The Stanes of Stofast were once one huge rock. The rock weighed around 2,000 tonnes and was transported by a glacier. The rock would have become embedded in the ice and flowed with it before being dumped on the hilltop when the glacier started to melt. The transport of rocks and other material in this way is called ice rafting.
Rocks like the Stanes of Stofast are called glacial erratics because they are different in either type, orientation or both from the rocks surrounding them. Glacial erratics help geologists to trace the direction of a glacier's flow by looking at the origin and the final location of the rock. Some erratics have been found to have been transported more than 300km (190 miles) from their source. Others have been transported by more than one glacier.
The Stanes of Stofast are composed of Valayre Gneiss. Valayre Gneiss is a metamorphic rock - this means a rock that has been transformed by heat and pressure in the earth's crust. It contains unusual and distinctive megacrysts (large crystals) of feldspar.
The Stanes are large, but erratics can be much bigger. The largest known is the Okotoks Erratic in Canada, 9 metres high, 41 metres long and 18 metres wide
The rock is now split in two as a result of frost action. First water gets into a fissure in the rock, then expands as it freezes, cracking the rock.
Rainwater collects in fissures in the erratic boulder
As the temperature drops the water freezes and expands, cracking the boulder further
Eventually, after many cycles of this freeze and thaw action, the boulder splits in two
At the co-ordinates answer the following questions:
1) Estimate the size of the gap where the rock has cracked (between the two tallest parts) in metres.
2) Just northwest of the Stanes is a southeast facing outcrop of rock. How does this rock compare to the Stanes and what does this tell us about how far the Stanes have been moved?
3) The outcrop contains some large megacrysts. What is the colour of these and what do you think they are?
4) (Optional) Take a photograph of yourself and/or your GPS with the stanes but taking care not to reveal the answer to Question 1.
E-mail your answers to Q1, 2 and 3 to the cache owner and post your photo in your log. No need to wait for a reply to the answers, but we will delete logs if the answers aren't satisfactory, so you may prefer to wait.