Schatkamer (Al-Khazneh), Petra

Petra in Jordan is – quite rightly – listed as one of the new Seven Wonders of the World, alongside the Taj Mahal, the Colosseum in Rome and Machu Picchu in Peru. In fact, Petra is such an important and popular site that it supports the entire Jordanian tourist industry.

The structure known as the Monastery (Al-Deir) high in the mountains above Petra

This ancient city of the Nabateans had been lost and forgotten for centuries, known only by the Bedouins who lived there, until it was rediscovered by chance in 1812 by the Swiss traveller and explorer Johann Burckhardt. Thanks to its remote and almost inaccessible location in a bare and mountainous landscape, Petra still retains its air of mystery. The site can only be reached by walking through the Siq, a 1.2 km long natural gorge between towering cliffs. At times the passage way is only 3 metres wide while the cliffs vary between 90 and 180 metres high. At the end of the Siq, visitors are rewarded by one of the most breathtaking views in the world as the sunlit façade of the Treasury suddenly comes into view.

View of the Treasury (Al-Khazneh) from the Siq, Petra

History

Petra has been inhabited since prehistory. The Nabatean city lies between the Red Sea and the Dead Sea and was an important junction for caravans travelling between Arabia, Egypt and Syria/Phoenicia. The Nabateans were a nomadic tribe who settled here in the 6th century BC. They were accomplished traders who controlled the caravan routes through western Arabia. The wealth they accumulated was invested in the city of Petra.

The buildings in Petra are half built and half hewn out of the rocks, surrounded by mountains full of canyons and narrow passages. In this landscape, formed mainly by red sandstone, the old eastern traditions are blended with Hellenistic architecture. Petra is extremely valuable not only because of the enormous number of tombs, temples and religious sites but also for the remains of water channels, tunnels and bypass dams. At its zenith around the year zero, the population numbered 30,000. The Nabateans developed their own form of writing which is a direct ancestor of modern-day Arabic.

Eroded, striped and coloured sandstone in the Street of Facades

Sometime around the year 100 the trade routes shifted, adversely affecting the city’s prosperity. After the Emperor Trajan annexed Petra into the Roman Empire in 106 AD, Nabatean trade and culture continued to flourish for several hundred years until it declined gradually after the 4th century AD.  A series of devastating earthquakes between the 6th and 8th centuries AD destroyed large parts of the city, after which Petra sank into oblivion until it was rediscovered in 1812.

Colourful sandstone

Petra covers an enormous area which includes not only the valley floor but also sacrificial and other sites high up in the surrounding cliffs, reached by long steep flights of steps. During the 2½ days which we spent here, we walked 73km and climbed up and down more than 2,000 metres.

The colourful mountain of Al-Habis in Petra

For anyone interested in geology, walking through Petra is a fascinating experience with its rocks and cliffs of multi-coloured sandstone – honey yellow, purple, brown and pink. The rocks have been shaped by erosion over the centuries and the walls of the canyon look as if they belong in a chocolate fountain.

Blue-grey, yellow, reddish brown and white sandstone in the remains of a tomb. Al-Habis, Petra

Even more splendid are the brightly coloured sandstone rocks found in many places within the tombs and temples. White, red, blue, yellow, grey – the colours alternate almost rhythmically in bands, in waves and sometimes in swirls. It is like looking at an abstract painting.

Geology

Petra is carved out of two geological sandstone formations: the Umm Ishrin formation of most of the monuments and the Disi formation found in a smaller number of monuments.

Obelisk tomb and Bal al-Siq triclinium, one of the few monuments carved out of sandstone of the Disi formation

The Umm Ishrin formation consists entirely of massive, multi-coloured (yellow, grey, red, brown and mauve-red) fine-, medium- and rough-grained, cross-bedded sandstone. The thickness of the formation is 300-350 metres and it dates from the Late Cambrian Era (500 million years old). The sands were deposited by river systems.

Yellow, white and red sandstone of the Umm Ishrin formation. Al-Habis, Petra

The Umm Ishrin formation consists of quartz sandstone which is mostly cemented together by iron oxides. In fact the formation is not very well cemented and is relatively easy to work with but strong enough to remain stable. This is possibly one of the reasons why the Nabateans chose the Umm Ishrin formation as the site for their settlement.

Spectacular colourful sandstone in tombs between Wadi Farasa and the Street of Facades

Colour variations

The variations in the colour of the sandstone are caused by the composition of the cement, the quartz granules themselves being almost colourless.

  • Light brown-yellow is a mixture of the clear quartz with the dark yellow feldspar.
  • Yellow from sulphur.
  • Pink to dark reddish tints (earth tones) from iron oxide.
  • Blue, dark purple and grey from manganese.
  • White sandstone has no cement, the sandstone is mostly rose-coloured
Colourful sandstone in tombs between Wadi Farasa and the Street of Facades

It is important to understand that the colours were created, after the sand was deposited, by the precipitation of minerals brought here by ground water. This is in contrast to the cross-bedding (criss-cross stratification) structures which were created during the time of deposition. These indicate the direction of flow of the river water which deposited the sand. The colours sometimes run across the cross-bedding structures.

Colourful sandstone of the Al Habis mountain, Petra

Liesegang rings

The regular patterns and rings which are so common – and so spectacular – in the Umm Ishrin formation here, as also in Wadi Rum and Wadi Mujib, are called Liesegang rings.

Liesegang rings in Umm Ishrin sandstone, Petra

The process by which Liesegang rings are formed is not completely understood and is still under investigation. However, the catalyst for Liesegang ring formation is thought to be a precipitation process which is referred to as the Ostwald-Liesegang supersaturation-nucleation-depletion cycle. This process forms a series of rings in the sandstone in a regularly repeated and alternating pattern which owes its colour – or lack of colour – to the presence of particular minerals.

Photo gallery

If you look at the brightly coloured sandstone in detail, you are reminded of modern abstract paintings, as shown in the photos below.

Red with white Umm Ishrin sandstone with fault lines which occurred after the colours had formed
Magenta and white stripes in the Umm Ishrin sandstone
Colourfully striped Umm Ishrin sandstone in Petra looks like an abstract painting
Earth tones in Umm Ishrin sandstone seen while climbing up the long steps leading up to the Monastery.
Colourful sandstone in tombs between Wadi Farasa and the Street of Facades
Colourful patterns in Umm Ishrin sandstone, detail

Text and photos          Jacob Otten

Translation                 Alun Harvey

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