Geology: the foundation of #Civilizations, Petra @McrMuseum

One of my highlights from the first episode of the stunning new BBC Civilizations series (available on BBC iPlayer) was Simon Schama’s visit to Petra.

Image Wikipedia Berthold Werner

This beautiful building, created during the 4th century Nabataean Kingdom was only possible because of the  the properties of the sandstone it is carved into.

Most buildings are made from rock that has been excavated, shaped into blocks which are then stacked to make walls. Petra is different, the columns, doors and rooms have all been excavated from the bedrock in the side of the gorge.

The Nubian Sandstone

The orange-brown sandstone at Petra was formed over a range of different time periods (from the Cambrian to the Upper Cretaceous) and was formed in a variety of different conditions from desert to shallow seas. The distinctive colour comes from the iron minerals that have reacted with oxygen forming the rusty-red.

Sand from Petra

The sandstone is very similar to the Triassic sandstone found in Cheshire and north-west England. This formed during desert conditions. The remains of the sand dunes can still be seen where the rock is exposed at the Stockport railway viaduct (on the M60) and the railway cutting going into Liverpool Limestreet station.

So what made it possible to create this building at Petra?

Not too hard, not too soft

Like the story of the Three Bears and the porridge, the sandstone here is just right. The strength and hardness come from how strong the cement is that holds the grains together to form the rock.

Too soft and it crumbles into sand. Softer parts of this sandstone have eroded away over the years and now form much of the sand of the Arabian Desert. Too hard and it is difficult to chisel out to form features (such as the details at the top of the columns) and rooms.

Strength

In contrast to buildings made from blocks of rock, Petra is pretty strong when under attack from armies, earthquakes and the ravages of time. Conventional block built buildings inevitably have points of weakness where the blocks meet.

It looks nice

One of the marks of culture and civilization is the desire to be surrounded by beautiful things. I guess it is a matter of personal taste, but the sandstone is a beautiful rusty-red colour reflecting the iron-minerals coating the quartz grains in the sandstone.

Right place at the right time

Petra was on a key trading route. It sits in a narrow sheltered gorge with little room for conventional buildings. The obvious thing to do is tunnel into the soft rock.

I didn’t want to miss an excuse to include a clip of one of my favorite films: Indiana Jones And The Last Crusade. Good opportunity to look at the Nubian Sandstone 🙂

Lamp from Petra, Manchester Museum