Millstones of the Peak District EarthCache

The Peak District is famous for the spectacular gritstone found throughout the area and for that reason the Peak District National Park opted to use millstones mounted on plinths to mark the entry points to the area on many of the major roads. There is one such millstone which is very easily accessible here as it is next to a layby and is a very good place to view an example of the rock found on the hills all around you. For those parking here for a walk on the hills this EarthCache will give you a basic introduction to the rock you will see throughout your walk. For those driving past, it is an easy driveby to give you a close up look at the most famous rock in the Peak District.

Firstly, we need to understand the basic formation of the rock you see here today. The northern Peak District is bounded on east and west sides by sandstone scarps. They are made of a rock often called "Millstone Grit". This is in fact a catchall for a variety of coarse sandstones, referred to here as "grits". The name "Millstone Grit" came about because certain varieties of Gritstone were commonly used to make millstones. Gritstone is actually a variation of sandstone and is made from sand, grit and rounded pebbles of quartz and some feldspar. The name ‘grit’ is used for many local sandstones and indicates the coarseness of the quartz grains in the rock, although the size of the particles in the sandstone is variable. Most of the sandstones of the Millstone Grit are coarse, with grains larger than 0.5mm and there are white quartz pebbles up to 10mm, especially in the Kinderscout Grit.

Gritstones were laid down in the delta of an immense river which flowed from a mountainous area. The land was below the sea, and every tide, every flood, dumped sand onto the sea floor. Generally the layers were thin and/or disturbed by currents. Occasionally some event would occur which laid down layers many metres thick in a very short time. All this happened around 300 million years ago, long before the dinosaurs. Eventually the grits were buried by muds, coal, and limestones. The layers of sand, under pressure and subject to chemical change, became rock. Eventually what is now England was pushed up out of the sea and erosion began creating all the gritstone formations you see up here today.

Sand and Grit

Gritstone is a type of sandstone and therefore contains sand grains as you would expect to find in sandstone. It also contains sand grains known as grit, this is a general term to describe larger angular sand grains.

Quartz Pebbles

Quartz is one of the most common minerals in the Earth’s crust. As a mineral name, quartz refers to a specific chemical compound (silicon dioxide, or silica, SiO2), having a specific crystalline form (hexagonal). It is found is all forms of rock: igneous, metamorphic and sedimentary. Quartz is physically and chemically resistant to weathering. When quartz-bearing rocks become weathered and eroded, the grains of resistant quartz are concentrated in the soil, in rivers, and on beaches. The white sands typically found in river beds and on beaches are usually composed mainly of quartz, with some white or pink feldspar as well. This is why you often see little white pebbles in the paths and peat on the hills around here, sometimes they looks like a white sand path. A pebble is a clast of rock with a particle size of 2 to 64 millimetres.

Quartz is typically light-grey and kind of looks like an opaque window. If you look at the crystals closely, they should stand out from the surrounding rocks. Quartz is pretty sturdy and you can’t scratch it with your fingernail (unlike other minerals). Usually in gritstone, the quartz has already eroded into pebble form and appears in the rock in pebble size. Sometimes these can be quite well eroded into white pebbles.

Feldspar

Feldspar is the name of a large group of rock-forming silicate minerals that make up over 50% of Earth’s crust. They are found in igneous, metamorphic, and sedimentary rocks in all parts of the world. Feldspar minerals have very similar structures, chemical compositions, and physical properties.

In sedimentary deposits produced from the weathering of feldspar-bearing igneous and metamorphic rocks, feldspars are usually most abundant close to the source area. Feldspars generally decline in abundance with distance from the source because during transport, they can be attacked by weathering and altered to clay minerals. In addition, their two directions of perfect cleavage make them vulnerable to mechanical weathering, which decreases their particle size and exposes a greater surface area to chemical weathering.

Feldspar is often similar to quartz and isn't always easy to identify but in gritstone there is a distinct difference. Feldspar grows (when allowed to) into rectangular crystals, and then usually breaks the same way. This breakage pattern is called cleavage, and feldspar has two directions of cleavage that meet at close to a 90 degree angle (just like a stairway). If you see little sparkles when you look at the rock, the flashes may be the light reflecting off the mirror-like cleavage surfaces. A torch can help you identify feldspar in gritstone as it does frequently sparkle.

Millstones are fairly famous in this area, to the extend that the good quality gritstone has become named Millstone Grit. A millstone is a special stone used in grist mills - for grinding wheat or other grains. Millstones come in pairs. The base (or bedstone) is stationary stone. Above the bedstone is the turning ‘runner stone’ and this actually does the grinding. The runner stone spins above the stationary bedstone creating the "scissoring" or grinding action of the stones. A runner stone is generally slightly concave, while the bedstone is slightly convex. This helps to channel the ground flour to the outer edges of the stones where it can be gathered up.

The millstones made in this area were usually created using a specific coarse sandstone which was created in the Namurian strata of the Carboniferous period mentioned above. This rock is commonly known as ‘Millstone Grit’. Generally, these stones came from the Peak District and were monoliths – that is, a single piece stone. The rival millstones were generally segmented millstones, usually French burrstones from near Paris - in the case of those, the rock was in sections and cemented together and bound with iron bands.

Let’s compare Millstones –

French burrstones: Consist of fine-grained re-crystallised quartz desposits from the Tertiary period (65.5 million to 2.6 million years ago - so much newer stone than our Carboniferous stone) – generally used for white flour as the stone was better for separating the wheat grain components

Peak District millstone grit: Fine/coarse sandstone, from the Carboniferous Period (360 and 285 million years ago) – generally used for barley and animal fodder.

The millstone you see here is a single piece of stone with no visible sedimentary layers (because it was made from a single sedimentary rock layer). Although there are layers of gritstone around Dovestones that have the size to create millstones I'm not aware that gritstones were ever made here. Therefore this particular millstone is likely to have been moved here from somewhere else in the Peak District, from one of the major millstone construction areas such as Hathersage. When millstones didn't come out perfectly they were frequently left lying around and some years ago, the Peak District National Park are likely to have collected a number of very good quality abandoned millstones all from the same place and used them to create these monuments. However the basic composition of the rock is very similar to the gritstone you can find on the hills all around this area.

Please visit the listed coordinates and examine the millstone you find here. The questions all relate directly to the information provided in the listing so you should be able to answer everything from GZ with no extra reading required. Please ensure you send in the answers at the time or soon after you log your find, as logs may be deleted if no attempt at the answers are made.

1) Firstly, what is the size of the millstone? Please give me the total diameter, the thickness and the size of the hole in the middle.

2) Now, please turn your attention to the texture of the stone. Please briefly describe it - rough/smooth? Colours?

3) Next, identify the sand and grit grains making up the stone. What sizes can you see (biggest/smallest) and what colours?

4) Quartz crystals or pebbles also form an important part of any gritstone. From the information above, please identify them. Describe them - rough/smooth? Are they all the same size? What is the size of the biggest / smallest bit of quartz you can see? Do they look more like crystals or pebbles (or both)?

5) Feldspar is also present in this gritstone. Using the description above, please identify some feldspar. Describe the crystals you can see in terms of colour and size. Do they glint in the light (a torch would help)?

Please note. Non spoiler photos are always welcomed. At the time of publication if you are in an area where there is no signal and you write your answers into the message center it will not queue the message to be sent later as it does with logs - instead it will be deleted immediately by the app if it fails to send. Please be aware of this as I need to receive your answers. If you don't get a pop up saying 'message sent' then it hasn't!

Thank you for visiting the Millstones of the Peak District EarthCache