What is dolerite?
It’s a rock that was once molten magma, like granite, but because it occurs in smaller and thinner volumes it cooled faster and so the mineral crystals in it are much smaller and you can only see them with a hand lens.
How do I recognize it?
It’s a dark grey, hard rock. It usually has vertical joints and fractures (caused by its cooling). You can recognize it in the landscape because creates a prominent scarp.
How was it made?
Dolerite was once a molten rock at a temperature of around 1,200 degrees C. When this part of the Earth was being stretched during an episode of mountain building caused by movement of the tectonic plates 295 million years ago, the molten magma was injected into weaker places in existing sedimentary rocks (rocks like limestone and sandstone) as “sheets” up to about 50 metres thick.
What does a dolerite landscape look like?
The injection of the dolerite as sheets into sedimentary rocks and then the tilting of all these rocks south and eastwards by around 10 degrees by earthquakes, gives a very characteristic “scarp and dip” or “cuesta” landscape.
What use is it?
Because of it’s hardness dolerite is used extensively for roadstone – the rock fragments that are mixed with asphalt in “tarmac”. More than 100 years ago it was also quarried and shaped into “sets” to make cobbled streets.
At Croy Hill The ditch is significantly shallower and narrower than elsewhere. There is also a 25m stretch where there is no ditch at all. In the 18th Century, the Romans were revered as invincible. William Maitland stated in his 1757 History and Antiquities of Scotland:
It cannot be reasonably imagined that [the Romans] would leave a rock undemolished in this part. Now as I am, for certain reasons, (too long to be inserted in this place) of opinion that rocks vegetate, the rock here, by its form, must have sprung up since the making of said ditch.
Question 1: Is Maitland's description on the formation of dolerite likely? If not, how do you believe these rocks were formed?
Question 2: From the given coordinates and with Bar Hill behind you look left and you can see the remains of the quarry.
- How high is the quarry?
- What colour is the rock?
- Are there any fissures?
Question 3: Again with Bar Hill behind you, look straight ahead and you can see the line of the Dolerite Scarp. Look along the scarp and you will see the peak of Croy Hill one of the highest points on the wall. Estimate the height of Croy Hill (hint - see map).
Question 4: What properties of dolerite may have resulted in it being quarried by the Romans or others?
Please send a photo of yourself or an identifying item with Croy Hill in the background.
Please send your answers by email or via the Message Centre. You may log your find without waiting for a response from me but if your answers are not received within 7 days of logging a find, then the log will be deleted.