THE CHANGING LANDSCAPE L1
THE HISTORY OF THE LANDSCAPE – the geology
It is hard to believe that 570 million years ago at the start of the Cambrian period the Ironbridge area was a shallow sea with only the Wrekin (it had no name then) emerging as an island above sea level.
In the Ercall Quarry by the Wrekin the remains of “ripple marks” can be clearly seen evidence of estuary tides and the sea. Over the next 225 million years the landscape continued to change; there were warm shallow seas filled with sea creatures, deserts, sandbanks, mountain ranges and even volcanic activity. The evidence of these events can be seen in the local rocks and fossils such as trilobites (a lobster like creature), corals and shellfish. During this time Wenlock Edge was a shallow sea very much like the Great Barrier Reef found off the coast of Australia today.
Around 345 million years ago things really started to change. It became warmer and the sea retreated to leave low lying swamps and land only a few metres above sea level. This is the time of great forests and swamps, very much like parts of the Everglades in America today. This was the start of the Carboniferous Period.
Coal swamp in the Carboniferous Fossil Trilobites
In these swamps dense forests began to grow. It is from the vegetation of these warm, damp jungles that we get much of the coal that was used at the start of the industrial revolution. Over millions of years the rotting vegetation compacted into peat then coal. Many layers of sands & muds can be found between the layers of coal, as the area was repeatedly flooded by rivers and the sea. It is these layers of sand, clay, coal and ironstone that provided the rich raw material for the development of iron at what is now Ironbridge.
The final major event that changed the area occurred towards the end of the Ice Age, less than 1 million years ago. Traveling east from Buildwas towards Ironbridge you start to see the spectacular narrowing of the Severn Valley as the river starts to enter a gorge. The gorge is thought to have been cut by the discharge of ice melt waters from a large lake (named Lake Lapworth) that was dammed by ice sheets that once existed in north Shropshire & further north and as a result a new route was provided for the now river Severn. It is useful to mention that serious landslips have taken place along the gorge in recent years particularly at Jackfield.
A GEOLOGICAL MAP OF THE IRONBRIDGE AREA.
KEY
Youngest Sedimentary Rocks / .d6 / COAL - Shrewsbury Coalfield (Carboniferous)
d6 / CLAY - Etruria Marl Clays (Carboniferous)
d5 / COAL – with IRONSTONE, sands & clays (Carboniferous)
d2 / LIMESTONE- (Carboniferous)
d2 / SANDSTONE - Lydebrook Sandstone (Carboniferous)
b7 / SHALE – Ludlow Shales (Silurian)
b6 / LIMESTONE – Wenlock Limestone (Silurian)
b6 / SHALE - Wenlock Shale (Silurian)
b5 / SHALE - Hughley Shale (Silurian)
b5 / SANDSTONES & GRITS (Ordovician)
a3 / SHALES - Sheinton Shales (Cambrian)
Oldest Sedimentary Rocks
Igneous Rocks / . .' .'." . ' . / BASALT & RHYOLITE TUFFS
The numbers in the key are from a geological map to show layers & bed changes.
Wenlock Limestone Shale
Carboniferous Fossils Carboniferous Coal
Ironstone Ironbridge in the Ice Age