Durango Bill’s
Ancestral Rivers of the World
Ancestral Rivers of
the Eastern United States
by
Bill Butler
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.
Featured Areas
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 (in Ala.) at Guntersville Dam
All pictures were generated via Delorme’s Topo USA computer
program
Hudson River cuts
through the Taconic Mountains between Newburgh and Peekskill, NY
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 England.)
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 Taconic Klippe.
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.
Delaware Water 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 surface.
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 the Appalachians.
Susquehanna River
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.
New River, Virginia
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 Tennessee River.
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 (in
Ala.) at Guntersville Dam
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 River.
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 existed 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 South America was in
the way, and again there were more mountains to the south. Thus
drainage was initially to the north, and then later it extended to the
west. Zircons in Mesozoic sandstones in the Colorado Plateau can be
traced back to their origin in the southern Appalachians.
Geologic maps of Tennessee (see http://www.state.tn.us/environment/tdg/bigmap.shtml
or http://geology.about.com/od/stategeologicmaps/
-> 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.
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