I was returning to my car after a nice walk around North Queensferry and as I got closer to the car park noticed the different types of weathering along the wall here - there's lots of different shapes to look at - but they're not all weathering in the same way or the same speed.
1) Are all the stones here from the same source? Does this explain the difference in weathering between stones? Is this an example of differential weathering?
2) What influence do you think the coastal location has on the weathering here?
3) Find a stone with dissolution weathering, take a photo and add it to your log.
4) Take a photo of yourself at GZ, or an identifying item such as your GPS with the Forth Railway Bridge in the background.
If logging as a group, all members of the group must be in the photo - if you don't want to be in the photo, you need to upload your own unique photo showing a personal item or your caching name. The person sending answers must mention in their message the cachers they are sending answers for.
Please submit your answers via message though the Geocaching website or by sending me an email - there is no need to wait for a response before logging your find.
Differential Weathering
At this spot, you can see an example of differential weathering. This means that different parts of the rocks, or different rocks next to each other, wear away at different speeds. Even though all the stones in this wall have been here for the same amount of time, you can see some are much more worn than others.
This happens because the stones aren't all exactly the same. Some might be naturally softer, have tiny cracks inside, or be made of minerals that dissolve more easily when wet. The constant exposure to the weather here by the sea, including rain and the salty air, attacks these weaker spots more quickly, causing them to erode faster than the tougher stones around them.
The differing make-up of the stones plays a crucial role. For instance, if some stones are a type of sandstone with a weak 'cement' holding the sand grains together, or if they contain minerals that are less stable when exposed to air and water, they will naturally break down more rapidly. Other stones might be denser, have a stronger internal structure, or contain tougher minerals like quartz, making them much more resistant to the forces of weathering. This variation in strength means that even when exposed to the exact same conditions, each stone responds differently over time.
Coastal Location and Weathering
One of the most significant processes in a coastal location like this is salt weathering. When sea spray hits the wall, the salty water seeps into tiny pores and existing cracks in the stones. As the water then evaporates, the dissolved salts are left behind and begin to crystallise. These growing salt crystals exert pressure on the surrounding rock, pushing the grains apart. Over time, repeated cycles of wetting, drying, and salt crystal growth can cause the rock to crumble, flake, or even develop distinctive hollows and pits, like a honeycomb pattern. If one particular stone is more porous or has more vulnerable minerals, it will suffer from this salt attack far more readily than a denser, more robust stone beside it.
Dissolution Weathering
A type of weathering you might observe, especially in rocks like sandstone, is dissolution weathering. This is a chemical process where certain minerals within the rock are dissolved by water. Rainwater, for example, is naturally slightly acidic, and when it comes into contact with minerals like calcite (which can act as a 'cement' in some sandstones), it can slowly dissolve them away. In a coastal location like this, the presence of salty water can sometimes speed up this process, as the salts themselves can attract and hold moisture, or contribute to chemical reactions that further break down soluble minerals. This removal of material often leaves behind hollows, grooves, or a generally pitted surface on the stone, as the more soluble parts are eaten away, while the more resistant grains or areas remain.
An example of dissolution weathering:
