Mull: Calgary Bay dyke EarthCache
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Calgary Bay dyke
One of the most popular places on Mull
The Island of Mull has been shaped by two main geological processes: volcanic activity and glaciation. Most of Mull is composed of basaltic igneous rocks which date to the start of the Palaeogene Period, about 65 million years ago. This was a time when the west coast of Scotland was tectonically very active – Mull is one of several volcanic centres. The intense igneous activity resulted in large quantities of basalt being poured out as lava. Basaltic lava can take on many forms and these are clearly seen in Mull today.
Fractures in the Earth’s crust that fed these volcanic eruptions acted as pathways by means of which molten magma reached the surface long after the surface volcanic activity had ceased, possibly until around 52 million years ago. The magma eventually solidified in the cracks to form vertical sheets of rock, known as ‘dykes’. Being harder than the surrounding rock, dykes, such as this at Calgary Bay, weather out as prominent features. This remarkable natural dyke, visible from afar, runs downhill to the shore where it was incorporated as part of an old building.
Dykes are what are known as ‘igneous intrusions’. Typically, these form when magma cools and solidifies before it reaches the surface.
The three common types of intrusion are sills, dykes, and batholiths.
Sills form when magma intrudes between the rock layers, forming a horizontal or gently-dipping sheet of igneous rock. The Whin Sill in northern England is an extended version of this, and provides a defensive cliff-line on which the Romans built Hadrian’s Wall.
Dykes form as magma pushes up towards the surface through cracks in the rock. Dykes are vertical or steeply-dipping sheets of igneous rock.
Batholiths are large, deep-seated intrusions (sometimes called plutons) that form as thick, viscous magma slowly makes its way toward the surface, but seldom gets there!
En route to the EarthCache you cross Calgary Bay sand dunes and machair, a Site of Special Scientific Interest, a quite remarkable place, not found elsewhere on Mull.
The old pier near the base of the dyke was built to facilitate small ships delivering coal and sheep to and from the Treshnish Isles; it is still maintained today.
To claim this EarthCache, email me with the answers to the following questions. There is no need to wait for a response from me before logging your visit, but I generally respond fairly quickly, and I will do my best to give you feedback, should any of your answers not be correct.
1. The Calgary dyke comes down to form part of an old building. Using your GPS, tell me the height above sea level of the base of the gate pillars immediately below the dyke.
2. What is your estimate of the maximum width of the dyke, and your estimate of the vertical height of the dyke directly above the prominent post (on the left-hand side as you look at it)?
3. The dyke is not truly vertical; it leans. Does it lean left or right? And what is your assessment of the angle of its lean…how many degrees?
4. The dyke clearly stands prominent from the surrounding rocks. What, do you think, is it about the substance of the dyke that makes it so prominent?
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