Geology of the Cairngorms National Park facts for kids
The Cairngorms National Park is a special place in the Scottish Highlands. It was made a national park in 2003 and got bigger in 2010. This park is much larger than just the Cairngorms mountains themselves, so it has many different kinds of rocks and landforms.
Most of the rocks in the park are part of something called the Dalradian Supergroup. Imagine thick layers of sand, mud, and limestone that settled at the bottom of ancient seas between 800 and 600 million years ago. These layers were on the edge of a huge old continent called Laurentia. You can also find rocks from the Moine Supergroup along the park's northwestern side.
These Dalradian and Moine rocks went through massive changes during an event called the Caledonian Orogeny. This was a time when huge landmasses collided, causing the rocks to be intensely folded, broken by faults, and changed by heat and pressure (this is called metamorphism). This happened between about 490 and 430 million years ago. Geologists call one part of this event the 'Grampian event', around 470 million years ago. It happened when a chain of volcanic islands crashed into Laurentia. Later, another continent called Baltica collided with Laurentia, causing more folding and faulting. Big faults like the Great Glen, Ericht-Laidon, and Glen Tilt faults were very active then. They might have helped huge blobs of granite rock push up into the Dalradian rocks and cool down.
The biggest of these granite blobs is what makes up the Cairngorms mountains themselves. This granite formed about 427 million years ago. Scientists think that within 20 million years, the rocks above this granite had worn away, and the landscape we see today in the Cairngorms started to form by 390 million years ago. The granite you see on the surface now was once buried deep underground, about 4 to 7 kilometers deep!
Other than a small area of Old Red Sandstone, there aren't many younger solid rocks in the park. However, the ice ages over the last 2.5 million years have really shaped the land. They carved out valleys and left behind piles of rock and dirt. After the ice melted, things like peat (decayed plant matter) and landslides also changed the landscape.
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Ancient Rocks: Moine Supergroup
The Moine rocks are metamorphic, meaning they've been changed by heat and pressure. While most Moine rocks are found west of the Great Glen Fault, some similar rocks east of the fault are also thought to be part of the Moine group.
One type of Moine rock is the Markie Gneiss. It's a type of sandy rock found in blocks along the western edge of the park. It's very old, from the Neoproterozoic era, but its exact age is still being debated.
Badenoch Group: The Oldest Rocks
The very oldest rocks in the park are part of the Badenoch Group. These rocks are found along the northwestern edge of the park and are about 900 million years old. They are mostly sandy rocks (psammites) and rocks made from mud (semipelites). They have been intensely folded and changed, often becoming 'migmatised', which means they started to melt and mix.
The Badenoch Group is split into two parts: the Dava Subgroup (older) and the Glen Banchor Subgroup (younger). These rock layers are believed to be several kilometers thick. They are found northwest of a line running from the Grantown Pluton towards the Boat of Garten. You can see these rocks well exposed near Carrbridge.
Dalradian Supergroup: A Mix of Rocks
Apart from the granite rocks, almost all the rest of the park is made of the Dalradian Supergroup. These rocks are a complex mix of different types, including psammites (sandy), phyllites and pelites (muddy), quartzites (very hard sandstone), and meta-limestones (changed limestone). They were originally laid down as sand, mud, and silt in an ancient ocean basin that kept getting deeper and then filling up over a long time.
The Dalradian sequence is estimated to be 25 kilometers thick or even more! The rocks were deposited between about 750 and 500 million years ago. The Dalradian is divided into four main groups, and all of them can be found within the Cairngorms National Park. From oldest to youngest, they are:
- Southern Highland Group
- Argyll Group
- Tayvallich Subgroup
- Crinan Subgroup
- Easdale Subgroup
- Islay Subgroup
- Appin Group
- Blair Atholl Subgroup
- Ballachulish Subgroup
- Lochaber Subgroup
- Grampian Group
- Glen Spean Subgroup
- Corrieyairack Subgroup
- Glenshirra Subgroup
Grampian Group: The Oldest Dalradian Rocks
The Grampian Group is the oldest part of the Dalradian rocks. It's found from Islay in the southwest of Scotland all the way to Nairn in the northeast, and even in Ireland. This group is about 7-8 kilometers thick and is made of sandy rocks (psammites), muddy rocks (semipelites), and quartzites.
The very oldest layers in this group are meta-limestones and pelites, which might have formed in shallow seas. Above these are the rocks of the Corrieyairick Subgroup, which are thought to have been turbidites – deposits from underwater landslides – showing a time when the ocean basin was getting deeper. The youngest rocks in the Grampian Group are the Glen Spean Subgroup, which were laid down in shallow coastal areas as the basin gradually filled up.
Appin Group: Tidal and Shelf Deposits
Above the Grampian Group is the Appin Group. This group is divided into three subgroups: Lochaber (oldest), Ballachulish, and Blair Atholl (youngest). Many of these rocks formed in tidal areas and on shallow shelves as the ocean basin slowly sank and deepened.
The Lochaber Subgroup includes rocks like the Tom na Fianaig and Dalvrecht Slate formations. The Ballachulish Subgroup has rocks like the Ailnack Phyllite and Limestone. The youngest Appin rocks are in the Blair Atholl Subgroup, which formed when the basin was filling up. These include the Glenfiddich Pelite and Inchrory Limestone formations.
Argyll Group: Diverse Layers
The Argyll Group sits above the Appin Group. The oldest part is the Islay Subgroup, which includes the Kymah Quartzite. Above this is the Easdale Subgroup, with rocks like the Badenyon Schist and Limestone. The Crinan Subgroup contains the Queens Hill Gneiss. The youngest part of the Argyll Group is the Tayvallich Subgroup, which has the Deeside Limestone. This group also includes other formations like the Craigievar and Blackwater Schist.
Southern Highland Group: The Youngest Dalradian Rocks
The Southern Highland Group is the youngest part of the Dalradian. You can find it in the southeast of the park, around Glen Prosen and Glen Clova. It stretches all the way from Northern Ireland to the North Sea. Mountains like Mayar and Driesh are made from these rocks. This group includes the Clashindarroch and Suie Hill formations.
Old Red Sandstone: Younger Sediments
There's a small patch of Old Red Sandstone rocks around the Tomintoul area. These rocks are mainly sandstones from the Devonian period, but they also have siltstones and a layer of pebbles at the bottom. They are called the Tomintoul Group. The lowest part of these rocks might even be from the very end of the Silurian period.
These rocks form hills like Carn na Farraidh and Carn Meadhonach near Tomintoul. There are no solid rocks from later periods (like the late Devonian or after) found within the National Park.
Plutons: Giant Granite Blobs
Many large blobs of molten rock, called plutons, pushed their way into the Dalradian rocks towards the end of the Caledonian Orogeny, from the late Silurian to early Devonian times. These plutons are grouped into different 'suites' based on their chemical makeup.
The biggest pluton in the park is the Cairngorm pluton, which forms the central Cairngorms mountains. It covers about 365 square kilometers and formed around 427 million years ago. The Mount Battock pluton is similar in size, but only a part of it is inside the park. These huge granite masses have a dramatic effect on the landscape.
How the Land is Shaped: Structure
The way the rocks are arranged in the park is very complex because they have been bent, broken, and changed by many different events over millions of years. Two major events happened in the late Proterozoic era, followed by another during the Ordovician and Silurian periods. Metamorphism (rock changing) happened during some of these events.
The Ericht-Laidon Fault is a big fault line running from northeast to southwest through the west of the park. You can't see much of it on the surface, but it's mapped under Grantown-on-Spey and extends to Loch Ericht and Loch Laidon. The Loch Tay Fault is another major fault that shapes the landscape, running through the center of the park. It follows the very straight Glen Tilt and defines the eastern edge of the Cairngorm pluton. These faults are part of a family of faults that run parallel to the famous Great Glen Fault.
The Grampian Shear Zone is another important feature. It's where the Badenoch Group rocks meet the Grampian Group rocks. This zone is about 750 million years old.
Ice Ages and Landforms: Quaternary Period
The Cairngorms landscape is incredibly important for understanding how glaciers shape the land. It has many features left behind by glaciers and periglacial processes (processes happening around glaciers). These include:
- Corries (bowl-shaped hollows carved by glaciers) and glacial troughs (U-shaped valleys).
- Moraines (piles of rock and dirt left by glaciers).
- Kames (mounds of sand and gravel).
- Eskers (long, winding ridges of sand and gravel).
- Meltwater channels (channels carved by water flowing under or from glaciers).
Many beautiful lakes in the park are of glacial origin, like Loch nan Eun, Lochnagar, Loch Brandy, and Loch Wharral. Famous U-shaped valleys include Loch Avon, Glen Einich, the Lairig Ghru, and Strath Nethy. Meltwater channels are common, with notable examples at Chalamain Gap and Ryvoan Pass. The deposits from the last ice age (called the Devensian ice age) are known as the Caledonia Glacigenic Group.
Tors: Granite Towers
Tors are common in the Cairngorms, especially on Ben Avon and Beinn Mheadhoin, and impressively high on Bynack More. Tors are tall, isolated rock formations, often looking like towers. They form because some parts of the granite are less cracked than the surrounding rock. This means water can't get into them as easily to cause weathering. Over a very long time, especially during the ice ages, the softer, more cracked rock around them wore away, leaving these harder granite towers standing tall.
