An Earthcache about Natural Arches and how they are formed
Walk along the Crawhill stretch of The River Avon Heritage Trail and you will come to this slightly out-of-the-way location, where there sits a feature of geological and (reputedly) historical interest. Wallace's Cave is said to have been a hiding place for Sir William Wallace after his defeat at the nearby battle of Falkirk in 1298, although there is no real evidence to confirm that. A quick look around the feature reveals it is not actually a cave at all - the riverside path passes right through it and immediately out the other side, but it probably was a cave once, and we know that because this is an excellent example of a Natural Arch landform.
This arch formed at the end of the last glacial period, when glacial meltwater created a huge raging torrent, laden with stones, and blasted its way along what is now part of the course of the River Avon. This glacial river was able to erode a tunnel through this stone bluff, probably because there were more joints in the rock right there than other locations nearby. At its simplest, a natural arch like Wallace's Cave is a rock exposure that has a hole completely through it formed by the natural, selective removal of rock, leaving a relatively intact frame. Wallace's Cave is an unusual natural arch in as much as most natural arches in Scotland are coastal - having been created by the effect of the sea - however this arch has glacial origins as described above. That has implications for it's future, as explained below.
The arch itself is formed in a thick layer of Passage Formation sandstone, which is found at this level in various parts of Central Scotland, and was laid down about 320 million years ago. The sandstone lies directly underneath the Scottish Lower Coal Measures Formation - a band of siltstone and mudstone with coal at the top, that is common in the Central Belt of Scotland. The sides of the arch show excellent examples of differing layers of sedimentary rock layered on top of each other in horizontal slices. Any layer of rock in a sedimentary sequence that can be distinguished from the layers above and below (like the ones here) is called a Bed.
Beds At Wallace's Cave
Wallace's Cave may not be coastal, but like other arches the natural process that removed some of the rock from this rock face, was Erosion. Erosion can selectively remove rock both macroscopically and microscopically, but the two types of erosion work on very different time scales.
Microsopic Erosion occurs when processes such as chemical weathering slowly dissolve the crystalline cement (formed by pressure) found in rocks, thus destroying the rock matrix and allowing other processes to disperse the remaining loose grains.
Macroscopic Erosion occurs when joints or fractures are first induced in this rock matrix through some usually high impact process that acts swiftly (in geological terms), and then widened through a variety of other processes. This splits the rock into distinct macroscopic pieces that can then move relative to each other under the forces of gravity or water pressure. The action of the glacial meltwater torrent that created this arch is an example of macroscopic erosion.
So why do we think this was once a cave? That's because due to the effect of erosion, a natural arch has its own lifecycle and usually goes through four stages:
Cave - this first stage occurs when an erosive force like water, interacts with cracks in the face of an outcrop like the one you see here. In this case, glacial meltwater containing sizeable stones as well as sill, rapidly eroded the rock. In addition, air may become trapped in joints or cracks in the rock. When the meltwater impacted the rock, this trapped air is compressed, which weakened the rock.
Arch - During the second stage, the meltwater erosion breaks through to the other side, and forms the distinctive arch shape, leaving support formations either side of the hole. This is because these adjacent areas to the hole must have had less weak points/joints in the rock.
Stack - If the arch continues to be subjected to the same erosional forces, this will result in the third stage where the hole will become bigger so that the layer at the top above it is no longer supported. When the arch collapses, this will leave the bluff/headland on one side and a stack of rock on the other.
Stump - In the fourth stage the top of the arch is long gone, and as a result the stack is exposed to the elements and erosional forces on all sides. It will typically erode at the base and eventually collapse to form a shorter rock stump with adjacent stone 'rubble'.
It's important to note that this is the lifecycle of a Natural Arch where the stone headland or bluff is continuing to be exposed to erosional forces.
To claim this Earthcache as found, send me your answers to the following questions, via the Geocaching app or the website. This must be done before logging the cache. I will reply to all your answers.
1) There are differing layers of rock here, most obviously displayed in the area of the arch right next to the water when viewed from the West/upstream. Look at the lower layer of rock on that portion of the arch next to the water (the area with the greatest green colouration), does it have more or less thin bedding than the area above it?
2) Stand at the Western side of the arch (upstream). Looking from this perspective do you think the river is likely to erode the arch hole any wider?
3) What does your answer to 2 and the origins of the arch, mean for the duration of its journey to stage 3 (stack), when compared to a coastal arch.
4) Given some of your answers above and the information on the cache page about the arch's method of creation, do you think this was a cave when Sir William Wallace supposedly hid here in 1298?