If you take a walk in these pleasant woods, you will come across a giant haggis! OK, it's not a haggis, it's a huge boulder. How did this boulder get here? Was it pushed or carried by some cavemen and the trees have grown around it or have modern day Neds brought it here for a laugh? No, it wasn't any of these, this is in fact an erratic and it has huge geological importance. Why?
In Tomfat woods, there are no rock formations so it couldn't have fallen off a mountain. Looking at the geology of the area, metamorphic rock would be common around here but that is not what this erratic is made of. An erratic is a rock that is different to the surrounding rock types that are commonly found in the area. So how did it get here?
It was carried to this spot by ice! Yes, ice! This ice has carried rocks for many hundreds of miles, some of them being carried to the UK (Scotland) from places like Scandanavia!
This erratic is formed of Conglomerate but what exactly is Conglomerate?: Conglomerate is a clastic sedimentary rock that contains large (greater than two millimeters in diameter) rounded clasts. The space between the clasts is generally filled with smaller particles and/or a chemical cement that binds the rock together.
What is the Composition of Conglomerate?: Conglomerate can have a variety of compositions. As a clastic sedimentary rock it can contain clasts of any rock material or weathering product that is washed downstream or down current. The rounded clasts of conglomerate can be mineral particles such as quartz or they can be sedimentary, metamorphic or igneous rock fragments. The matrix that binds the large clasts together can be a mixture of sand, mud and chemical cement.
How Does Conglomerate Form?:Conglomerate forms where a sediment of rounded clasts at least two millimeters in diameter accumulates. It takes a strong water current to transport and shape particles this large. So the environment of deposition might be along a swiftly flowing stream or a beach with strong waves. There must also be a source of large-size sediment particles somewhere up current. The rounded shape of the clasts reveal that they were tumbled by running water or moving waves.
In September, 2012, NASA's Mars rover Curiosity discovered an outcrop of conglomerate exposed on the surface of Mars. The rounded clasts within the conglomerate provide evidence that a stream or a beach had moved the rocks and tumbled them into rounded pebbles. This conglomerate was the most convincing evidence that water once flowed on the surface of Mars.
Conglomerates often begin by being deposited as a sediment consisting mainly of pebble and cobble-size clasts. The finer-size sand and clay which fill the spaces between the larger clasts is often deposited later on top of the large clasts and then sifts down between them to fill the interstitial spaces. The deposition of a chemical cement then binds the sediment into a rock.
Conglomerates may be clast-supported (clasts are in contact) or matrix-supported (clasts are separated from one another by fine-grained matrix that binds the clasts together).
To log this cache please complete the following tasks:
Task 1 - Describe the clasts contained within this erratic: what shape are they, why do you think they are this shape?
Task 2 - Are all the clasts the same? Describe the different clasts that can you see. Can you name the specific types of rock that they have come from?
Task 3 - What sort of conglomerate do you think it is? Why?
Task 4 - Can you find out where this rock has come from and therefore the direction of the ice movement?
Please send me the answers via my profile. Do not post these answers in your log.
You are free to log the cache once you have emailed me the answers but I will get back to you if there is a problem. Any logs from cachers who haven't sent the answers will be deleted!
Optional task: It would be nice to see that you have enjoyed your walk and this earthcache by posting a photo of your visit.
You must carry out these educational tasks as required by the Earthcaching organisation as a condition of logging the cache. See www.earthcache.org