This EarthCache is set around the Stone Circle at the Brockholes Nature Reserve. The Stone Circle was constructed between the summer solstice on the 21st June 2014 and the winter solstice on the 21st December 2014. There are 14 stones in the circle plus two entrance stones and a central stone, making 17 stones in total.
Below is a plan of the stone circle you will need to follow to answer the questions
It may be a good idea to print this plan before you go for the EarthCache
The stones in this circle include examples of the major types of exposed hard rock in Lancashire, limestones and sandstones.
The three types of rocks you will find are:
Carboniferous Limestone
Pendle Grit (sandstone).
Haslingden Flags (Sandstone)
Carboniferous Limestone
Carboniferous Limestone is a collective term for the succession of limestones occurring widely throughout Great Britain and Ireland. The Carboniferous Limestone of the Lancashire area was formed on the bed of a warm, shallow sea which covered this area some 340 million years ago.
Limestone is a sedimentary rock, which may vary in colour from pale grey to almost black. It is the rock type most likely to contain fossils. Limestone consists predominantly of calcium carbonate. The fragmentary ‘hard parts’ of marine animals and algae, both of which are made of calcium carbonate, are ‘cemented’ together by more calcium carbonate to make limestone. Nearly pure limestone, of which chalk is an example, is almost white. When contaminated with terrestrial sediment or with organic carbon, the colour of limestone is darker. Limestone originates as sediment in warm tropical seas, remote from significant sources of sediment coming from the land. The limestone- producing animals need clean water. The organisms evolved over geological time, so the fossils found within limestone give a very good indication of its age.
Sandstone
Sandstone is a sedimentary rock composed of sand-size grains of mineral, rock, or organic material. It also contains a cementing material that binds the sand grains together and may contain a matrix of silt- or clay-size particles that occupy the spaces between the sand grains.
Sandstone is one of the most common types of sedimentary rock and is found in sedimentary basins throughout the world. It is often mined for use as a construction material or as a raw material used in manufacturing. In the subsurface, sandstone often serves as an aquifer for groundwater or as a reservoir for oil and natural gas.
Pendle Grit
Pendle Grit is a geologic formation in England. It is a coarse-grained Carboniferous age sandstone assigned to the Millstone Grit Group. This coarse grained sedimentary rock, perhaps containing pebbles, these are are known as gritstones, in the past they have been called ‘grits’ leading to the name Millstone grit. The formation takes its name from Pendle Hill in Lancashire.
Pendle Hill, to the east of Clitheroe is a striking and prominent hill rising to 557m. The hill is formed by Upper Carboniferous shales and sandstones with the Pendle Grit Formation, it’s the oldest Millstone Grit sandstone seen in the central Pennines
Haslingden Flag
Haslingden Flag is a type of sedimentary rock, It is a fine grained sandstone composed of fairly small grains of sand in thin beds. it’s relatively easy to split or quarry in slabs, and so is ideal for paving. It is also used for making fences and roofing. There are Upper and Lower Haslingden Flagstones and the name is given to layers of the stone that appear throughout the area (not only in Haslingden) although the most spectacular examples are in the Rossendale valley. It was quarried and mined in Rossendale.
These rocks that lie underneath the valleys and hills of Rossendale were formed millions of years ago at a time when the whole of what is now the North of England was covered by huge river deltas and lagoons. Sediments, mainly sands, silts and muds, were eroded from hills in an area that now includes Scandinavia and Greenland and were swept into vast river deltas and lagoons in a central basin in a position now occupied by the Pennines. The sediment settled to the bottom as the water slowed down in the deltas and lagoons. This sediment built up until it was hundreds of metres thick and was gradually compacted and cemented into the “sedimentary” rocks we know today.
Slickenside
Slickensides are smooth rock surfaces with parallel grooves or scratches formed by frictional wear during the sliding and movement along a fault This surface is normally striated in the direction of movement. A fault slip may polish smooth the walls of the fault plane, marking them with striations called slickensides The plane may be coated by mineral fibres that grew during the fault movement, these are known as slickenfibers, this also shows the direction of movement. The surface feels smoother when your hand is moved in the same direction as the eroded side of the fault. As you slide your hand over the rock in the direction the surface steps down it feels slick and smooth, similar to the scales on a fish when stroked from the head.
When you see a rock with slickensides, an easy way to remember it, is to think (Ah this side looks very slick)
Limestone Pavement
During the last ice age. The weight of the ice stripped away the topsoil that lay over the limestone, exposing the bare rock underneath Limestone pavement is a landform consisting of a horizontal surface of exposed limestone showing the effects of chemical weathering. many of these landforms have developed a distinctive surface pattern resembling paving blocks. Rainfall is naturally slightly acidic and acidic water is able to dissolve limestone. When it rains, the water flows down the cracks. The rock at the crack dissolves and the crack gets wider and deeper until you can’t call it a crack any more – it’s a gryke! This leaves exposed blocks of limestone called clints. The resulting pattern of blocky rock is called a limestone pavement
Clints and Grykes
Clints are the blocks of limestone that form the pavement. They are chemically weathered so that their surface is covered by a series of pits and hollows (called karren ). Grykes are fissures or crevices separating the clints in a limestone pavement. They can be quite a few feet in depth, they are formed when the cracks and crevices in the limestone are widened by chemical weathering.
Malham Cove in the Yorkshire Dales National Park is a good example of this, and is amongst the top ten geological wonders of Britain.
Although the clints have a relatively level surface overall, if you look in detail they have many little pits and small channels on them. As these depressions collect water and rainwater is naturally slightly acidic it is able to dissolve the limestone, causing the depressions to gradually get bigger, meaning they collect even more water and so on. These depressions or pools are known as kamenitza. When rain falls on limestone pavement, some will collect in the kamenitza, while the rest of the water flows to the edge of the clint and disappears down the gryke. The flowing water will also dissolve the limestone beneath it, so where the water flows most, will be dissolved most, forming a channel. These channels are called karren.
To claim a find for this EarthCache, you can use information from the cache page and your observations at GZ.
It is very important to use the diagram on the cache page, it might be a good idea to print this out before you go.
Please send your answers via message centre or email. I assure you I will read all your answers and logs and will reply to each one.
Please do not include answers in your log.
1) Compare the grain size of stones 15 and 16 with the grain size of stones 3 and 4 then tell me which two stones are formed from Pendle Grit? What brings you to that conclusion?
2) Stone 13 has a large expanse of slickensides on one of its sides. Explain how slickenside is formed and what angle do the striations orientate in this stone?
3) Which two stones do you think were at one time covered by a layer of soil which was later stripped away, and what feature did this form?
4) What are the dimensions of the centre stone?
Optional extra: Please feel free to add any photos of your visit that don’t show any specific features from the logging tasks
Note: There is a car parking charge on the Brockholes car park :
April 1st – October 31st £5.00 all day parking
November 1st – March 31st £3,00 all day parking
I have added waypoints for free parking spots - 0.5 miles away
Also No dogs are allowed on Brockholes Nature Reserve