Stirling Sill facts for kids
The Stirling Sill is a huge sheet of hard rock found deep under central Scotland. It's like a giant, flat rock layer that pushed its way into older rocks. This rock is called quartz-dolerite, which is a type of igneous rock. The Stirling Sill is very old, forming around the late Carboniferous or Permian periods. That's about 250 to 300 million years ago!
This sill formed when hot, melted rock, called magma, squeezed into cracks and layers of existing rocks. It often spread out between layers of coal measures, which are rock layers that contain coal. Sometimes, the magma even pushed upwards through cracks, forming features called dikes.
Contents
What is the Stirling Sill?
The Stirling Sill is a large example of a geological feature known as a sill. A sill forms when magma pushes its way between layers of rock, spreading out horizontally. Imagine pushing a playing card between the pages of a book – the card is like the sill, and the pages are the rock layers.
How it Formed
The rock that makes up the Stirling Sill is called quartz-dolerite. This is a very tough, dark-colored rock. It was formed when magma cooled down and hardened slowly underground. Because it cooled slowly, it developed small crystals.
The sill pushed into rocks that were already there, especially the layers that contain coal. These coal-bearing layers are known as "coal measures." The sill often followed the flat "bedding planes" between these rock layers. Sometimes, it broke through these flat layers and pushed upwards through cracks, forming vertical walls of rock called "dikes."
Where Can You See the Sill?
You can see parts of the Stirling Sill sticking out of the ground in several places. These places are called "outcrops." In the western part of the area, the sill is usually found near the bottom of the Carboniferous rock layers. You can spot it above older sandstone and lava deposits, for example, near the Bannock Burn valley.
Visible Locations
Some of the most famous places where you can see the Stirling Sill include:
- The Abbey Craig: This is the hill where the Wallace Monument stands.
- Stirling Castle: The rock that Stirling Castle sits on is part of the sill.
- Kings Park
- Gillies Hill
- Sauchieburn
These places give you a good idea of how thick the sill is. It's about 100 meters (or 330 feet) thick in some areas!
How the Sill Affected Coal
The Stirling Sill had a big impact on the coal found in the area. In some places, the hot magma from the sill actually burned or "destroyed" the coal. This was not good news for miners!
Creating Anthracite
However, in other areas, the heat from the sill had a different effect. It "baked" the coal, turning it into a more valuable type of coal called anthracite. Anthracite is a very hard, shiny coal that burns much cleaner and hotter than regular coal. Geologists believe this baking effect was strongest near a large crack in the Earth's crust called the Ochil Fault.
Where Did the Magma Come From?
Scientists are not entirely sure where the magma that formed the Stirling Sill came from deep underground. They know that there are some places where the magma pushed up, called "risers," especially near the Ochil Fault.
Possible Source
One idea is that the magma came from the same deep source that created the Ochil Hills much earlier. Geologists think this source might be somewhere east of Stirling, possibly near Kincardine. This would mean that after the first big volcanic activity that formed the Ochil Hills, there was a later burst of magma that created the sill. Finding the exact source is very difficult because it would require drilling many deep holes through very hard rock.
Uses of Stirling Sill Rock
The tough quartz-dolerite rock from the Stirling Sill has been used by people for a long time. In the past, it was dug out, or "quarried," in places like Cambusbarron and Murrayshall.
Modern Uses
Today, the rock is still quarried at a place called Northfield. It is mainly used as "roadstone." This means it's crushed up and used to build roads, making them strong and durable.
