Caerketton Hill (pronounced Care-ketton), with its dry ski slopes, dominates the southern views of Edinburgh and marks the eastern end of the Pentland Hills. These hills were formed when two plates of the Earth's crust collided around 400 million years ago (early Devonian times), joining “England” to “Scotland”.
Caerketton was part of one of the active volcanoes at this time. The hill is composed of what would have been a very sticky (viscous) lava known as rhyolite and of broken fragments derived from it called rhyolitic tuff. These rocks are part of the Pentland Hills Volcanic Formation.
This hill, which may once have been over 2000 metres above sea level, has been eroded over hundreds of millions of years now to the 478 metre crag we see today. On the north side of Caerketton, the lower slopes below the crags are covered in rock debris, or talus (pronounced tay-lus), but commonly called scree.
The geological processes of weathering or erosion which produce scree slopes are chemical, when water from rain or snow reacts with minerals and chemicals in the rock which breaks into fragments; mechanical, when the water within the cracks of rocks freezes, expands and causes the rock to split; and thermal, when heated rock expands while cooling rock contracts.
Often screes are a product of all three processes. Then comes “transport” when the rock fragments tumble down, mainly by gravity but also assisted by rainwater, ice and snow, to form scree slopes.
Eventually the scree production stops because the rock slope becomes completely covered in its own scree. Here you should be able to identify screes at different stages of production and transport.
The global average erosion of soil is about one centimetre every 300 years. That does not sound much but that rate of erosion could flatten a normal hill in a mere 10 million years, but Caerketton Hill is formed from lava, which erodes much more slowly.
QUESTIONS AND ANSWERS
1. Why are the rocky crags themselves so prominent and hard?
2. When were these volcanic rocks erupted?
3. What is the main type of erosion which has created these scree slopes? Explain your choice.
4. How can you distinguish between the three types of scree slope here?
5. What is the vegetation stabilising the third or older scree slopes?
6. At an average erosion rate of 1 cm per 300 years how long would it take to flatten a typical Munro (3000 feet or 910 metres high)?
Please now go ahead and log this cache. A photo is always welcome.
For more information about the geology of the Pentland Hills please go to Pentland Hills Regional Park
This geocache has been provided by the Lothian and Borders GeoConservation.