1 Snake Pit
Under the Silurian seas
Start Point – the Car Park
Suggested start and end point for the Wren’s Nest Voyage. DY1 3SB for your Sat Nav.
Leave the car park to your right, walking along Wrens Hill Rd.
The car park here is free, but does close at 4:00pm and is of fairly limited capacity. There are no other official car parks, but with consideration you could park on nearby residential roads, for example, the appropriately named Fossil View.
NOTE: This trail is not the same one as that in the printed leaflet. That trail is shown by blue dots on the MAP along with its Geopoints. It is those Geopoints that are shown by the numbered posts you will come across around the Wren’s Nest area, ignore those when using the Voyager trail.
To the Snake Pit (there are no snakes!)
From the car park, follow the road past The Caves pub and the new houses. Carefully cross the road and walk into the wooded area on your left (use the MAP and photos in Voyager to guide you). When using the app touch ‘+’ for important extra information.
The Wren’s Nest is the worlds first National Nature Reserve.
The Wren’s Nest is a Geosite in the planned UNESCO Black Country GeoPark.
As you walk the app will tell you when you have moved over a different rock, by the sound alert ‘ENVIRONMENT CHANGE’. Touching the THEN control will provide information about the environment that existed here when the rock you are walking over formed millions of years ago.
Currently the app may be playing underwater sounds. Why is that happening?
When you get to each of the 11 sites, make sure you answer the TASK questions for each site.
From time to time you may get a THREAT alert (only when using the Voyager app). You have 60 seconds to respond to the threat or lose a lot of score. To know the best answer to a threat, you need to have looked up under THEN the key information about what things were like when the rock you are over was forming. For example, was there a possible threat from volcanoes, or some dangerous predator of the time.
Entrance to the Snake Pit – Task site 1
The rock currently beneath you is around 427 million years old.
In the Voyager app touching the CLOCK face, will open a timeline for Earth history from the present day until 4.6 billion years ago when the planet first formed. Scroll the timeline to around 420 million years and read off the percentage of OXYGEN in the atmosphere, also the mean temperature of the planet, sea level and carbon dioxide.
Oxygen 17% of the atmosphere (21% at present)
Carbon dioxide 3300 ppm (around 415 ppm at present and rising rapidly) – ppm stands for parts per million
Sea Level around 200 metres higher than today
Mean global temperature 18’C (14’C now and rising)
If you could travel back through time and try to survive in this place 427 million years ago, you would need an extra oxygen supply and in some cases a water supply. The Voyager app provides you with virtual supplies of oxygen and water (you can see the levels by touching HEALTH in the app). These supplies will be draining as you carry out your walk (Voyage) around the 11 sites. The less you can avoid draining your supplies the higher your final score. The FUEL supply is based on your devices battery level. Voyager will trigger alerts when your supplies are getting dangerously low.
Now into the Snake Pit and Task site 1 questions, which can be answered by working through the information Voyager provides for the site, along with your own observations of the rocks and other features of the site.
This is an old quarry where pure limestone was extracted for hundreds of years. A remaining rock face on the left hand side shows this limestone layer. It is called the Upper Quarried Limestone. The pure limestone was used as a building material or burned to make lime for farming, but its biggest use was as a flux for iron making during the Industrial Revolution times.
In the Wren’s Nest area there is a second band of extensively quarried hard limestone, called the Lower Quarried Limestone. When using the app, find these two limestones on the MAP – touch the different colored areas to find the names of the rocks the colours represent.
At the information board in the quarry, look in the opposite direction from the rock face to your left and observe the long narrow trench which disturbs the natural lines of the hill. This is where they quarried the limestone along its outcrop. Look back towards the rockface, see the layers that are steeply inclined and notice that they look different to each other, why do you think that they look different? Have a closer look and see what features you can find in each rock bed.
These layers are a series of limey muds and shelly limestones that formed on seabeds that were once flat (horizontal). In the app select THEN for a picture of how this place may have looked when these rocks were first forming.
Close up you can see many thin beds of limestone with a soft limey pale green mudstone between. What might cause these changes in the type of rock which forms?
Earth when these rocks were forming
This is how geologists believe the oceans and continents were arranged around the time the rocks here were forming. What today we know as England, Wales and southern Ireland were part of a land mass called Avalonia, which lay around 30 degrees south of the equator on the edge of an ocean called Iapetus. The Iapetus Ocean was being slowly closed due to the motion of tectonic plates – vast slabs of continental crust and ocean floor that form the outermost part of the planet.
When the Iapetus finally closed (around 420 million years ago) it fused what today we know as Scotland and Northern Ireland to the northern edge of eastern Avalonia (England, Wales and southern Ireland).
The bentonite layers (fossil volcanic ash) we find in the rocks of the Wren’s Nest would have originated from volcanoes possibly associated with the closure of the Iapetus Ocean, or with another lot of tectonic plate activity stirring to the south of Avalonia.
Closer view of the plate collision
The Avalonia plate is being overridden by the Laurentia plate, with the Iapetus (once 5000 km wide) reduced to a narrow finger of deeper water. The spot that will be Dudley 420 million years in the future, is situated in the shallow continental shelf sea on the eastern side of the Iapetus. The oceanic crust attached to the leading edge of Avalonia is plunging into the mantle below Laurentia where it breaks up and melts, giving rise to volcanic activity in the mountains of eastern Laurentia. Scotland and N. Ireland are part of the Laurentia plate.
By the end of the Silurian the Iapetus was no more and Avalonia formed one large landmass with Laurentia and also Baltica (Scandinavia). Late in the Silurian we see an end to marine rocks and their replacement by the red sandstone river floodplain deposits of the Devonian.
The leading edge of eastern Avalonia had a ‘soft collision’ with the Laurentian plate, unlike that between Baltica and Laurentia and the western section of Avalonia (now across in Newfoundland) and Laurentia. Had the plate interaction been harder, the Silurian rocks of England and Wales could have been crumpled into mountains of Himalayan proportions!
Continental crust is not as dense as oceanic crust, so does not so readliy get pulled down into the mantle (subducted) when plates collide. From about the mid-Silurian the Laurentian plate started to override the Avalonian plate, causing it to depress into the mantle. The resulting depression produced the deeper sea into which the deeper water sediments (muds and silts) were deposited. During the rest of the Silurian this depressed area was nudged south eastward, filling with sediments that eventually were no longer of marine origin. The plate collision event was over, Laurentia was sutured to Avalonia. The next plate collision came from the south and was responsible for folding the rocks of the Wren’s Nest.
Environment for the Wren’s Nest rocks
Zooming in closer, the environment in which the Wren’s Nest rocks formed was one of shallow clear warm seas with many areas of coral reef (patch reefs). Out to the west (about where Wales is now) the sea floor was deeper. In this deeper water no corals lived and mud and silt washed into the sea would build up on the sea bed. Down the slope into the deeper water, slides of sediment scoured out channels, forming underwater canyons.
The rocks of Wales of around the same age as those of the Wren’s Nest (geologists call them the Wenlock Series) are thick layers of mudstone and siltstone, with non of the corals and shells of the creatures that lived in the shallower water. The area shown as land on the diagram, indicates that land tended to be present to the east during times when sea levels were lower (the shape of the land is arbitrary). This land is shown as barren, land plants were still quite ‘new’ with limited colonisation of land. No trees, no flowers, no grasses at this time. For more information about when various plant and animal groups emerged touch the CLOCK in the app, then in the timeline which opens, swipe the grey information panel to the left.
On the diagram the North arrow is skewed to the NE, which reflects that the area (Avalonia plate) was swung around to the north west in the Silurian.
Back to the Snake Pit rocks. The beds deeper down were first to form and are therefore older than those on top. When you look closely (lower photo) at the layers you can see that shelly fossils are present. These fossils are often broken and scattered on the surfaces of rock layers or tumbled and jumbled within the layers. This suggests that they were washed by waves or surges in the sea caused by storms.
Between these hard limestone layers are soft shales and mudstones. These often have very thin layers that we call laminae and have grains that are very small indeed. Such grains would easily be washed away by any water movements and so we believe that they were deposited in quiet still waters between the storms.
In geology we look at the world today to work out how things may have formed in the past. We call this ‘The Principle of Uniformitarianism’. In other words, the way things happen today is pretty much the same as it would have happened in the past – i.e. it is ‘uniform’. When we look at the seas of today we see very fine sediment settling out in seas where waters are too deep to be affected by the waves (below wave base). So we think that the thin sheet like limestones and the muddy layers in between them were formed in waters just below wave base in the seas of the times. These rocks belong to a period of geological time called ‘The Silurian Period’ which occurred between 443 and 418 million years ago.
Youngest rocks of the Wren’s Nest
On the eastern side of the Snake Pit quarry these rock are visible. They sit above the thick Upper Quarried Limestone so must be younger. Geologists name them the Lower Elton Formation and they are around 425 million years old. In the app select MAP, find and touch the Lower Elton Formation, next touch THEN to see a reconstruction of the environment here when the beds of the Lower Elton Formation were bring deposited.
When using Voyager, based on the sound playing (assuming you have not muted audio, or turned off background audio in the HELP section of Voyager), what would you say about the sea depth when the Lower Elton Formation was forming, as against the sea depth when the Upper Quarried Limestone was forming?
You are looking here at the bottom surface of a rock bed, the sediment of which has infilled hollows left in the bed that was once below it millions of years ago. The hollows were formed by sea currents (during a storm surge) scouring out the sediment when it was still soft limey mud on the sea floor.
Route to Task site 2
When using the app, now would be a time to select ‘DO IT’ and answer the TASK questions for this site.
In this case the task 1 question are presented below, but only in the app can you provide your answers, receive a score and gain feedback.
BEWARE: Sometimes you will only be allowed to answer TASKS when located close to the site (the ‘DO IT’ control flashes green when questions can be answered). Once you are done walk back to the road, carefully cross, then go through the gate on the opposite side of the road. Voyager should trigger the Site 2 info as you approach the site.
Task Site 1 Questions
The shape of the Snake Pit location indicates that it was formed by?
a) Erosion in a desert
b) Glacial action
c) Quarrying limestone
d) Volcanic activity
The Silurian Period of geological time is between:
a) 20,000 to 50,000 years ago
b) 100,000 to 1 Million years ago
c) 300 million to 318 million years ago
d) 440 million to 418 million years ago
The rocks here are limestones that formed in a shallow tropical sea. Which of the following things tells us that it was underwater?
a) Fossils of sea creatures in some layers
b) The layers are tilted
c) The layers are both thick and thin
d) The limestone is hard