrace 

on Bear Island. The deep clayey soils formed by 

chemical weathering of the underlying rocks in adjacent parts of the Piedmont upland have not yet had 

time to form on the recently river-scoured surface of 

the bedrock terrace. The only soils are sand and silt 

deposited by recent floods in low hollows and in 

nooks and crannies among the rocks. Rainwater runs 

almost immediately off the dry rock ridges and collects in the undrained potholes and abandoned channels to form ponds and swamps.

Return to Billy Goat Trail and continue downstream 

In about 60 meters the trail rounds a rocky ridge and 

turns north.

3.06 Trail dips into valley containing elongated pond on 

left. This part of the trail is impassable during floods 

because water flows from the river northward. Note 

the flood debris that litters the valley bottom and that 

is caught on the south sides of shrubs and tree trunks. 

Many trunks lean to the north, showing that high 

water flows from the river to your right and sweeps 

around the hill in front of you.

Trail climbs steeply over 15-meter hill. It then 

descends into another deep valley, actually a continuation of the one you just crossed. Note here that

poison-sumac

71
"t.'-^ 

"\ "

 v*"

Prickly pear cactus at Stop 8.

flood debris on shrubs and trees and leaning trunks 

show that high water flowed from your left to the river 

on your right.

Trail crosses the valley and follows gulley to top of 

rocky plateau. Just before descending again, it passes 

to the right of a low ledge extending diagonally away 

to the left.

3.32 Stop 9. ANDALUSITE. On top of this ledge a few 

meters west of the trail, the schist contains abundant

72
stubby, gray, 1- to 2-centimeter-long crystals of an- 

< dalusite, a metamorphic mineral with the chemical 

formula AI 2 SiO 5 . Andalusite is fairly common in the 

schist at this end of Bear Island; near Stop 6, kyanite, 

another mineral with the same chemical composition, 

is found. Microscopic examination shows that both 

the kyanite and andalusite are partly changed to silli- 

' manite, a third form of Al 2 SiO5 . Laboratory studies show 

that these three minerals with the same composition 

can form together only at temperatures of about 650°C 

and pressures of about 5.5 kilobars, corresponding 

v to depths of 20 to 30 kilometers. Since all three 

minerals occur together within a very small area, we 

conclude that metamorphism of the rocks now exposed here on Bear Island took place at approximately 

that temperature and depth.

Trail descends steeply into another valley, follows 

river bank a few meters, then climbs back to level surr- face of bedrock terrace.

3,55 Billy Coat Trail emerges on C and O Canal towpath. 

( Turn left 2.08 kilometers to Great Falls Tavern or right 

0.65 kilometers to causeway across canal leading to 

parking area at Anglers Inn.

sugar maple

73
Suggested Reading

Botanical evidence of floods and flood-plain deposition, by Robert S. 

Sigafoos: U.S. Geological Survey Professional Paper 485-A, Washington, 1964, 35 p.

The geology of Howard and Montgomery Counties, by Ernst Cloos, C. A. 

Hopson, C. W. Fisher, and Emery T. Cleaves: Maryland Geological 

Survey, Baltimore, 1964, 373 p.

Crystalline rocks of the Potomac River Gorge near Washington, D.C., by 

John C. Reed, Jr., and Janice Jolly: U.S. Geological Survey Professional Paper 414-H, Washington, 1963, 16 p.

The metamorphosed sedimentary rocks along the Potomac River near 

Washington, D.C., by George W. Fisher, in Studies of Appalachian 

geology Central and southern: Wiley-lnterscience Publishers, New 

York, 1970, p. 299-316.

The Piedmont crystalline rocks at Bear Island, Potomac River 

Maryland, by George W. Fisher: Maryland Geological Survey Guidebook No. 4, 32 p.

Gold veins near Great Falls, Maryland by John C. Reed, Jr., and John C. 

Reed: U.S. Geological Survey Bulletin 1286, Washington, 1970, 22 p.

The District of Columbia, its rocks and their geologic history, by Martha 

S. Carr: U.S. Geological Survey Bulletin 967. Washington, 1950, 59 p.

Geology and ground-water resources of Washington, D.C., and vicinity, 

by Paul M. Johnston, with-a section on Chemical quality of the water, 

by D. E. Weaver and Leonard Siu: U.S. Geological Survey WaterSupply Paper 1776, Washington, 1964, 97 p.

The Fairfax family in "Fairfax County, by Kenton Kilmer and Donald 

Sweig: Fairfax Co. Office of Comprehensive Planning, Fairfax Co., 

Va., 1975, 119 p.

74
Acknowledgments

The original edition was prepared under the auspices of the Geological 

Society of Washington. C. B. Hunt of the Johns Hopkins University; W. 

L. Newman of the U.S. Geological Survey; and S.W. Schiffman, A.T. 

Outlaw, and S.Q. Smith of the National Park Service gave valuable advice on format and content. The discussion of vegetation changes in the, 

past 16,000 years is based on pollen studies by Isabel A. Griffith of 

Aldie, Va. Kathleen G. Muth and Joseph G. Arth kindly made available 

unpublished age determinations on mica samples from pegmatite dikes.

Drawings are by J. R. Stacy of the Geological Survey. Photographs on 

page viii are by the National Park Service; those on pages iii, 2, and 12 

are by James Q. Reber of Bethesda, Md.; and those on pages 7, 18, 27, 

29, 31, 33, 42, 44, 45, 53, 55, 58, 62, 63, and 72 are by David F. Usher of 

the U.S. Geological Survey; and those on pages 37 are by Thomas M. 

Yanosky, also of the U.S. Geological Survey.

Metric-English Conversion

(The following table gives the factors used to convert metric units to English units.)

Multiply metric units

millimeters

centimeters

meters

kilometer

hectares

liters

metric ton

kilobar

Celsius (°C)

by

0.03937

0.3937

3.281

0.6214

2.471

0.2642

1.102

14.50

1.8 and +32

to obtain English units

inches

inches

feet

mile

acres

gallons

ton

pound-force per square

Fahrenheit (°F)

inch

£U.S. GOVERNMENT PRINTING OFFICE: I980 O 311-344/20

75
Reed, Sigafoos, and Fisher GEOLOGIC STORY OF GREAT FALLS AND POTOMAC R