THE RIVER AND THE ROCKS
The Geologic Story of Great Falls and the Potomac River Gorge
TRAIL LOGGREAT FALLS, VIRGINIA
This walking tour is designed to acquaint you with
the important geological features that can be seen from the park trails.
To help you find the stops that are indicated, small light-colored oval
signs have been placed in strategic places to point your way. At each
stop there will be similar signs that have the stop number on it. Length
Leave south entrance of Visitor Center. Cross old
Patowmack Canal and walk to nearest overlook at the brink of the
(About 250 paces.) Stop G1. A closeup of
Great Falls. The total drop of the river here from the uppermost
rapids to the channel below this overlook is about 40 feet; the cascade
directly in front of you is 16 feet high. The narrowest part of the
channel below the falls here is 75 feet wide at normal stages of the
river. The foam commonly seen below the falls is detergent sudsevidence
of water pollution from towns and cities upstream.
The rock here is metagraywacke, a type of
metamorphosed sandstone. The conspicuous layers in this outcrop are the
original layers or beds in which the sand was 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 B-B'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
Notice also the straight fractures or joints that cut
across the rock layers. These joints formed as the rocks were uplifted
and cooled. The most conspicuous joints are nearly vertical and
perpendicular to the rock layers. These joints break the rock into
blocks, making it easier for the river to quarry away the resistant
layers. Each of the cascades in the falls marks a place where the river
passes over a group of joints.
Return to path along old Patowmack Canal, turn left
and walk south (downstream).
(About 320 paces.) Marker here shows level of
1936, 1942, and 1937 floods. In 1936 the water in the middle of the
river was more than 80 feet deep. Turn left here and walk about 50 feet
(About 20 paces.) Stop G2. Vista of the falls
and view of the old riverbed. 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 yards above the falls.
Notice the nearly right-angle turn in the river at
this point. Above this point the river is following the rock layers;
here it turns to follow a zone of weakness caused by closely spaced
joints like those visible at Stop G1.
Return to path along Patowmack Canal and continue
(About 300 paces.) Large outcrop of rock in the
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.
This surface is part of the old riverbed. The exposed
rock was rounded and smoothed by the river.
Continue along same path.
(About 400 paces.) Trail intersection at wooden
bridge over old canal bed. Turn left off main trail. Walk about 125
yards and turn right on steep side trail leading down into small stream
valley. Go about 10 yards upstream and cross creek. The narrow valley
that you face in crossing the stream was man made, an early attempt to
provide an outlet for the Patowmack Canal. The main valley was cut along
a series of thin lamprophyre dikes, several of which are exposed in a
small waterfall about 15 yards farther upstream. Scramble up steep trail
on south side of valley. About 30 yards after crossing stream, turn
right and follow trail south (downstream).
(About 300 paces.) Stop G3. The Mather Gorge. The cliff
below is about 65 feet high. The river here is about 220 feet wide and
is 25 feet deep at normal stages of the river.
Sight along the flat rock face toward the Maryland
shore. You can see a series of deep vertical clefts in the cliff face
which are dikes of lamprophyre. They 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 80 feet 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. The fault zone is more easily eroded than the solid rocks on
The rock here is mica schist. About 25 feet to the
north (upstream) is a vein of quartz more than a foot thick. Gold was
mined from similar veins at several nearby places on the Maryland side
of the river.
Lamprophyre dikes (indicated by white dots) on the Maryland side of the
river as seen from Stop G3 on the Virginia side. Location of Stop G5 on
the Maryland side is also shown.
(About 275 paces.) Follow narrow but distinct
trail downstream along brink of gorge past river-smoothed outcrops of
At the junction of two trails, continue on 40 yards
where a twin-trunked tree is living testimony of the flood of 1889 (the
year of the Johnstown flood). The single inclined trunk grew vertically
but was felled. The vertical trunks started to grow when the parent
tree was felled. Their age tells us that the tree was felled in 1889 by
a flood as large as that in 1936.
Retrace steps to trail junction, turn left on
crosstrail for about 40 yards to main trail along Patowmack Canal. Turn
left (down stream).
Ruins of lift lock on the Patowmack Canal. The
lock is built chiefly of reddish-brown sandstone of Triassic age that
was quarried near Seneca, 8 miles upstream from Great Falls, and carried
downriver by boat. The same "Seneca Stone" was used in many of the older
buildings in downtown Washington.
At trail fork at lower end of lock follow right
(upper) branch of trail.
Beyond the trail entering on the left and to the next
stop, you pass through a red oak and post oak forest, typical of the
ancient riverbed which is now a bedrock terrace some 60 feet above the
river. Also growing here are Virginia pine, cedar, pignut hickory,
white oak, and chestnut oak. This type of forest is present on the flat
surface above the cliffs across the river.
River-worn pebbles wedged in a rock cranny more than 50 feet above the
normal water level.
(About 350 paces.) Stop G4. Potholes.
Here are several unusually good examples of potholescircular
holes ground in the rock by pebbles and boulders churned by the
river when it was at this level. (See Stop G2, Maryland trail
log, for a discussion of the formation of these holes.) Some of the
pebbles left by the river can still be seen in the potholes and wedged
in crannies in the rock outcrops. The small pond to the west, between
here and the steep hill, was once a side channel of the river.
Leave trail and walk over to edge of the 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 20 to 30 feet 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 10 or 20 feet above normal water
level, the rocks have a yellowish cast which is due to yellow and
orange lichens (Candelariella vitellina). This zone is flooded
several times each year.
Near the base of the cliffs, just above normal water
level, the rocks are covered with a metallic purplish-black coating that
consists of iron and manganese oxides.
Return to trail and continue south (downstream).
(About 150 paces.) Stop G5. 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, 5 feet in diameter and estimated to weigh over 6 tons, is the
granddaddy of them all! It is composed of a dark, heavy igneous rock
called diabase, and the nearest outcrop of such rock is near Blockhouse
Point, more than 7 miles upstream. This boulder is far too large to have
been moved by even the most violent flood. Could it have floated in an
ice jam or in a tangle of logs?
Continue downstream about 20 yards, turn right on
faint trail leading up steep hill. Scramble to top of hill and follow
trail to the right, skirting brink of abandoned stone quarry on the
left. Follow this trail for about 100 yards, then turn right again on
main horse trail.
Virginia pines are growing on disturbed ground along
the edge of the quarry. Beyond and along the trail the forest consists
primarily of oak specieswhite, black, scarlet, southern red,
and chestntut oaks. Note that post oak is not present, and red oak and
pignut hickories are rare. This forest is typical of a well-drained
(About 320 paces.) Stop G6. An ancient gravel
deposit. The ground here is littered with gravel aud large rounded
boulders, part of a river deposit even more ancient than those along the
trail below. This deposit lies almost 120 feet above the present level
of the river and must have been deposited during a very early stage of
the cutting of the valley, perhaps 2 to 3 million years ago.
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 ran down the ruts left by wagon wheels, eroding
gullies in the deep clay soil. When the ruts became too deep, it was
easier to move the road to one side than repair the gullies. In some
places this happened several times, leaving a series of parallel
Follow trail north (upstream).
(About 230 paces.) Deep gullies crossing road
were probably caused by drainage from old field, now completely
The old field to the west is marked not only by a
wire fence embedded in tree trunks, but also by the presence of Virginia
pine. The pines are dying and falling, thus permitting the hardwood
treesoaksto 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.
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 honey locust found only here. Many cultivated plants such as
daffodils, day lilies, privet, and others indicate human activity.
PIEDMONT UPLAND TREES
(About 500 paces.) Chimney and ruins of
buildings at Matildaville. Turn right and follow trail back to path
along old Patowmack Canal. Turn left and return to Visitor Center.
(About 1,060 paces.) Visitor Center.
Last Updated: 01-Mar-2005