(About 60 paces.) Stop G4. A closeup view of
Great Falls. The total drop here at normal stages of the river is
about 40 feet. The highest cascades are about 16 feet. The distance
froun here to the overlooks on the Virginia shore is about 500 feet. In
the narrowest part of the channel below the falls the river is about
75 feet wide at normal stages, but at higher stages it floods to the
base of the cliff below you. About once every 2 years floods cover this
overlook to depths of 1 to 2 feet.
Some of the scattered shrubs growing among the rocks
below are sycamore and river birch that normally grow to be large trees.
Here they are kept small and shrubby by frequent flood damage. The
detergent suds commonly seen in the river just below the falls are
evidence of pollution from towns and cities upstream.
Notice the broad flat surface at the top of the
cliffs on the Virginia side. This is part of the same ancient riverbed
across which you have been walking. This ancient river bed, which lies
40 to 50 feet higher than the river below the falls, can be traced
upstream to the present level of the river above the falls. The river
occupied this bed before the gorge was cut and the falls were formed.
The forest on the old riverbed downriver from the
falls is different from that on the same surface upriver from the falls.
This trail passed through a red oak and post oak forest with pine,
juniper, pignut hickory, and a few other species. Upriver, this surface
stupports a sycamore and ash forest with river birch, silver maple,
boxelder, and cottonwood. Red and post oaks are not present.
The rocks here are chiefly metagraywacke (a type of
metamorphosed sandstone). On the outcrop directly east of you is an
8-inch-thick layer of metagraywacke interlayered with mica schist. Some
of the larger original sand grains are visible in the lower part of the
layer. When these layers were deposited, they lay nearly horizontal on
the sea bottom, but they were crumpled into folds at the time the rocks
were deeply buried and metamorphosed. The many knots and layers of
white quartz were sweated out of the rocks during metamorphism and
outline some of these folds. Notice the schistosity cutting across the
The straight fractures or joints displayed in the
rocky islands in the river were formed when the rocks were uplifted. The
most conspicuous joints are nearly vertical and perpendicular to the
folded rock layers. Notice how these structures control the details of
the falls. The several river channels tend to closely parallel the rock
layers, and the cascades or steps occur where the channels cross a
closely spaced group of joints.
Continue along trail for about 30 yards to lower
overlook. Note conspicuous folds in rock layers to the right 15 feet
beyond the lower overlook guardrail.