Hi guys, today I want to continue our journey to Petra, the ancient rock hewn city. This marvelous
creation contains over 800 sandstone caves. While looking at photos of Petra’s beautiful facades,
we notice many caves’ lower portions are more damaged and sometimes we see a horizontal rock
layer has been stripped off. Why are these monuments more eroded on the bottom than on
top? Some have proposed that the severe lower erosions we see at Petra were caused by floods.
Petra is in a semi-arid and mountainous region, and it is susceptible to flash floods like most
dry areas. But are flash floods really the cause of the lower erosions of the caves?
After researching and reading several scientific papers,
I’ve come to the conclusion that floods are unlikely to be the main factor of this curious
lower erosion of the rock structures. The real culprit is something or someone else.
Before I argue further, let’s clarify two important points.
First, is the sandstone in Petra soft and easy to carve? Due to the extreme
age of Petra’s rock caves, they look somewhat melted and soft. But this appearance is deceiving
because almost all caves were carved in real HARD sandstone.
According to papers and reports, which links are in the video description section, Petra has two
major types of stone formations: Disi sandstone and Umm Ishrin Sandstone. The area around the
Visitor Center to the entrance of the Siq, is made of Disi sandstone. This layer is hard,
pale gray and usually erodes into dome-like shapes. The Siq and all of the famous monuments of
Petra, from the Treasury, the Street of Facades, Royal tombs all the way to the Monastery were
carved out in Umm Ishrin Sandstone. This darker colored, iron-rich sandstone is easily recognized
by its beautiful, abstract painting like patterns. Geologists have found this stone to be harder and
more erosion resistant than the lighter colored Disi formation. The Umm Ishrin sandstone is mostly
composed of medium to coarse grained, well-sorted quartz in percentage as high as 80%, and contains
layers with amounts of hematite cement as high as 50%. High quartz content indicates high hardness
and the abundant iron oxides in this stone are not only what made the iconic red color of Petra,
but also work well with silica in the sandstone matrix which dramatically decrease weathering.
Sandstones usually range from 6-7 on the Moh’s scale on hardness. In comparison, the ancient
Greeks and Romans mostly used marble which is a much softer stone, rated 3 on the scale.
Since sandstone in Petra has high quartz content I would say this red sandstone
will be very close to a 7 which signifies it’s almost as hard as granite and steel.
That also means the creators of Petra planned and carved their magnificent work in the hardest stone
in the region. This is remarkable. Stronger stone is much more arduous to work with – you can ask
any stonemason who will tell you there is a huge difference between working on marble and granite.
Durability and workability of the stone are on the opposite spectrum. Harder stone requires
better tools along with much more force. It’s interesting that the ancient people apparently
weren’t dismayed by the difficulty of the work because they would rather make extra efforts
to ensure their incredible stone creations can withstand the test of time.
Their ingenuity and hard work paid off because Petra is a timeless masterpiece for millennia.
This culture valued beauty and quality of work and so deserves our deepest respect.
So, Petra’s gigantic caves are carved in sandstone with a similar hardness of steel.
The mainstream academia suggests these caves were created by the Nabataeans over 2,000 years ago
during a time when there was no steel. Even if steel tools existed then, it would be impossible
to carve the stone which is basically the same hardness. Assuming the ancients somehow had hand
tools made of materials harder than steel, there still would be no reason for masons to exert
themselves to dress the enormous caves such as the Urn tomb, from floors to ceilings in precise,
uniformed parallel markings. What's the point or necessity of this? In my last video,
I presented close-up photos of the Treasury’s facade showing distinct machine-like markings,
if you want to check out that link is at the upper right corner. These intriguing, tightly
placed markings can be found in other caves as well, such as the column capital of the Monastery,
and even in a small cave where tourists can stay over in Little Petra. These delicate and
accurate markings are consistent and uniformed, looking very much like today’s machine marks.
Now let’s talk about the floods in Petra. The city of Petra is hilly and rugged
and lies at an average elevation of 820-850 meters above sea level, whereas surrounding mountains
rise up to some 1400 meters. The site is in a semi arid area which gets 45 centimeters or 18”
of rain on average per year. Most rain falls in winter months and heavy storms sporadically
happen which lead to flash floods. Even though the sandstone in Petra is strong and hard to erode,
over a long period of time eventually the water wins. Flash floods created Petra’s canyons
millions of years ago. The entrance to the city, the narrow winding gorge of the Siq
is a flood channel that started with a crack that split and deepened over time.
On this map, the darker areas are mountains and the lighter colors represent low grounds.
The paths that tourists walk today are wadis; wadis are dry river beds or valleys which might
get flooded when serious storms inundate the region. When it rains in Petra, the water would
flow down towards the center valley and then drain out through Wadi Siyyagh to the west.
That being said, I want to point out that Petra caves are located
along cliffs on various altitudes; many are higher up and above the flood zone.
Flash floods might impact some lower-laying caves but shouldn’t have affected caves that are located
higher than the floodway. However, the monuments which are way above the flood zone such as the
Urn tomb and the Palace tomb still show the worst damages at the lower portions
such as the doorways. Since they were not flooded such damages or erosions are not caused by floods.
Also, at the Siq there are sculptures carved on the side missing upper bodies while the
lower feet are preserved. If the erosion was caused by floods, then we should have seen the
opposite effect wherein the feet are eroded away and the upper bodies remain intact.
So floods are not the main factor which caused these lower portion damages. Then what was the
reason? For decades, groups of researchers have worked on in situ investigation and lab analysis
to study the weathering pattern on the rock-cut monuments in Petra. There are two primary
categories which affect the deterioration rate in stone: intrinsic effects and external influences.
Intrinsic ones are related to the characteristics of the material itself. We know that Petra’s
caves were mostly carved in hard, quartz rich sandstone. Now let’s see the external elements
that damage the stone. They include weathering such as exposure to wind and sunlight, climate,
earthquakes, running water, and human contact. Less exposed areas would be less affected by
heating and cooling cycles and wetting and drying cycles thus better preserved. For example,
the Treasury is tucked into a narrow valley with towering cliffs therefore protected from gusting
wind and brutal solar rays; leaving its exterior in great shape, while the Royal tombs are out in
the open and exposed to elements which over time made them appear melted. One side of
the Palace tomb is usually in the shade because of a slight setback from the Corinthian tomb;
this small difference has helped protecting the original stonework in the shaded area.
Researchers also observed that northern facing facades were the least weathered
while western and eastern ones showed greatest surface recession instead of the southern ones.
Two groups of published researchers have commented as follows: These differences were attributed to
the different microclimatic conditions of the orientations, mainly greater daily variation of
surface temperatures on western and eastern facades. These west and east faces had been
subject to both high surface moisture (from morning dew and rainfall) and high insolation
exposure such as half-day sunlight and heating, summer surface temperatures exceeding 50˚C,
whereas the southern aspects received full-day sunlight, but less surface moisture.
The upper and lower portions of a monument’s facade usually have
endured the same degree of solar, wind, wetting and drying process.
This leads us to the question, then why do we see more deterioration at the lower
portions of the facade? If these were not caused by floods, then what was the reason?
Besides natural causes, there is one critical factor that has greatly damaged Petra’s
monuments – the human factor. For one thing, Petra is one of most famous tourist sites in the world
and has attracted a great flow of visitors each year. When people are admiring the
fantastic monuments, they might intentionally or unintentionally touch the stone surfaces.
There is a reason why the Treasury is now closed to entry because guards, tourists and tour guides
were leaning, touching, rubbing and propping their feet against the chamber walls and these
contacts over time produced small loose sand piles at the base of the walls. Geomorphologist Thomas
Paradise’s team have monitored the Treasury’s interior and found that where the tour groups
usually sit have led to a loss of half a cubic meter of sandstone over just a few
years. The most receded area was the portion most frequently leaned against and touched by visitors.
Where do people most likely touch and lean? The doorways. Everyone enters and
exits through doorways. Guards and police officers stand near the doorways. A building’s entrance
usually has the most wear and tear. As a result, we see many Petra caves, especially the big
and well known ones which would have received more visits in both ancient and modern times,
showing the most damages at the doorways and lower portions on the exterior. It’s possible that these
areas have not only been leaned on by humans and animals through thousands of years, but also used
as walls to support tents or other structures in ancient times. Btw tents are still widely-used
by today’s local Bedouin tribes in the larger Petra area. Therefore, I think human contact is
the real culprit of the greater erosions at the doors and lower areas of Petra’s caves.
On another note, the expansion in the development of tourism amenities and services
and the local population increase, have resulted in rapid urban growth. The loss of soil causes
less water retention in the upper slopes thus increasing the chances of floods downstream.
The combination of weathering, earthquakes, flash floods, and hundreds of thousands of
human visitors per year is gradually wearing away Petra’s rocks. Eventually erosion will
remove this city which would be an unthinkable tragedy. We really need to rethink in what
manner we ought to protect and preserve this wondrous rock city before it is lost for good.
Other than educating tourists to resist touching the precious cave facades and interior,
I think it would be beneficial to restore and rehabilitate the original water system.
This may help in saving the monuments in the long run. Even though flash floods might
not be the reason for those lower cave erosions, they are still a real issue.
The recurrence of flash floods over the past few years has made an astounding impact on
Petra and its monuments. Flash floods even occurred in front of the Treasury.
The flooding problem we have today probably wouldn’t happen if the original water system
was still working. For instance, there once was a designated dam at the entrance of the
Siq to block floods, meanwhile the water was diverted through a channel Wadi Muthlim,
which is also called the little Siq. If the dam at the Siq entrance was intact and the little
Siq tunnel was maintained clear, there might not be floods coming down to the Treasury because the
water could then be directed to completely bypass the Treasury and Street of facades.
This ancient water system contains rock cut channels, dams, cisterns and water conduits
to control and collect annual flash floods. It was essential to the livelihood of the
ancient desert inhabitants and provided water for people’s daily use, irrigation and agriculture.
It also prevented floods damaging the rock hewn city by controlling and diverting water.
Unfortunately, this ingenious system broke down due to a major earthquake over 1,600 years ago.
The city declined and people moved out. Without any maintenance, Petra’s water system crumbled;
dams collapsed; conduits and tunnels clogged and cisterns filled with sediment and
mud. This highly sophisticated engineering feat no longer diverted water away from the caves
or properly irrigated the site; this likely changed the regional microclimate and created
more polarized temperature and humidity throughout the day
which might have made the monuments more susceptible to accelerated weathering.
Restoring Petra’s water system might offer us a better opportunity to protect the city from
rapid deterioration. Now, for my final remark. The other, more exciting reason
of cleaning up the floodway in Petra is there might be an entire hidden level
submerged beneath centuries of sediments, which accumulated after the failing of the water system.
It’s quite obvious that the Siq and the outer Siq are covered with sediments.
This thick layer is substantial because we can see certain areas where it covers
up the original doorways of some caves at the outer Siq. The original ground and path could
very well be several meters lower. What might be still buried down there? Imagine what could be
hiding right underneath people’s feet? Don’t you wanna to know? If you have any insights,
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to share with you. This is Curious Being, I’m Tina. Thanks for watching and see you next time.