The 'Grey Bools' are a well known feature to the people of Wick. A collection of large boulders (bool is local dialect for boulder) they sit heaped on the cliff top between the town's harbour and the 12th Century Castle of Old Wick. The definition of a boulder is often given as a large, rounded rock that has been smoothed by the action of the weather or water.
To visit the cache site there is a public road leading along the coast from Wick with car parking at the end.
Stormy Seas
The Grey Bools are a good example of cliff-top storm deposits (CTSD). Another example can be found at Crosskirk on the north coast of Caithness though this site is the more easily accessed.
CTSDs are accumulations of boulders occurring well above the high-water mark. They are examples of coastal erosion and their subsequent transportation by large waves during exceptionally major storms is testimony to the power of the sea.
It is understood that the factors significant in CTSD occurrence are:
1) Cliff profile: In simple terms this means the height and angle of a cliff face, plus its features such as any wave-cut notches or changes in slope angle. With regard to CTSD an angular ramp-like profile facilitates the run-up of water necessary to shift large chunks of rock.
2) Bathymetry: Bathymetry is the measurement of depth of water.
3) Skerries: A skerry is a rock or rocky island often situated near the coast - Caithness has some fine examples. The presence of skerries prevents the formation of CTSD.
The boulders at the cache site lie above sea level at the head of a rock ramp that slopes to the north. The water depth is between 10 to 20 metres only a few tens of metres offshore. The largest boulder - known as Charlie's stone - has an estimated mass of 500 tonnes. A step at around 5 metres above sea level on the ramp is understood to be from where the blocks were torn from the rock.
Dating the Past
Dating the event that resulted in the CTSD is inherently difficult. Work carried out in the 2000s compared the boulders with headstones made of the same rock used in local graveyards. Through examination of the amount of weathering it was estimated that a date between 1800 and 1850 was likely. The earliest record of the Grey Bools is from the 1840's and an extreme North Sea storm occurred in January 1830.
Weathering is the breaking down of rocks on Earth's surface - water, ice, salts, vegetation, animals, temperature change are among some of the potential causes. There are three types of weathering: physical, chemical and biological.
Physical weathering: Caused by the effects of temperature change on the rock usually assisted by water. Freeze - thaw occurs when rain water seeps into cracks in the rock then freezes and expands breaking the rock apart.
Chemical weathering: Occurs when rain water reacts with the mineral grains of the rock, new minerals and soluble salts are formed. This process happens more quickly with higher temperatures so a warm, damp climate is most conducive.
Biological weathering: The effect of living organisms on the rock. An example might be the roots of a tree growing in a crack and eventually prizing rock apart.
Prehistoric Lake
The Grey Bools are made from the Lybster subgroup of the Lower Flagstone Group a type of Middle Old Red Sandstone (ORS). Flagstone is the dominant stone of Caithness and indeed has, at times, been quarried in other parts of the county on a sizeable commercial scale. It was formed 370 million years ago when what is now northern Scotland was covered by the Orcadian Basin within which sat a lake known by us today as Lake Orcadie.
Flagstone is a finely bedded sedimentary rock, often containing feldspar and quartz. The colour can be determined by the 'binding' material. These can include silica, iron oxide or calcite.
Looking back from the car park along the cliffs towards Wick the cyclicity that is a feature of the Lybster subgroup may be apparent. Subtle changes in colour can be seen (easier in good light!) that demonstrate the changing conditions of the times when the rock was being formed. The water level in the lake varied and ripples and polygonal dessication cracks found at certain exposures indicate this.
Time for Tasks
From the above and being at the site please answer the following to claim the earthcache:
1) Please describe, in your own words, the general shape of the majority of the boulders.
2)) Regarding the cliff profile: what angle from horizontal would you estimate the cliff face to be? How might this have contributed to the boulders being deposited at the cache site?
3) How much weathering has affected the boulders? Which of the three types of weathering would you consider has had the most impact? How might their location have played a role in this?
Whilst by no means necessary a photo of your visit to the EarthCache site will always be welcome..
To complete this EarthCache please send us your answers either via email or through the Message Centre. Feel free to log the find before we reply as whilst we will check all the answers we wouldn't want to delay a successful visitor recording the EarthCache.