Millstone Grind EarthCache

This EarthCache is published for GeoWeek 2018. It is based on an old millstone which is preserved on a grassed area on the bank of the Micker Brook to the north side of Broadway, Cheadle. This is near to Downs Bridge on Wilmslow Road, just south of Cheadle village. Access is easy (but watch out for possible nettles!).

(This EarthCache is now part of the Cheadle GeoTrail Audio guide which provides audio commentary on the geology of 4 EarthCache locations in the Cheadle area: 'Millstone Grind' (GC7MEBC), 'Rock of Ages' (GC7DHPN), 'Death in Paradise' (GC7MM93) and 'Memorial Rocks' (GC7NAFQ). This trail can be accessed by downloading the "SITUATE" app to a smart phone from the App Store and searching for 'Cheadle GeoTrail'. (https://situate.io/). This was produced to celebrate GEOweek 2023)

In the 12th century, Cheadle was served by two water-driven corn mills on the banks of the Micker Brook. Cheadle Higher Mill was situated not far from here on the far side of the brook and this millstone is a relic from that mill; probably the manorial mill of Cheadle Moseley back in the 1500s. However, the rock from which this millstone is made (aptly named Millstone Grit) is a relic of a much longer past; 320 million years ago. (Figure 1)

Figure 1

Millstones made of gritstone were quarried for milling flour and were termed "Peaks" by millers after the Peak District from where these sandstones were often sourced. Up to the 18th century, millers in England used millstones quarried from a single gritstone block. 

THE GEOLOGICAL BACKGROUND

During the Carboniferous Period in Britain, between a time of shallow limestone seas and swampy coal forests, large rivers flowed from the Scottish Highlands to finally cover what is now the Peak District, Pennines and parts of Northern England. (Figure 2). These rivers deposited up to 2000m of sediments in deltas and floodplains, similar to the Mahanadi River Delta in India (Figure 3), to form the coarse sandstones of the Millstone Grit Series – the gritstones.

Figure 2

Figure 3

A close look at the gritstone shows it to be a coarse-grained sedimentary rock made predominantly of quartz sand grains cemented together. Quartz is a hard silica-rich mineral (with a hardness of 7 on Mohs scale), which does not easily break (cleave) or readily dissolve in acidic waters, unlike other minerals. The quartz originated from the weathering of granite outcrops in the Scottish Highlands and was transported by fast flowing rivers before being deposited quickly in the deltas. The range in size of the quartz fragments, and their angular shape suggest the fragments have travelled far (there are few other granitic minerals that have survived the journey), and fast (there is little time to round the grains) and deposited quickly (as the energy of the rivers dropped relatively quickly resulting in moderate sorting of the fragments).

Despite being resistant to weathering, the millstone is nevertheless being slowly broken down by physical, chemical and biological agents as it stands (in situ) in this environment. Whilst the processes of physical weathering (freeze-thaw of ice and effects of the surface heating by the sun) and chemical weathering (action of acidic rainwater) are slow, biological weathering can be seen to be operating today. The presence of moss and lichens is a clear indication of this and even the tiniest bacteria and algae produce chemicals that help break down the rock on which they live, so they can get the nutrients they need. This gives the green tinge to the rock in places whilst the brown colours are the result of oxidation of small quantities of iron in the rock (rust).

To log this EarthCache please answer the following questions and send by e-mail or through the Message Centre;

1. Suggest which side of the millstone is currently being affected most by biological weathering. Briefly explain your reasons why.
2. Suggest one piece of evidence from the size, shape or sorting of the quartz grains that would support the theory that the sediment forming the gritstone was deposited in a river delta.
3. The volume of this millstone has been calculated to be 0.38 m³ (cubic metres).
   The density of gritstone is 2650 kg m^-3 (kilograms per cubic metre).
   Calculate the mass of this millstone (in metric tonnes).
4. Suggest at least two reasons why gritstone was a good stone for milling grains.

Please feel free to add photos to the gallery.