INTRODUCTION
Aloba Arch, in addition to
being an unusually large and magnificent arch, seems to have a very
unusual origin as well. Aloba Arch is located in the remote Ennedi Range
of the Sahara Desert in the country of Chad.
Attempting to classify a natural arch without actually examining
it first hand in the field is fraught with risk. Photographs focus in
on certain aspects of the arch and filter out others. Shapes and spatial
relationships are distorted. Key attributes may be hidden from view.
Some feature in the photo might appear to be one thing, but actually
be something quite different. Distances and scale are easily misinterpreted.
Therefore, it is with considerable trepidation that I attempt to
classify Aloba Arch here. I have never visited the arch and can only
draw conclusions from the photographs and field data of others. Nevertheless,
the set of still photos, videos, and other data that is available to
me for interpretation paints a rather consistent picture. Therefore,
I am sufficiently confident in what follows to risk the possibility
of being proven wrong by a hoped for closer examination at some point
in the future.
My classification of Aloba Arch uses the genetic taxonomy that was
partially developed by the Standards and Definitions Working Group (SDWG)
of the Natural Arch and Bridge Society (NABS) in the early 1990's, and
is being gradually completed by me. I have documented this taxonomy
on this website (Arch Info). This taxonomy
is based extensively on the three references
listed on this website.
Since I have chosen to use a genetic taxonomy, what follows is mostly
a description of how the arch formed. It includes identification of
the erosional processes that led to arch formation, the processes that
governed subsequent development of the arch, and the sequence these
processes followed. This scenario is, of course, hypothetical. It is
of necessity sketchy and not very rigorous.
The field data on Aloba Arch that I examined are the still photos,
video, and maps provided to NABS by various members. In addition, I
consulted two technical papers on the general geology and climate history
of the Sahara. Although these provide some background information, they
may or may not apply specifically to the Ennedi region.
OBSERVATIONS
Any classification of a natural arch must be based on its observable
attributes. The attributes that contribute most to a genetic classification
are the contextual, morphologic, and geologic attributes described on
the website given above. Using this framework, here are the relevant
observations I extracted from the examined data.
Aloba Arch is located at the SE extremity of a sandstone outcrop.
The sandstone that makes up this outcrop appears to be layered in two
strata. The bottom layer appears to be more heavily cross-bedded than
the top layer. The boundary between these two strata is at about the
vertical mid-point of the arch. I do not know if these two strata are
different formations, different members, or just different layers of
the same member. However, nearby outcrops that consist only of the lower
layer of sandstone appear to have eroded much more than this same layer
in the outcrop capped by the upper layer. This suggests that the lower
layer is softer than the upper layer.
The top layer of sandstone has been fractured into a dense field
of parallel fins similar in appearance to the "Behind the Rocks" area
southwest of Moab, Utah. The top of Aloba Arch, the lintel, is in one
of these fins. If the joints that separate these fins extend down into
the lower layer of sandstone, they have not widened in that layer as
they have in the upper layer. In other words, the gaps between the fins
only appear to extend down through the upper layer to the top of the
lower layer. This suggests that the upper layer has been exposed to
erosion for a much longer time than the lower layer, especially if the
lower layer is indeed softer than the upper layer.
The fins all run in a NW to SE parallel direction. At its SE extreme,
the field of fins narrows to a point. The fin that the arch is in is
the SE-most fin in the field and has no flanking fins. This fin, including
the SE abutment of the arch, projects into a bowed valley that has the
appearance of an abandoned stream meander. Along the SW flank of the
sandstone outcrop is an incised drainage valley.
The SW bank of this valley consists of heavily eroded outcrops of
the lower layer sandstone intermingled with sand dunes. Of course the
NE bank consists of the outcrop capped by fins. The fact that the upper
layer is ubiquitous on one bank and absent on the other suggests that
the drainage valley was incised into the lower layer only as it became
exposed to erosion through uplift, and that this drainage did not exist
during the time when only the upper layer was exposed, i.e., when the
fins formed. The drainage now empties through the base of the arch into
a sand flat.
The lintel of the arch is in the shape of a half catenary and is
arched both on top and bottom. The sides of the lintel conform to the
sides of the fin the arch is in. The opening of the arch is an upright,
oval aperture with the long axis of the oval aligned with the vertical.
The height of the opening, approximately 90 meters, is remarkable for
more than its sheer magnitude. It is very uncommon for a large upright
aperture to have a height twice its span. This alone suggests an unusual
history for the arch.
Interestingly, if you divide the aperture in two along the short
axis of the oval, i.e., horizontally by extending the boundary between
the two sandstone layers, and consider these two halves of the opening
separately, you are left with two large, but otherwise unremarkable
openings. The top half of the opening is a half semicircular aperture,
while the bottom half looks like a cross-section of the drainage valley.
Viewed separately, either would have to be considered fairly common.
But together as a single aperture, the opening is extremely unusual,
perhaps unique.
All this data, taken together, is very suggestive of how Aloba Arch
came into existence. It's unusual morphology is likely the result of
a coincidence. This coincidence led to the sequential combination of
two processes that normally do not appear together in the genesis of
an individual natural arch.
GENESIS SCENARIO
Geologically, Aloba Arch is a very recent phenomenon. However, the
sandstone it is in is very old. The scenario below attempts to explain
the observed attributes of the arch, while remaining consistent with
my sketchy understanding of the geologic and climatologic history of
the Ennedi area.
Between 450 and 350 million years ago (mya), the two layers of sandstone
were laid down in a continental shelf. This may have occurred during
two separate depositional eras or during a single continuous one. Regardless,
the result was a layer of harder sandstone over an older, softer layer
of sandstone. In subsequent eras, these layers were buried beneath various
younger strata.
Roughly 70 mya, volcanic doming occurred in this area. The flexing
of these two layers, still buried under other strata, ranged from severe,
over the central parts of the domes, to negligible, between the domes.
In some places near the edges of these domes, mild flexing led to a
series of long, parallel, evenly spaced, vertical joints in these sandstone
layers. Where there was severe flexing, these layers became highly fractured.
About 6 mya, a period of general uplift and erosion began in this
region that continues to today. About 30 thousand years ago (kya), erosion
reached and exposed the upper layer of sandstone. This was during the
last ice age. Glaciers covered much of Europe to the north. The climate
of the Ennedi, although south of the ice pack, was cold and arid. Nevertheless,
as uplift exposed more and more of this upper layer, three rates of
erosion resulted. Where doming had caused it to be highly fractured,
erosion was rapid and complete. In many places the layer disappeared
altogether. Where doming had not occurred, the layer was very resistant
to erosion and remains visible today. Where parallel vertical joints
existed, water seepage, freeze expansion, and weathering widened these
joints creating the field of sandstone fins that contains Aloba Arch.
About 15 kya, a buttress
natural arch formed in the SE-most fin of this field. By the time
it reached a mature stage, this buttress natural arch resembled modern
day Corona Arch both in general appearance and size. Compression strengthening
made its lintel very resistant to subsequent erosion. The top half of
Aloba Arch had formed.
Uplift and erosion continued. About 10 kya, the coincidence occurred.
Just as erosion began exposing the top of the lower layer of sandstone,
the climate of the Ennedi changed dramatically. For the next four thousand
or so years the Ennedi was warm and wet. Drainage channels became permanent
streams. Where these flowed over the newly exposed softer layer of sandstone
they incised into that rock as it was uplifted. One such stream formed
along the SW flank of the outcrop of fins. As the stream cut downward
into the softer layer, the harder layer above it stabilized and protected
the NE bank of the stream. The opposite bank, lacking this protective
cap, eroded faster.
For the first several of the 40 centuries that the wet climate lasted,
this stream flowed around the fin that contained Aloba Arch. At some
point, however, it found and widened a passage through the softer rock
under the arch. Eventually, lateral stream piracy diverted the stream
through the arch. Aloba Arch became a meander
natural bridge!
Uplift and erosion continued. By about 6 kya, the climate of the
Ennedi began transitioning to what it is today. At that time, the upper
half of Aloba Arch looked pretty much like it does today. The lower
half of the opening, however, looked quite different.
Between the time the stream diverted through the arch (approximately
8 kya) and the time the climate began to change to desert (6 kya), the
volume of water carried by the stream was pretty heavy. The stream filled
the base of the opening. But as the climate became more arid, the stream
diminished. Erosion became more pronounced toward the center of the
streambed. As uplift combined with stream erosion to lower the base
of the natural arch, the abutments broadened back toward the center
of the stream. There was less water flow, hence the channel narrowed.
By about 3 kya, desertification was complete. The climate was not
much different from that of today. Still, there was enough occasional
flow in the stream to keep lowering the base of the opening as uplift
continued. During this last short period of a few thousand years, dunes
crept into the area and weathering polished the sandstone more. Today,
Aloba Arch is the result.
Does this scenario pass any reasonableness tests? For example, could
uplift and stream erosion cut the lower half of the opening in the time
that the scenario allots, about 8,000 years? Uplift rates for the Ennedi
would have to be about 4 to 6 mm per year for this to have happened.
The only information I have been able to find for the eastern Sahara
ranges from 1 to 10 mm per year. Without specific data for the Ennedi,
all I can claim is general consistency.
How likely is it that lateral stream piracy would re-channel the
stream directly under an existing opening? Isn't this asking a lot of
coincidence? The answer is that it wasn't just chance. The weight of
the existing buttress arch would have preferentially compressed and
strengthened the lower, softer sandstone layer that was directly under
its abutments. The sandstone directly under the opening would not have
been compressed and would therefore have been more likely to experience
the seepage that eventually led to lateral stream piracy.
The scenario requires that the buttress arch formed about 15 kya,
and that the lintel experienced little change over the past 10 thousand
years while the lower half of the opening was cut. Is this reasonable?
An age of 15 thousand years is consistent with age estimates made for
the large sandstone arches of the Colorado Plateau. Without more detailed
information about the specific sandstone formation/member involved,
it is reasonable to assume that Ennedi arches last about as long. Further,
the shape of the lintel is quite strong structurally and is the result
of compression strengthening. Therefore, the lintel is much more resistant
to erosion even than the fins in the same layer to the NW of the arch.
Once a buttress natural arch has formed, the subsequent development
of its lintel is much slower. Here again the scenario seems reasonable.
CLASSIFICATION
Even if the above scenario is essentially correct, does that necessarily
imply that Aloba Arch should be classified as a meander natural bridge?
Why not classify it as a buttress natural arch or as a hybrid of the
two types? These questions are discussed next.
We start by pointing out that, regardless of the correctness of
the genesis scenario, Aloba Arch must be classified as one of the following
four possibilities:
- Buttress Natural Arch
- Meander Natural Bridge
- Hybrid of above two types
- Irregular Natural Arch
No other option makes any sense at all. Unfortunately, there are
problems to varying degrees with these four choices as well.
If one does not accept the above genesis scenario, then Aloba Arch
must be classified as an irregular natural arch. As discussed in the
following paragraphs, its observed attributes are not fully consistent
with either the buttress type by itself or the meander type by itself.
One is left with having to speculate that some unique, complex, and
undetermined set of erosional processes led to its formation. But if
one does accept the genesis scenario, resorting to the irregular type
in the taxonomy is unnecessary. Unless the scenario is shown to be fundamentally
incorrect, classification based on the scenario is preferable to an
alternative that amounts to giving up on trying to explain how the arch
formed.
Classifying Aloba Arch as a buttress natural arch is problematic
at best. Although the lintel is very consistent with this type, the
lower halves of both abutments and the opening are not. Both abutments
broaden outward significantly toward the base. The opening narrows toward
the base and becomes much less aperture like. It is hard to believe
that wall collapse, the key process of formation for a buttress natural
arch, could account for this morphology. Wall collapse requires the
wall to be uniformly thin. This is only true of the top half. Wall
collapse usually acts to maintain a catenary or half-catenary shape.
Again, this is only true of the top half.
Classifying Aloba Arch as a meander natural bridge also has at least
one problem. Although there appears to be an abandoned meander and lateral
stream piracy, it is also very obvious that the sandstone fin was not
formed through this process. Rather, it is one member of a field of
fins that formed in some global way, most likely the way described in
the scenario. That there was a stream meander around the fin must be
viewed as a coincidence, not as cause and effect.
At first, classifying Aloba Arch as a hybrid of a buttress natural
arch and a meander natural bridge seems to have a great deal of appeal.
But there is a significant problem with this approach as well. Although
the genesis scenario strongly suggests a hybrid classification, the
taxonomy does not allow this, and for good reason. Once hybrids are
allowed in a genetic taxonomy its utility becomes virtually nil. Almost
every natural arch will show some evidence for multiple genetic processes.
Thus, most of them would wind up being classified as a hybrid of two,
three, or more types. This would be confusing at best. It would certainly
obviate the primary benefit of having a taxonomy in the first place.
To avoid this, the taxonomy requires classification in accordance with
the principle or primary process of formation. Hybrids are not allowed.
Of all the choices, classifying Aloba Arch as a meander natural
bridge seems best. This choice has the fewest problems. Moreover, it
best describes the current state of the arch, i.e., what lifecycle the
arch is following now. Having been a buttress natural arch before it
was a meander natural bridge makes Aloba Arch very unusual, perhaps
unique, but does not really change it from being a meander natural bridge
now.
CONCLUSION
Based on the whole set of data available to me, including geology
data on the eastern Sahara and copies of photos, videos, and maps of
the arch and its immediate surroundings, I have hypothesized a genesis
scenario for Aloba Arch. In essence, this scenario claims that the arch
began as a buttress natural arch, and remained that way for about the
first half of its life. About 10 kya a coincidence resulted in the transformation
of this arch into a meander natural bridge. These classification types
are defined in the standard taxonomy referenced above.
If the scenario is correct, Aloba Arch is now best classified as
a meander natural bridge. If the scenario is incorrect, Aloba Arch does
not fit into the standard taxonomy other than as an irregular natural
arch. Again, these conclusions must be qualified since they are not
based on firsthand field observations. I would be happy to personally
validate them should funding for such an expedition be found. I welcome
all comments, questions, complaints, contrary opinions, and, of course,
contributions.
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