Ancestral Rivers of the World
in Australia and New Zealand
Antecedence and superimposition are geologic
processes that explain how and why rivers can cut through mountain
systems instead of going around them. The examples here (including
pictures) are from Australia and New Zealand, but other examples
can be found throughout the world.
Alice Springs - Todd River
Arroyos - the Great Outback (3 pictures)
Fitzroy River ( 2 pictures)
Kawarau River/Gorge, South Island, New Zealand
Alice Springs -
The above Google Earth view looks west northwest
across central Australia’s Alice Springs. The long ridge is part
of the MacDonnell Ranges. (See below for geological history) The
item of interest is the Todd “River” which cuts through an
upturned sandstone ridge to form the 600-foot deep Heavitree Gap.
There are several other “gaps” within a 10 mile radius of Alice
Springs where other streams/rivers cut through similar upturned
Rivers in central Australia are unusual as most of
the time they don’t have any flowing water. When a significant
rain does occur, the Todd does flow; and more importantly, it
transports a load of silt and sand. The silt and sand are abrasive
elements that allow the Todd to keep cutting down through the hard
sandstone ridge. The process has been going on for hundreds of
millions of years, and it started when local topography and
rainfall amounts were much different from what we see today.
Arroyos - the
Great Outback (3 pictures), Northern Territory
Australia does have rivers with
real water, but Australia would not be the same if it did not have
the “Great Outback”. You could spend days trying to drive across
the “Outback”, and it might seem as though there was nothing there
except “miles and miles” of nothing but “miles and miles”.
Aerial views reveal a different and fascinating
world. The pictures below are an excerpt from a geologic story
that started long ago when the “Outback” was different.
The picture above looks west-northwest over the
western end of the MacDonnell Ranges some 75 miles west of Alice
Springs. There is a small motel at Glen Helen just below the
center of the picture where the dry stream bed enters the main
The dry steam bed actually has a name, the Finke
River, and on rare occasions it does flow (from right to left).
Close examination of the Google Earth photographs show there was
no flowing water at the time the picture was taken, and there were
just a few stagnant mud puddles.
Many hundreds of millions of years ago, there were
horizontal layers of sand and silt laid down in what is the
current field of view. Gradually they hardened into layers of
sandstone and siltstone. The MacDonnell Ranges were subsequently
uplifted some 300 to 400 million years ago. (The highest portions
were off the right side of the picture.) What had been horizontal
sedimentary layers were bent upward over the core of the uplifted
mountain range. Mt. Sonder (right edge of the picture) is a minor
remnant of this former mountain range. River drainage that
developed off the south side of these mountains eroded valleys,
and the remnants of these rivers persist to today. One of these
ancient drainage valleys is the ancestor of today’s “Finke River”.
As the upper layers of the range erode away, the
upturned layers along the flanks are gradually exposed. The harder
sandstone layers resist erosion and are exposed as long ridges.
Even though the Finke “River” only flows
occasionally, it still has enough erosion power to abrade down
through these harder ridges. Thus today, it occupies a series of
gorges that cut through the upturned sandstone layers. The gorge
just below the center of the picture is over 500 feet deep.
The picture above is some 40 miles south-southeast of
the first picture, and looks westward where the Finke River cuts
across more upturned ridges. (In this view it again flows from
right to left.) The landscape was crumpled many millions of years
ago, and the structural deformations probably date back to the
uplift of the MacDonnell Ranges. A couple of these beveled
crumples can be classified.
The structure in the lower right hand corner is the
beveled remnant of an anticline. In an anticline, the center is
lifted upward, and the overlying sedimentary layers are tilted so
that they tilt upwards toward the center of the anticline.
The next “crumple”, that runs vertically through the
center of the picture is a syncline. Here the layers are bent down
toward the center of the structure. In-between the anticline and
the syncline, the layers dip down sharply as you go from right to
Finally there is another beveled anticline in the
lower left corner. (The portion of the picture in this area has a
different tint and lower resolution.) Once again, the layers were
bent over the top of this anticline similar to the anticline in
the lower right corner.
The structural folds can be traced for many miles
into the distance. The Finke River cuts across these folds since
it was in place first, and has had enough erosion power to keep
its ancestral path.
The view above looks east-northeast in another area
some 125 miles west-northwest of Ayers Rock (Uluru). Another dry
arroyo is cutting through a mountain range. The mountain range is
identified by Google Earth as the Dean Range (part of the
Petermann Ranges), and some searching through atlases found the
name for the small town in the lower left quadrant as “Docker
River”. (A dirt airstrip is somewhat more visible on the far side
of the “river” almost directly above the town.)
The Dean Range is an upturned sandstone ridge that
was bent upward over the top of a larger dome structure centered
some 10 to 20 miles further to the south-southeast (off the right
edge). There is very little left of the dome except slightly
higher ground that serves as a source drainage area for the
“Docker River”, and exposed rock layers that look much older than
the sandstone in the Dean Ranges.
As for the “Docker River”, a close examination of the
Google Earth (satellite) photos couldn’t even find a mud puddle.
However, given the very occasional times the river does flow
combined with millions of years of geologic time, the Docker River
has cut a gap through the Dean Range that is over 1,000 feet deep.
The majority of Australia’s “Ancestral Rivers” are
found in Western Australia. The picture above looks north, and
shows the Coongan River (left) and a tributary cutting through
what appears to be another upturned sandstone ridge north of the
The thin dark line that snakes upward in the right
side of the picture appears to be a volcanic dike. At some point
after the sedimentary layers were laid down, and before erosion
exposed the current surface, there was some kind of force that
pulled the upper layers of the crust slightly apart. Molten lava
seeped upward into the resulting crack, and formed a thin wall of
basaltic rock when it cooled. When it is exposed, it is more
resistant to erosion, and thus produces a small ridge. The dark
rock in the lower right corner looks as though it is part of the
When a mountain range is uplifted, river systems
develop radially outward. They thus ignore layers of sedimentary
rock that were bent up over the rising range. With time, erosion
removes the upper layers, and the lower layers are revealed.
Frequently, river systems are able to maintain their paths, and
cut gaps through the more resistant elements within these
If you stood where the rivers are cutting through the
ridge, you would see a cross section of these resistant layers. In
this particular view, they would be tilted up toward the viewpoint
for the picture, and tilted downward toward the distant horizon.
Fitzroy River (2
pictures), Western Australia
The picture above is a vertical view of a limestone
dome (the Gieke Range) where the Fitzroy River has eroded down
through the dome to form Geike Gorge (Geikie Gorge). The Fitzroy
enters the field of view from the right edge and flows left
through the dome. The small town of Fitzroy Crossing is downstream
off the left edge.
The thin dark line for the river is bigger than you
might think. The area is a local tourist attraction where tour
boats take tourists through the gorge.
The lighter rocks in the picture are Devonian age
limestones - part of an ancient 350 million to 400 million year
old reef system. The uplift history of the reefs (which are on the
southern edge of the King Leopold Ranges) is not known, nor is the
history of the Fitzroy River known. However, the river has cut a
gorge over 100 feet deep in the old limestone reefs.
About 35 linear miles upstream, the Fitzroy has cut
down through other limestone layers. The limestone layers in Geike
Gorge (first picture) are essentially horizontal as they are found
on the top of a dome. The layers in this picture have been tilted
as they are on the southwest side of a syncline. (The axis of the
syncline runs from the top to the bottom of the picture to the
right of the center.)
The tilting has exposed a sequence of layers, and
where these more erosion resistant layers are exposed, they
produce long ridges. The Fitzroy enters from the right edge and
has eroded down through these ridges before it exits the field of
view on the left edge. There are other gorge systems and caves in
these limestone reefs that may be of interest if you ever take a
trip to the area.
Ord River, Lake
Argyle, Western Australia
The picture above shows the extreme north end
of Lake Argyle and its outlet, the Ord River. Lake Argyle is a
large man-made reservoir that backs up from the Ord Dam.
The Ord River cuts through the Carr Boyd Ranges (left
and above the lake) at about 150 feet above sea level. The tops of
the Carr Boyd Mountains are 900 to 1,200 feet above seal level.
There is an alternate path that the Ord River could have
originally taken to the right of the fault (suture) zone (extends
through the center of the picture) where the river would never
have to travel across terrain that exceeded 340 feet above sea
level. The overflow spillway for the lake was built through this
lower alternate route. If you look closely at the extreme north
end of the lake, you can trace the route of this overflow
Why did the river take a path through the higher mountains?
There are 2 possible reasons for the illogical course
of the Ord River. In both cases the mountain route would have been
lower at the time the river established its course.
Case 1) The area to the right of the fault looks like
the soil/rock is less resistant to erosion than the rock to the
left of the fault. Thus it erodes more rapidly. If we ran the
clock backward and replaced everything that was eroded, it’s quite
possible that not too far in the distant past, that this area was
higher than the current Carr Boyd Mountains. This seems possible,
but there are multiple other potential escape routes for the Ord
River that stay under the 900-foot level, and these would also
have to be blocked in the past.
Case 2) A more logical explanation is that the old
fault was reactivated at some time in the last 10 to 20 million
years. In this scenario the current height of the Carr Boyd
Mountains is due to relatively recent uplift. Thus the Ord River
was in place first at a low elevation to the left of the fault. As
the mountains were uplifted, the Ord River eroded away material
that kept getting in the way. Thus it could keep its original
There is a piece of evidence for case two. The Ord
Canyon below the dam has steep sides. This is characteristic of
something that has been cut in the last 10 million years.
Northern Territory, Australia
The picture above looks to the north where the
Victoria River follows a relatively narrow passageway into
and through a low plateau in the northern part of the
Northern Territory. The passageway is nowhere nears as spectacular
as other canyons that we have looked at, and the view has used a
vertical exaggeration factor of 3 to emphasize the relief. The
river elevation as it enters the passageway is slightly under 200
feet above sea level while the top of the very flat plateau is
constant between 750 and 800 feet.
What is of interest is why did the river take this path instead of
going somewhere else?
The terrain to the south (in back of the field of
view) is exceedingly flat. You can travel 100 miles to the south
from the observation point and never exceed 600 feet above sea
level. (Remember the plateau is 750 to 800 feet.) It seems highly
unlikely that this huge area to the south could have eroded down
to under 600 feet while the plateau was essentially untouched by
erosion except in the immediate vicinity of the river.
There is an alternate escape route for the river. You
could go 30 to 40 miles to the east of the observation point and
then follow virtually level ground north into another drainage
system and stay under 600 feet. The river didn’t take that route
either. Instead it went north over a plateau that is currently
more than 750 feet above sea level until you erode down to the
river’s current elevation.
It appears that the river established its path when
the plateau was somewhat lower than it is today. The plateau has
been subsequently uplifted a few hundred feet, but the river was
able to erode down as fast as the plateau rose.
The river never was at the 750-foot elevation of the
current top of the plateau. It was always at its current elevation
of just a couple of hundred feet above sea level. As the plateau
rose, the river simply cut a groove into the rising block of
River/Gorge, South Island, New Zealand
The picture above looks SSW near Queenstown, South
Island, New Zealand. Lake Wakatipu is in the upper right quadrant,
with the lake winding back into the mountains for a considerable
distance off the right edge of the picture. The lake is some 50
miles long and very deep with parts of it as much as 300 feet
below sea level. The yellow line in the lower left portion of the
picture is the Gibbston Highway which follows the Kawarau River
down through Kawarau Gorge.
The surface elevation of the lake is about 985 feet
above sea level. If you follow the valley toward the south from
the far end of the lake (go up in the picture), the maximum
elevation for what would appear to be a normal exit route for the
lake’s outlet never exceeds about 1200 feet above sea level.
Instead, the lake’s outlet is the Kawarau River which drains the
lake starting near Frankton, and then continues left through the
Remarkable Mountains to exit off the left side of the picture. The
Remarkables are over 5,500 feet high in the extreme lower left
corner and over 7,000 feet high near the center of the picture.
Why would the river pick a route that flows through a
mountain range that is a mile higher than the lake level when
present topography allows an alternate route that has a maximum
elevation that is less than 250 feet higher than the lake level?
The answer again appears to be an example of
antecedence. If we go back some 20 (+/-) million years, the
Remarkables didn’t exist. The ancestor of today’s Kawarau River
flowed lazily from right to left in approximately the same path as
the river’s current course. There are even some embedded meanders
in Kawarau Gorge that imply a former lazy path across a relatively
Over the last 20 million years one block of the
earth’s crust has been forced upward to create the Remarkables
mountain range. Another block of the earth’s crust (underlying the
current lake) has sunk to form a graben. (The graben probably also
includes the failed exit route to the south - up in the picture.)
The river was able to maintain its path as it eroded
away the rising material that kept trying to block its path. The
canyon that the river carved through this rising block of rock is
today’s Kawarau Gorge. Meanwhile the bedrock beneath the current
lake dropped far below sea level. It appears that recent
glaciation has scoured out some of the sediments that may have
accumulated in the graben which allows the bottom of the lake to
still be well below sea level. As the glacial ice melted away in
the last 20,000 years, the “hole” filled with water to form
main Ancestral Rivers Page
Web page generated via Sea Monkey's Composer
within a Linux Cinnamon Mint 18 operating system.