This EarthCache will take you to the lovely and very impressive rock basins on Kestor. The Earth Science Lesson of this EarthCache will focus on the secondary phase of weathering of the granite which makes rock basins. This is the second in a series of EarthCaches focusing on the weathering of the granite tors.
Kestor is located on Chagford Common, which lies on the very east of the moorland part of Dartmoor National Park, between Batworthy, to the north, and Fernworthy Forest, to the south. The name of Kestor has been somewhat confusing through the centuries because today it appears on the OS map with apparent Tor status, but it is often referred to as a rock. An example of this is on the 1888 OS map this rocky outcrop is named as Castor Rock, apparently combining the two.
Kestor is a special place and this area of the Moor is one of my favourites, partly for the geology that is shown at this Tor, but also for the wealth of archaeology that this Tor overlooks. Surrounding this Tor is the Shovel Down and Thornworthy Tor Reave System. This is a prehistoric field system, which in this area has been reused into the Medieval, and looking off the Moor from Kestor, there is evidence for Fossilised Reaves (which are the continued used of reave alignment into the modern fields). This settlement and agricultural landscape was created during the Bronze Age (c.2000-700 BC). As well as this extensive evidence for Bronze Age activity the area is also incredibly significant as it was the location of the first irrefutable proof that that was reuse of the upland area of Dartmoor into the Iron Age (c.700-54 BC). This was identified by Lady Aileen Fox in her 1956 work at Roundy Pound, Kestor. From here the knowledge of Dartmoor during the 'dark age' of Dartmoor, the Iron Age, has grown significantly. To compliment this agricultural and settlement landscape the Shovel Down area is also significant as a ceremonial complex. This ceremonial complex includes a number of burial cairns, as well as three stone rows. These ceremonial features show that even before Bronze Age, during the later Neolithic, this area was important to those inhabiting the area.
This Tor has witnessed the creation of the wonderful archaeological landscape that all of who visit Dartmoor love, but this Tor has also witnessed the process of erosion that has created the very landscape of Dartmoor itself. As with most of the granite tors on Dartmoor Kestor has undergone the same processes that have created the granite outcrop that forms the tor, which have been explained in the first of this series Pil Tor - A Natural Avenue. After the granite outcrop was formed, therefore exposing the granite to the elements, another feature of weathering started to form.
This feature is a Rock Basin. Rock Basins are, as their name suggests, dish like hollows on the top of a rock. Over the years there have been fierce arguments over how they are formed. Both of these would be fitting for tor erosion, but only one is through geological processes. The less geologic, and scientific, explanation was that these were formed either by man (druids) or mythical beings (such as the devil/dewer). The latter has long been held responsible for the Rock Basin on Great Mis Tor, called the 'Devils Frying Pan'. The legend states that after a night out on the Moor the devil stopped at Great Mis for something to eat, and the resulting action formed the frying pan shaped Rock Basin. Similarly druids were thought to have carved our these basins for their rituals. The explanation includes carving them to collect pure rainwater or for placing blood or other spiritual fluids in.
Thanks to advances in geology we now know that these basins are in fact naturally occurring and are a result of weathering. Granite is largely comprised of Quartz and Feldspar Crystals, as minor amounts of Mica, Amphiboles and other minerals. As mentioned in the Pil Tor - A Natural Avenue even rocks of the same type can have variation in their formation, resulting in variation to their resistance to weather. In the case of this EarthCache weaker Feldspar Crystals within the granite are the key to the formation of these basins. These weaker Feldspar Crystals are more susceptible to weathering by frost. This is similar to freeze thaw but on a smaller scale. Instead of large, visible to the naked eye, cracks within the rock this frost action specifically breaks down the crystalline structure of the Feldspar Crystal. The resulting small fragments that have been eroded from the rock are subsequently blown away by the wind. Over time this process repeats itself leaving a smooth hollow. This process continues to erode away at the base of the basin, as well as the sides, ever widening and deepening the basin.
Rock basin on Chinkwell Tor, Dartmoor
Rock Basins are a fairly common feature on Dartmoor, with a central belt across the Moor (roughly east to west) container most of the known specimens. The widespread nature of this phenomenon indicates that weaker Feldspar Crystals commonly occur in the granite that is found across this central band of Dartmoor. Based on the concentration on Kestor, and the knowledge that they indicate weak Feldspar Crystals, it could be concluded that Kestor must have 'weaker' granite than elsewhere on Dartmoor. However, it is important to remember that, like freeze thaw weather, the creation of Rock Basins in reliant on water collecting on the surface, and later inside the hollow. For a significantly large basin to form, as are found on Kestor, the surface of the granite must also be flat, as well as containing these weak Feldspar Crystals.
To log this EarthCache go to both of the waypoints and send me a message or email answering these following questions
Go to the posted coordinates and look at the group of rock cut basins in front of you.
1) How many basins are they?
2) Describe the diameter and depth of all of these rock basins.
Go to the second rock basin group waypoint and look at the two basins
3) Will both of these rock basins hold water and if not why not?
Either waypoint
4) How did these rock basins form?
5) What do the large number of rock basins on Kestor suggest about the granite forming this tor?