Ancestral Rivers of the World
of the Eastern United States
Antecedence and superimposition are geologic
processes that explain how and why rivers can cut through mountain
systems instead of going around them. Examples (with pictures) are
from the eastern United States.
Hudson River (Taconic Mountains)
between Peekskill and Newburgh, New York.
Delaware Water Gap (Delaware River
on the New Jersey/Pennsylvania Border)
Susquehanna River north of Harrisburg, Pennsylvania
Potomac River at Harpers Ferry
New River near the Narrows, Virginia
French Broad and Pigeon Rivers, North Carolina to Tennessee
Tennessee River – Alabama/Mississippi/Tennessee section
All pictures were generated via Delorme’s Topo USA computer
Hudson River cuts
through the Taconic Mountains between Newburgh and Peekskill,
The picture above shows the Hudson River where it
cuts through the Taconic Mountains in eastern New York State. The
Taconic Mountains are one of the oldest recognizable mountain
systems in the United States.
Some 500 million years ago, movements of the earth’s
crust (plate tectonics) began squeezing the Iapetus Ocean (long
vanished predecessor of today’s Atlantic Ocean) out of existence.
At the time, North America was an island continent (much like
today’s Australia), centered a little south of the equator, and
rotated from today’s orientation such that due north from New York
city would take you toward today’s extreme southwestern Canada.
Over the next 250 million years, most of the earth’s
land masses would converge to form the super-continent Pangaea.
The Iapetus Ocean separated the east coast of what
would become the United States and the continent of Africa. As the
content of Africa closed in, there were a couple of island arcs
(similar to today’s Aleutian Islands) that crumpled into North
America as well as the final collision with Africa. The resultant
crumpling of the earth’s crust produced today’s Appalachian
Mountains. There were three distinct phases of mountain building
within this process. These were the Taconic (~470 to 450 million
years ago), the Acadian (~400 to 350 million years ago), and the
Appalachian (~300 to 275 million years ago) orogenies.
In the Taconic event, the earth’s crust was broken
into a series of horizontal sheets/layers which were then shoved
on top of each other. The process was similar to a deck of cards
that initially is spread out with the cards slightly overlapping.
If you then push the cards together, the individual cards will
slide over each other and build up into a stack.
One of these sheets/layers of rock that was stacked
was a hard erosion resistant layer called the “Taconic Klippe”.
The “Taconic Klippe” itself did not get pushed to high elevations,
but other layers were stacked on top of the Taconic Klippe, and
they formed an impressive mountain range. The Taconic Klippe
extends north-northeastward to include southwestern Vermont. There
were also limestone layers that were stacked and buried. It gets
hotter as you go deeper into the earth, and with time these
limestone layers were “cooked” to become Vermont’s famous marble.
In the last 200 million years the super-continent
Pangaea rifted apart. The Atlantic Ocean has filled in this rift.
The new rift that opened was slightly to the east of the old
Iapetus Ocean with the result that a section of what came in as
part Africa remained attached to North America when the new rift
opened. The “technical name” for this adopted chunk of land is
“New Hampshire”. Thus New Hampshire geology is much different than
that for Vermont. (Ditto for much of the rest of eastern New
The Appalachians are old mountains and with time
erosion beveled off the high mountains/layers and filled in some
of the valleys with sand and gravel (some of which hardened into
conglomerates). The end result was a nearly flat surface. South
and southeastward flowing rivers developed across this flat
surface. One of these rivers (which would become the Hudson)
happened to be above a portion of the old erosion resistant
Over the last few tens of million years there has
been a mild regional uplift which has allowed erosion to set in
again. Rivers (and more recently, glaciers) have stripped off
enough material to expose the Taconic Klippe. The Hudson River was
over the Klippe, but it was able to erode down fast enough to stay
in its original path.
Today, the erosion resistant Taconic Klippe has
emerged as a low range of mountains. The Hudson River was
“superimposed” above it millions of years ago, but has been able
to erode down fast enough to maintain its original path. It thus
cuts through the Taconic Mountains instead of “finding” an easier
path somewhere else.
Gap (Delaware River on the New Jersey/Pennsylvania Border)
The Delaware Water Gap was an important route for
early settlers that were headed west. Over geologic time the
Delaware River had eroded a path though upturned layers of hard
rock, and it was much easier to build an early road near the river
as opposed to climbing over 1,000 feet to go over the mountain.
Today Interstate Route 80 follows the same principle.
The geologic history of the area is similar to that
given above for the Hudson River. Here surface layers of rock were
simply folded into long ridges as opposed to the fracturing in the
Taconic Mountains. Again, a long period of erosion followed the
original mountain building, and a nearly flat surface resulted.
The Delaware River developed southeastward across this flat
Recent mild regional uplift has allowed erosion to
set in again. The upturned edges of the harder rock layers in the
old folds have resisted erosion and now produce long ridges. There
are many streams and rivers that had developed across the old flat
surface. As erosion set in again, if they had enough erosion
power, they were able to maintain their paths by cutting down into
the harder ridges. Thus, mountain gaps are common in this area of
north of Harrisburg, Pennsylvania
The geologic history for the Susquehanna River near
Harrisburg, PA (lower right corner) is very similar to that for
the Delaware Water Gap. The ridges are uniformly about 1,000 feet
higher than the river. As with the other pictures, the view is
toward the north and river flow direction is from north to south
(top edge toward the bottom edge).
Potomac River at
Harper Ferry, Maryland / Virginia
These ridges in the Appalachian Mountains that cross
the path of the Potomac River have a history similar to that of
the previous rivers. Here the Potomac River enters from the top
left corner and flows toward the lower right corner. The
Shenandoah River enters from the lower left edge and joins the
Potomac at Harpers Ferry just before the Potomac cuts through the
first ridge. Washington, D.C. is well off the lower right corner.
As in the previous views, the ridges rise about 1,000
feet above the river level.
View of the New River from just north of Radford, Va to the
Narrows, Va. Here the New River enters from the lower right corner
and flows northwestward through the ridges and off the top edge of
the picture into the 3,000+ foot-high Appalachian Plateau.
While the previous locations were examples of
“superimposition” where the mountains were in place first and
rivers subsequently developed on a superimposed flat surface, the
New River is an example of “antecedence” as its ancestor was in
place before the Appalachian Mountains were crumpled into
existence. As such, the section of the New River that flows
northwestward across the Appalachians has to be one of the oldest
rivers in North America.
Some ~300 million years ago, river drainage in this
section of the New River developed from southeast to northwest. As
the Appalachian Mountains rose across the path of the ancestor to
the New River, the old river had enough erosive power to maintain
its path. Eventually Africa collided with North America, and the
only route to the sea for this ancient river solidified as a
southeast to northwest path.
Subsequently, as land in the southwestern United
States rose above sea level, this ancestral drainage extended
westward. Zircons (microscopic rock crystals) that originally came
from the southern Appalachians were transported to the American
southwest where they are found today in Mesozoic sandstone layers.
French Broad and
Pigeon Rivers, North Carolina to Tennessee
The New River isn’t the only river that flows from
southeast to northwest across the highest portions of the
Appalachian Mountains. The picture above shows an area stretching
from Asheville, NC in the lower right corner to Douglas Lake, TN
(slightly above the center of the left edge.)
Drainage for the French Broad River originates on the
South Carolina border (well south of Asheville) and continues
north-northwest from Asheville to cross the highest part of the
Appalachians near the center of the picture. From there, the
French Broad turns west to Douglas Lake, Tennessee. Below Douglas
Lake, the French Broad merges with the Holsten River to form the
Interstate 40 (Lower left corner) roughly follows the
Pigeon River which flows from Clyde (lower left corner) across the
Appalachians to where the Pigeon River joins the French Broad just
before reaching Douglas Lake.
Both rivers appear to date back to Pangaea time and
appear to be every bit as old as the New River.
Tennessee River –
The picture above shows Guntersville Lake at the
southernmost point in the Tennessee River’s path. Logically, the
shortest distance for the Tennessee River to continue to the ocean
would be to continue flowing south to the Gulf of Mexico. Instead,
the Tennessee River turns west-northwest and then north to join
the Ohio River. Why the river takes this illogical path has been
subject to much controversy.
The current Tennessee River looks like it has been
pieced together from at least three sections that have different
origins in topography and time. Section one consists of multiple
headwater tributaries (also see French Broad, above) down to
Guntersville Lake and Dam. In section two, the river cuts through
the ridge on the west side of the lake (see the above picture) and
flows west-northwest to the Alabama/Mississippi/Tennessee border.
In section three, it flows due north to finally join the Ohio
The origin of section three appears highly illogical
if you just look at current drainage patterns. There is
considerable difference of opinion regarding this portion of the
river’s path, but it is the author’s conclusion that it is a
remnant of a drainage system that began 250 million years ago.
250 million years ago river drainage in western
Tennessee could not go east as the Appalachians and Africa blocked
any path to an ocean. Similarly, drainage could not go south as
there were more mountains blocking that potential route. Thus
drainage was initially to the north with an eventual extension to
the western U. S. Zircons in late Paleozoic and Mesozoic
sandstones in the Colorado Plateau can be traced back to their
origin in the southern Appalachians. (For example, see: http://gsa.confex.com/gsa/2010AM/finalprogram/abstract_178548.htm
Geologic maps of Tennessee (see http://www.state.tn.us/environment/tdg/bigmap.shtml
-> Tennessee) indicate bedrock in this area dates back this
far, and the more recent Cretaceous rocks could easily be deposits
by a sluggish ancestral river.
Another model for the Tennessee River proposes that
at one time the Tennessee River continued west to the Mississippi
via the current route of the Hatchie River, and shifted to its
present northward course via “stream capture”. (See http://gsa.confex.com/gsa/2003SC/finalprogram/abstract_48098.htm
) However, topographic maps show no evidence of any relatively low
ancestral valley pathway that would support this alternate model,
and the terrain surrounding the current northward portion of the
river “looks very old” on topographic maps.
(Click on map for a large version)
The topo map above shows the terrain to the west of
the Tennessee River where the river turns north to eventually join
the Ohio River. The Tennessee River and Pickwick Dam/Reservoir are
in the upper right corner. The blue line extending southward from
the Pickwick Reservoir is the Tennessee-Tombigbee Waterway – a
man-made canal that allows barges to take a shortcut from the
Tennessee River to the Gulf of Mexico. Streams and rivers on the
left edge of the map flow northwestward into the Hatchie-Tuscumbia
River system. The drainage divide between the Tennessee and
Hatchie Rivers is of interest.
If there were an ancestral connection between the
Tennessee River and the Hatchie River, there would have to be an
ancestral east to west valley system that crossed the “relatively
high” ridge that separates the two drainage systems. The topo map
doesn’t show any sign of an ancestral connection.
The small community of Gravel Hill in the upper left
corner may be of interest. There is little information about the
gravel at Gravel Hill, but there is no sign of any recent river
system that could have left a gravel deposit on this relatively
high ground. It would appear that the gravel is left over cobbles
and deposits of the north-flowing drainage that existed over one
hundred million years ago.
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