The Iron Bridge - A World First
The Iron Bridge, named after its cast iron construction, is the first bridge of its kind ever built. Prior to the bridge’s construction, the only way across the river was by ferry. The bridge is still in use today and has been since its construction in the late 18th century. No vehicles are allowed to cross it, though pedestrian and tourist traffic is permitted.
The Iron Bridge is an arch bridge, made completely with cast iron with sandstone abutments at either end. Previously, cast iron was too costly to produce, but in the late 1700’s it became more affordable. In the case of the Iron Bridge, it was especially easy to bring the iron to the construction site due to the abundance of raw materials found in the Gorge and the availability of a furnace located very nearby. Though the bridge is quite short – only two hundred feet long and sixty feet tall - it has five arch ribs.
The original bridge architect, Thomas Pritchard, died soon after work had begun on the bridge in 1777. Making do with who was available at the time, control of construction was transferred to Abraham Darby III, who was an iron master at the foundry his grandfather first owned. He was in part commissioned for the project because the iron was cast at that very foundry.
In total, the bridge used three hundred and seventy nine tons of iron, at an expense of seven British pounds sterling per ton. Darby took all of the original and additional unexpected costs on himself and the company, causing him to live in debt for the rest of his life. Future iron bridges were made with far less cast iron at less cost, making the first iron bridge unique.
The Sandstone Abutments
The bridge itself is supported by sandstone abutments on either bank. The one on the north (Tontine) bank features a brick-lined tunnel to allow pedestrians through. The sandstone used for the abutments was sourced from the nearby quarry in Ladywood, a little further along the river on the south (Tollhouse) bank. The sandstone comes from the Little Flint Coal Measures and is whitish in colour. Many of the bands of sandstone found between the coal measures were used in local building projects.
Sandstone is a sedimentary rock. It is formed from layers of sand grains which were deposited in the sea, in rivers or in deserts and later cemented together. Sandstone is very porous and therefore water can penetrate it easily. It is therefore susceptible to weathering. The stone weathers best when it is naturally-bedded, ie the end grains face the weather. (Imagine looking at a pile of paper end on, so that you can see all of the pieces of paper). Face-bedded stone weathers much more poorly and is subject to much greater deterioration. Water damages a face-bedded stone by causing entire sheets of sandstone to flake off. Also, freeze/thaw cycles allow water to get into the stone and then freeze and expand, causing some of the top layer to split off.
Your tasks
To complete these tasks you will need to go to the coordinates at the pedestrian arch on the north (Tontine) bank and stand next to the large wooden arched doors (on the power station side of the bridge). You will need to examine the stonework around the pedestrian archway, shown on the picture below.
Look at the archway and examine the stones (which are numbered on the picture above). Some of them have been naturally-bedded and some face bedded. If your GPS doesn't support pictures, the stones of the arch are numbered 1-13 from left to right, with the keystone being No 7.
- Which of the archway stones have been face-bedded? (Use the numbers on the picture to explain which stones.)
- Describe the damage to these stones and how it differs from the naturally-bedded stones.
- What do you think has caused this damage?
- Walk through the tunnel to the other side of the sandstone arch (on the War Memorial side). Describe how the stonework appears on this side. Is it in better or worse condition than the first side you examined? How can you explain this?
- As an optional extra, post a picture of you and/or your GPSr at the Iron Bridge.