Just when you thought that the Romans were ancient history enough for this area, come with me on a walk into the deeper history of the site of Hadrian’s Wall…
AD122, and the hills are ringing with the sound of Roman legionnaires constructing the Wall, quarrying the stone from small local quarries and making use of natural features such as the Great Whin Sill to aid their defensive positions.
But let’s journey back to another time when there was a lot of life in this place, back to the Carboniferous Era, between 360 and 300 million years ago. At this point in time, this area was much nearer the equator and underneath a shallow tropical sea. Corals were common, and their skeletons would form limestone. Changes in climate would lead to layers of sandstones, shales and limestones being deposited.
Fast forward to the end of the Carboniferous period and we find the layers of sedimentary rock being tilted southwards. Magma is intruded between some of the layers. Over time it solidifies to form the hard dolerite that makes up the Great Whin Sill. The Great Whin Sill (as also described in the nearby GC1YH9E, and at GC3KGB1) was formed by dolerite intruding into Carboniferous limestone, sandstone and shale, around 300 million years ago. The dolerite is harder than the other rocks, so has been left exposed after the others have been eroded.
Fast forward for the last time to the Quaternary (around 1 million years ago) when the area was much quieter, and covered with ice. The massive erosion forces scoured the softer limestones and shales, leaving the harder Whin Sill exposed. As the ice retreated, around 12,000 years ago, enormous amounts of meltwater flowed from the glaciers, carrying sediment that is laid down across the north of England. These huge fast-flowing rivers did what the glaciers were unable to do and cut through the dolerite of the Great Whin Sill. The steep-sided meltwater channels can be seen in various places including Sycamore Gap.
This cache requires visiting two sites which can be reached from the Steel Rigg car park (N 55 00.110 W 2 23.269), or you can park at the permissive car park at (N 55 00.539 W 002 23.446). The first site requires walking on rough ground with no marked paths and then on a boardwalk.
Please keep to the boardwalk at all times, and take particular care when it is wet or flooded.
The first location is Greenlee Lough.
A National Park information sheet can be found here. This walk is described in more detail on the sheet, which is also available at the Information Centre at Once Brewed (N 54 59.447 W 002 23.189).
A “lough” (pronounced loff) is the local word for a lake (compare the Scottish “loch”). Glaciers in this area moved from west to east, exposing the Great Whin Sill and accentuating the cuesta landforms. A cuesta is a ridge with a steep slope (escarpment or cliff) on one side and a gentle (dip) slope on the other. They are often formed by sedimentary rocks that have been tilted, and then the softer layers erode faster to create the dip slopes.
Question 1: Identify the location of a cuesta landform elsewhere, near your home location if possible.
The movement of the ice eroded the softer rocks, forming hollows which have filled with water to become loughs. Greenlee Lough is only 1.8m at its deepest point. Shallower hollows have wet, acidic conditions loved by mosses and these have formed peat bogs though time. Here, the glacier’s scouring has resulted in a variety of soil types in a small area.
If space is available, you may park at the permissive car park at (N 55 00.539 W 002 23.446). If this car park is full, you can also park at Steel Rigg (N 55 00.110 W 2 23.269).
From the permissive car park, follow the minor road uphill and south east to the signpost for “Greenlee Nature Reserve”, go through the wicket gate, then follow the line of electricity poles to the start of the boardwalk. You are walking across fell ground, rough pasture with rushes and tough grasses. Traditional sheep and cattle graze here. Continue through three wicket gates and alongside the wood to the start of the boardwalk.
Here you will see that the soil type changes from the fell ground to deep peat. The boardwalk allows you to continue on ground that would be too fragile or wet to walk on. Peat is formed from vegetation that is partially decayed. The wetland here prevents oxygen reaching the decomposing plant material, resulting in slow anerobic decomposition and the build up of the peat.
Question 2: Identify the predominant plant in the bog. Do you think that peat will continue to accumulate here or not and give a reason for your answer.
Question 3: What does the presence of the round-leaved sundew in the peat bog tell you about the soil?
At the junction in the boardwalk, keep to the right and follow it around the lough. You are entering a reedbed, an uncommon habitat at this altitude. The common reed can grow to 4m high - you may find yourself walking down a “plant corridor”.
Question 4: Estimate the height of the reeds when you visit.
Question 5: The native white-clawed crayfish requires very clean water to thrive. Given the surrounding soils, why would Greenlee Lough be a suitable habitat?
At this point you can choose how to return to the parking area. The first option is to continue on the boardwalk towards the bird hide beside the lough (N 55 01.258 W 002 21.475). Greenlee Lough is a Special Area of Conservation, a Site of Special Scientific Interest and a National Nature Reserve. The lough is important for wildfowl and ospreys have also been seen. The path continues across hay meadows . At the finger post turn left and follow the grass track uphill towards Greenlee Farm House. Follow the footpath around the farm and turn left onto the farm track. This track leads back to Gibbs Hill and the permissive car park. The second option is to return to the junction in the boardwalk and follow the branch northwest. There is a right of way across the fell ground which connects with the farm track from Greenlee Farm.
Location 2 (N 55 00.128 W 002 22.260) is the famous Sycamore Gap in the Great Whin Sill near Crag Lough.
The glaciers may not have been able to erode the dolerite sill as much as the surrounding rocks, but here, and at other places, there is a sharp cut or gap in the sill. Water has accomplished erosion where the ice could not.
As the ice retreated, the huge amount of melting ice water or meltwater deposited sand and gravel from the glaciers across the country. This meltwater also eroded the rock that the glaciers had left, cutting through the Whin Sill at places like this. There may have been different stages to this erosion as the ice retreated and advanced before finally retreating.
To complete the cache, please answer the following questions:
Question 6: Estimate the height of the meltwater channel, from the base of the tree to the top of the Sill
Question 7: How many similar channels can you identify between the Steel Rigg car park and Sycamore Gap?
In summary:
Question 1: Identify the location of a cuesta landform elsewhere, near your home location if possible.
Question 2: Identify the predominant plant in the bog. Do you think that peat will continue to accumulate here or not and give a reason for your answer. Question 3: What does the presence of the round-leaved sundew in the peat bog tell you about the soil?
Question 4: Estimate the height of the reeds when you visit.
Question 5: The native white-clawed crayfish requires very clean water to thrive. Given the surrounding soils, why would Greenlee Lough be a suitable habitat?
Question 6: Estimate the height of the meltwater channel, from the base of the tree to the top of the Sill
Question 7: How many similar channels can you identify between the Steel Rigg car park and Sycamore Gap?